What's the difference between an interface and an abstract class?
A. An abstract class may contain code in method bodies, which is not allowed in an interface. With abstract classes, you have to inherit your class from it and Java does not allow multiple inheritance. On the other hand, you can implement multiple interfaces in your class.
. Why would you use a synchronized block vs. synchronized method?
A. Synchronized blocks place locks for shorter periods than synchronized methods.
A. This keyword indicates that the value of this member variable does not have to be serialized with the object. When the class will be de-serialized, this variable will be initialized with a default value of its data type (i.e. zero for integers).
A. You can't force GC, but could request it by calling System.gc(). JVM does not guarantee that GC will be started immediately.
How do you know if an explicit object casting is needed?
A. If you assign a superclass object to a variable of a subclass's data type, you need to do explicit casting. For example:
Object a; Customer b; b = (Customer) a;
When you assign a subclass to a variable having a supeclass type, the casting is performed automatically.
CIO, CTO & Developer Resources
What's the difference between the methods sleep() and wait()
A. The code sleep(1000); puts thread aside for exactly one second. The code wait(1000), causes a wait of up to one second. A thread could stop waiting earlier if it receives the notify() or notifyAll() call. The method wait() is defined in the class Object and the method sleep() is defined in the class Thread.
A. Yes. Add a main() method to the applet.
What's the difference between constructors and other methods?
A. Constructors must have the same name as the class and can not return a value. They are only called once while regular methods could be called many times.
Can you call one constructor from another if a class has multiple constructors
A. Yes. Use this() syntax.
Explain the usage of Java packages.
A. This is a way to organize files when a project consists of multiple modules. It also helps resolve naming conflicts when different packages have classes with the same names. Packages access level also allows you to protect data from being used by the non-authorized classes.
A. You need to add a directory or a jar file that contains the package directories to the CLASSPATH environment variable. Let's say a class Employee belongs to a package com.xyz.hr; and is located in the file c:\dev\com\xyz\hr\Employee.java. In this case, you'd need to add c:\dev to the variable CLASSPATH. If this class contains the method main(), you could test it from a command prompt window as follows:
c:\>java com.xyz.hr.Employee
A.There's no difference, Sun Microsystems just re-branded this version.
A. I'd use the method equals() to compare the values of the Strings and the == to check if two variables point at the same instance of a String object.
A. Yes, it does. The FileNoFoundException is inherited from the IOException. Exception's subclasses have to be caught first.
A. It's possible if these variables are final.
* Q17. What can go wrong if you replace && with & in the following code:
String a=null; if (a!=null && a.length()>10) {...}
A. A single ampersand here would lead to a NullPointerException.
A. Java Vector class is internally synchronized and ArrayList is not.
A. This method is used to ensure that Swing components are updated through the event-dispatching thread.
A. Use the following syntax: super.myMethod(); To call a constructor of the superclass, just write super(); in the first line of the subclass's constructor.
What is the difference between the instanceof and getclass, these two are same or not ?
Answer: instanceof is a operator, not a function while getClass is a method of java.lang.Object class. Consider a condition where we use
if(o.getClass().getName().equals("java.lang.Math")){ }
This method only checks if the classname we have passed is equal to java.lang.Math. The class java.lang.Math is loaded by the bootstrap ClassLoader. This class is an abstract class.This class loader is responsible for loading classes. Every Class object contains a reference to the ClassLoader that defines. getClass() method returns the runtime class of an object. It fetches the java instance of the given fully qualified type name. The code we have written is not necessary, because we should not compare getClass.getName(). The reason behind it is that if the two different class loaders load the same class but for the JVM, it will consider both classes as different classes so, we can't compare their names. It can only gives the implementing class but can't compare a interface, but instanceof operator can.
The instanceof operator compares an object to a specified type. We can use it to test if an object is an instance of a class, an instance of a subclass, or an instance of a class that implements a particular interface. We should try to use instanceof operator in place of getClass() method. Remember instanceof opeator and getClass are not same. Try this example, it will help you to better understand the difference between the two.
Interface one{
}
Class Two implements one {
}
Class Three implements one {
}
public class Test {
public static void main(String args[]) {
one test1 = new Two();
one test2 = new Three();
System.out.println(test1 instanceof one); //true
System.out.println(test2 instanceof one); //true
System.out.println(Test.getClass().equals(test2.getClass())); //false
}
}
Does it matter in what order catch statements for FileNotFoundException and IOExceptipon are written?
Answer: Yes, it does. The FileNoFoundException is inherited from the IOException. Exception's subclasses have to be caught first.
Ans) In Java, "thread" means two different things:
An instance of class java.lang.Thread.
A thread of execution.
An instance of Thread is just…an object. Like any other object in Java, it has variables and methods, and lives and dies on the heap. But a thread of execution is an individual process (a "lightweight" process) that has its own call stack. In Java, there is one thread per call stack—or, to think of it in reverse, one call stack per thread. Even if you don't create any new threads in your program, threads are back there running.
The main() method, that starts the whole ball rolling, runs in one thread, called (surprisingly) the main thread. If you looked at the main call stack (and you can, any time you get a stack trace from something that happens after main begins, but not within another thread), you'd see that main() is the first method on the stack— the method at the bottom. But as soon as you create a new thread, a new stack materializes and methods called from that thread run in a call stack that's separate from the main() call stack.
What is difference between thread and process?
Ans) Differences between threads and processes are:-
1. Threads share the address space of the process that created it; processes have their own address.
2. Threads have direct access to the data segment of its process; processes have their own copy of the data segment of the parent process.
3. Threads can directly communicate with other threads of its process; processes must use interprocess communication to communicate with sibling processes.
4. Threads have almost no overhead; processes have considerable overhead.
5. New threads are easily created; new processes require duplication of the parent process.
6. Threads can exercise considerable control over threads of the same process; processes can only exercise control over child processes.
7. Changes to the main thread (cancellation, priority change, etc.) may affect the behavior of the other threads of the process; changes to the parent process do not affect child processes.
What are the advantages or usage of threads?
Ans)
Threads support concurrent operations. For example,
• Multiple requests by a client on a server can be handled as an individual client thread.
• Long computations or high-latency disk and network operations can be handled in the background without disturbing foreground computations or screen updates.
Threads often result in simpler programs.
• In sequential programming, updating multiple displays normally requires a big while-loop that performs small parts of each display update. Unfortunately, this loop basically simulates an operating system scheduler. In Java, each view can be assigned a thread to provide continuous updates.
• Programs that need to respond to user-initiated events can set up service routines to handle the events without having to insert code in the main routine to look for these events.
Threads provide a high degree of control.
• Imagine launching a complex computation that occasionally takes longer than is satisfactory. A "watchdog" thread can be activated that will "kill" the computation if it becomes costly, perhaps in favor of an alternate, approximate solution. Note that sequential programs must muddy the computation with termination code, whereas, a Java program can use thread control to non-intrusively supervise any operation.
Threaded applications exploit parallelism.
• A computer with multiple CPUs can literally execute multiple threads on different functional units without having to simulating multi-tasking ("time sharing").
• On some computers, one CPU handles the display while another handles computations or database accesses, thus, providing extremely fast user interface response times.
What are the two ways of creating thread?
Ans) There are two ways to create a new thread.
1)Extend the Thread class and override the run() method in your class. Create an instance of the subclass and invoke the start() method on it, which will create a new thread of execution. e.g.
public class NewThread extends Thread{
public void run(){
// the code that has to be executed in a separate new thread goes here
}
public static void main(String [] args){
NewThread c = new NewThread();
c.start();
}
}
2)Implements the Runnable interface.The class will have to implement the run() method in the Runnable interface. Create an instance of this class. Pass the reference of this instance to the Thread constructor a new thread of execution will be created. e.g. class
public class NewThread implements Runnable{
public void run(){
// the code that has to be executed in a separate new thread goes here
}
public static void main(String [] args){
NewThread c = new NewThread();
Thread t = new Thread(c);
t.start();
}
}
What are the different states of a thread's lifecycle?
Ans) The different states of threads are as follows:
1) New – When a thread is instantiated it is in New state until the start() method is called on the thread instance. In this state the thread is not considered to be alive.
2) Runnable – The thread enters into this state after the start method is called in the thread instance. The thread may enter into the Runnable state from Running state. In this state the thread is considered to be alive.
3) Running – When the thread scheduler picks up the thread from the Runnable thread’s pool, the thread starts running and the thread is said to be in Running state.
4) Waiting/Blocked/Sleeping – In these states the thread is said to be alive but not runnable. The thread switches to this state because of reasons like wait method called or sleep method has been called on the running thread or thread might be waiting for some i/o resource so blocked. 5) Dead – When the thread finishes its execution i.e. the run() method execution completes, it is said to be in dead state. A dead state can not be started again. If a start() method is invoked on a dead thread a runtime exception will occur.
What is use of synchronized keyword?
Ans) synchronized keyword can be applied to static/non-static methods or a block of code. Only one thread at a time can access synchronized methods and if there are multiple threads trying to access the same method then other threads have to wait for the execution of method by one thread. Synchronized keyword provides a lock on the object and thus prevents race condition. E.g.
public void synchronized method(){}
public void synchronized staticmethod(){}
public void myMethod(){
synchronized (this){ // synchronized keyword on block of code
}
}
What is the difference when the synchronized keyword is applied to a static method or to a non static method?
Ans) When a synch non static method is called a lock is obtained on the object. When a synch static method is called a lock is obtained on the class and not on the object. The lock on the object and the lock on the class don’t interfere with each other. It means, a thread accessing a synch non static method, then the other thread can access the synch static method at the same time but can’t access the synch non static method.
What is a volatile keyword?
Ans) In general each thread has its own copy of variable, such that one thread is not concerned with the value of same variable in the other thread. But sometime this may not be the case. Consider a scenario in which the count variable is holding the number of times a method is called for a given class irrespective of any thread calling, in this case irrespective of thread access the count has to be increased so the count variable is declared as volatile. The copy of volatile variable is stored in the main memory, so every time a thread access the variable even for reading purpose the local copy is updated each time from the main memory. The volatile variable also have performance issues.
What is the difference between yield() and sleep()?
Ans) yield() allows the current the thread to release its lock from the object and scheduler gives the lock of the object to the other thread with same priority.
sleep() allows the thread to go to sleep state for x milliseconds. When a thread goes into sleep state it doesn’t release the lock.
What is the difference between wait() and sleep()?
Ans)
1) wait() is a method of Object class. sleep() is a method of Object class.
2) sleep() allows the thread to go to sleep state for x milliseconds. When a thread goes into sleep state it doesn’t release the lock. wait() allows thread to release the lock and goes to suspended state. The thread is only active when a notify() or notifAll() method is called for the same object.
What is difference between notify() and notfiyAll()?
Ans) notify( ) wakes up the first thread that called wait( ) on the same object.
notifyAll( ) wakes up all the threads that called wait( ) on the same object. The
highest priority thread will run first.
What happens if a start method is not invoked and the run method is directly invoked?
Ans) If a thread has been instantiated but not started its is said to be in new state. Unless until a start() method is invoked on the instance of the thread, it will not said to be alive. If you do not call a start() method on the newly created thread instance thread is not considered to be alive. If the start() method is not invoked and the run() method is directly called on the Thread instance, the code inside the run() method will not run in a separate new thread but it will start running in the existing thread.
What happens when start() is called?
Ans) A new thread of execution with a new call stack starts. The state of thread changes from new to runnable. When the thread gets chance to execute its target run() method starts to run.
If code running is a thread creates a new thread what will be the initial priority of the newly created thread?
Ans) When a code running in a thread creates a new thread object , the priority of the new thread is set equal to the priority of the thread which has created it.
When jvm starts up, which thread will be started up first?
Ans) When jvm starts up the thread executing main method is started.
What are the daemon threads?
Ans) Daemon thread are service provider threads run in the background,these not used to run the application code generally.When all user threads(non-daemon threads) complete their execution the jvm exit the application whatever may be the state of the daemon threads. Jvm does not wait for the daemon threads to complete their execution if all user threads have completed their execution.
To create Daemon thread set the daemon value of Thread using setDaemon(boolean value) method. By default all the threads created by user are user thread. To check whether a thread is a Daemon thread or a user thread use isDaemon() method.
Example of the Daemon thread is the Garbage Collector run by jvm to reclaim the unused memory by the application. The Garbage collector code runs in a Daemon thread which terminates as all the user threads are done with their execution.
What all constructors are present in the Thread class?
Ans) Thread()
Thread(Runnable target)
Thread(Runnable target, String name)
Thread(String name)
Can the variables or classes be Synchronized?
Ans) No. Only methods can be synchronized.
Q19) How many locks does an object have?
Ans) Each object has only one lock.
Can a class have both Synchronized and non-synchronized methods?
Ans) Yes a class can have both synchronized and non-synchronized methods.
Java Keywords
Ans) There are four different types of modifiers:
Modifer
Accessible in the same package
Accessible in different package
Private
No
No
Protected
Yes
Yes, only if the class extends the main class
Default
Yes
No
Public
Yes
Yes
What is the use of final keyword?
Ans) The final keyword can be assigned to
Class level variable
method
class
Objects
If a final is assigned to a variable, the variable behaves as a constant. It means that the value of variable once set cannot be changed.
final int i=1;
i =5; // error
If a final is assigned to a method then it cannot be overridden in its child class.
class Parent {
final void print(){
System.out.println(“Inside”);
}
}
class Child extends Parent{
public final void print(){ // error cannot override final method
System.out.println(“Inside”);
}
}
If a class is made as final, then no other class can extend it and make it as parent class. E.g. String Class.
Final objects are instantiated only once. i.e
final Map map = new HashMap();
map.put(“key”,”value”);
map = new HashMap(); // error
What is use of synchronized keyword?
Ans) This keyword is used to prevent concurrency. Synchronized keyword can be applied to static/non-static methods or a block of code. Only one thread at a time can access synchronized methods and if there are multiple threads trying to access the same method then other threads have to wait for the execution of method by one thread. Synchronized keyword provides a lock on the object and thus prevents race condition. E.g.
public void synchronized method(){}
public void synchronized staticmethod(){}
public void myMethod(){
synchronized (this){ // synchronized keyword on block of code
}
}
What is volatile keyword?
Ans) In general each thread has its own copy of variable, such that one thread is not concerned with the value of same variable in the other thread. But sometime this may not be the case. Consider a scenario in which the count variable is holding the number of times a method is called for a given class irrespective of any thread calling, in this case irrespective of thread access the count has to be increased. In this case the count variable is declared as volatile. The copy of volatile variable is stored in the main memory, so every time a thread access the variable even for reading purpose the local copy is updated each time from the main memory. The volatile variable also have performance issues.
What is a transient variable?
Ans) If some of the properties of a class are not required to be serialized then the varaibles are marked as transient. When an object is deserialized the transient variables retains the default value depending on the type of variable declared and hence lost its original value.
What is a strictfp modifier?
Ans) Strictfp is used with variable only . It is used to restrict floating point calculations ( fp ) to ensure portability ( platform Independent ). When this modifier is specified, the JVM adheres to the Java specifications ( IEEE-754 floating-point specification ) and returns the consistent value independent of the platform. That is, if you want the answers from your code (which uses floating point values) to be consistent in all platforms, then you need to specify the strictfp modifier.
What is a static variable?
Ans) Static keyword can be used with the variables and methods but not with the class but there are static class. Anything declared as static is related to class and not objects.
Static variable : Multiples objects of a class shares the same instance of a static variable.Consider the example:
public class Counter{
private static int count=0;
private int nonStaticcount=0;
public void incrementCounter(){
count++;
nonStaticcount++;
}
public int getCount(){
return count;
}
public int getNonStaticcount(){
return nonStaticcount;
}
public static void main(String args[]){
Counter countObj1 = new Counter();
Counter countObj2 = new Counter();
countObj1.incrementCounter();
countObj1.incrementCounter();
System.out.println("Static count for Obj1: "+countObj1.getCount());
System.out.println("NonStatic count for Obj1: "+countObj1.getNonStaticcount());
System.out.println("Static count for Obj2: "+countObj2.getCount())
System.out.println("NonStatic count for Obj2: "+countObj2.getNonStaticcount())
}
Output
Static count for Obj1: 2
NonStatic count for Obj1: 2
Static count for Obj2: 2
NonStatic count for Obj2: 0
In the above program obj1 and obj2 share the same instance of static variable count hence if the value is incremented by one object , the incremented value will be reflected across the other objects.
What is a static method?
Ans)A method defined as static is called static method. A static method can be accessed without creating the objects. Just by using the Class name the method can be accessed.
Static method can only access static variables and not local or global non-static variables. For eg:
public class Test{
public static void printMe(){
System.out.println("Hello World");
}
}
public class MainClass{
public static void main(String args[]){
Test.printMe()
}
}
OutPut:
Hello World
Also static method can call only static methods and not non static methods. But non-static methods can call static mehtods.
Why static methods cannot access non static variables or methods?
Ans) A static method cannot access non static variables or methods because static methods doesnt need the object to be accessed. So if a static method has non static variables or non static methods which has instantiated variables they will no be intialized since the object is not created and this could result in an error.
What is static class ?
Ans) A class cannot be declared static. But a class can be said a static class if all the variables and methods of the class are static and the constructor is private. Making the constructor private will prevent the class to be instantiated. So the only possibility to access is using Class name only
What is throw keyword?
Ans) Throw keyword is used to throw the exception manually. It is mainly used when the program fails to satisfy the given condition and it wants to warn the application.The exception thrown should be subclass of Throwable.
public void parent(){
try{
child();
}catch(MyCustomException e){ }
}
public void child{
String iAmMandatory=null;
if(iAmMandatory == null){
throw (new MyCustomException("Throwing exception using throw keyword");
}
}
What is use of throws keyword?
Ans) If the function is not capable of handling the exception then it can ask the calling method to handle it by simply putting the throws clauseat the function declaration.
public void parent(){
try{
child();
}catch(MyCustomException e){ }
}
public void child throws MyCustomException{
//put some logic so that the exception occurs.
}
Key Concepts in Java
Why is main() method static?
Ans) To access the static method the object of the class is not needed. The method can be access directly with the help of ClassName. So when a program is started the jvm search for the class with main method and calls it without creating an object of the class.
What is the difference between static methods and instance methods?
Ans) instance method belongs to the instance of a class therefore it requires an instance before it can be invoked, whereas static method belongs to the class itself and not to any class instance so it doesn’t need an instance to be invoked.
Instance methods use dynamic (late) binding, whereas static methods use static (early) binding.
When the JVM invokes a class instance method, it selects the method to invoke based on the type of the object reference, which is always known at run-time. On the other hand, when the JVM invokes a static method, it selects the method to invoke based on the actual class of the object, which may only be known at compile time.
Can static block throw exception?
Ans) Yes, static block can throw only Runtime exception or can use a try-catch block to catch checked exception.
Typically scenario will be if JDBC connection is created in static block and it fails then exception can be caught, logged and application can exit. If System.exit() is not done, then application may continue and next time if the class is referred JVM will throw NoClassDefFounderror since the class was not loaded by the Classloader.
What is difference between abstract class and interface?
Ans) 1) A class is called abstract when it contains at least one abstract method. It can also contain n numbers of concrete method.Interface can contain only abstract( non implemented) methods.
2) The abstract class can have public,private,protect or default variables and also constants. In interface the variable is by default public final. In nutshell the interface doesnt have any variables it only has constants.
3) A class can extend only one abstract class but a class can implement multiple interfaces.
4) If an interface is implemented its compulsory to implement all of its methods but if an abstract class is extended its not compulsory to implement all methods. 5) The problem with an interface is, if you want to add a new feature (method) in its contract, then you MUST implement those method in all of the classes which implement that interface. However, in the case of an abstract class, the method can be simply implemented in the abstract class and the same can be called by its subclass.
Explain with example to describe when to use abstract class and interface?
Ans) Consider a scenario where all Cars will have 4 tyres and other features can be different.
In this case any subclass of Car has to have 4 tyres. This is a case where abstract class will be used and a default implementaion for tyres will be provided.
public abstract class Car{
public abstract String getCarName();
public final int getNoOfTyres(){
return 4;
}
}
Consider a scenario where Cars can have any number of tyres and other features can also be different. In this case interface will be created.
public interface Car{
public abstract String getCarName();
public abstract int getNoOfTyres();
}
Does java support multiple interitance? Why?
Ans) Java doesnt support multiple inheritance but it provide a way through which it can enact it.
Consider the scenario is C++
Class A{
public void add(){
// some text
}
}
Class B{
public void add(){
// some text
}
}
Class C extends A,B{
public static void main(String arg[]){
C objC = new C();
objC.add(); // problem, compiler gets confused and cant
decide to call Class A or B method.
}
This problem is called Diamond problem.
This problem in java is taken care with the use of interfaces
In Java similar problem would look like:
interface A{
add();
}
interface B{
add();
}
class C implements A,B{
add(){
// doesnt matter which interface it belong to
}
}
Can this keyword be assigned null value?
Ans) No
What are the different types of references in java?
Ans) Java has a more expressive system of reference than most other garbage-collected programming languages, which allows for special behavior for garbage collection. A normal reference in Java is known as a strong reference. The java.lang.ref package defines three other types of references—soft, weak, and phantom references. Each type of reference is designed for a specific use.
A SoftReference can be used to implement a cache. An object that is not reachable by a strong reference (that is, not strongly reachable), but is referenced by a soft reference is called softly reachable. A softly reachable object may be garbage collected at the discretion of the garbage collector. This generally means that softly reachable objects will only be garbage collected when free memory is low, but again, it is at the discretion of the garbage collector. Semantically, a soft reference means "keep this object unless the memory is needed."
A WeakReference is used to implement weak maps. An object that is not strongly or softly reachable, but is referenced by a weak reference is called weakly reachable. A weakly reachable object will be garbage collected during the next collection cycle. This behavior is used in the class java.util.WeakHashMap. A weak map allows the programmer to put key/value pairs in the map and not worry about the objects taking up memory when the key is no longer reachable anywhere else. Another possible application of weak references is the string intern pool. Semantically, a weak reference means "get rid of this object when nothing else references it."
A PhantomReference is used to reference objects that have been marked for garbage collection and have been finalized, but have not yet been reclaimed. An object that is not strongly, softly or weakly reachable, but is referenced by a phantom reference is called phantom reachable. This allows for more flexible cleanup than is possible with the finalization mechanism alone. Semantically, a phantom reference means "this object is no longer needed and has been finalized in preparation for being collected."
How to change the heap size of a JVM?
Ans) The old generation's default heap size can be overridden by using the -Xms and -Xmx switches to specify the initial and maximum sizes respectively:
java -Xms -Xmx program
For example:
java -Xms64m -Xmx128m program
What is difference between instanceof and isInstance(Object obj)?
Ans) Differences are as follows:
1) instanceof is a reserved word of Java, but isInstance(Object obj) is a method of java.lang.Class.
if (obj instanceof MyType) {
...
}else if (MyType.class.isInstance(obj)) {
...
}
2) instanceof is used of identify whether the object is type of a particular class or its subclass but isInstance(obj) is used to identify object of a particular class.
Java supports pass by value or pass by reference?
Ans) Java supports only pass by value. The arguments passed as a parameter to a method is mainly primitive data types or objects. For the data type the actual value is passed.
Java passes the references by value just like any other parameter. This means the references passed to the method are actually copies of the original references.Java copies and passes the reference by value, not the object. Thus, method manipulation will alter the objects, since the references point to the original objects.Consider the example:
public void tricky(Point arg1, Point arg2)
{
arg1.x = 100;
arg1.y = 100;
Point temp = arg1;
arg1 = arg2;
arg2 = temp;
}
public static void main(String [] args)
{
Point pnt1 = new Point(0,0);
Point pnt2 = new Point(0,0);
System.out.println("X: " + pnt1.x + " Y: " +pnt1.y);
System.out.println("X: " + pnt2.x + " Y: " +pnt2.y);
System.out.println(" ");
tricky(pnt1,pnt2);
System.out.println("X: " + pnt1.x + " Y:" + pnt1.y);
System.out.println("X: " + pnt2.x + " Y: " +pnt2.y);
}
OutPut:
X: 0 Y: 0
X: 0 Y: 0
X: 100 Y: 100
X: 0 Y: 0
The method successfully alters the value of pnt1, even though it is passed by value; however, a swap of pnt1 and pnt2 fails! This is the major source of confusion. In the main() method, pnt1 and pnt2 are nothing more than object references. When you pass pnt1 and pnt2 to the tricky() method, Java passes the references by value just like any other parameter. This means the references passed to the method are actually copies of the original references.
What is memory leak?
Ans) A memory leak is where an unreferenced object that will never be used again still hangs around in memory and doesnt get garbage collected.
What is the difference between equals() and ==?
Ans) == operator is used to compare the references of the objects.
public bollean equals(Object o) is the method provided by the Object class. The default implementation uses == operator to compare two objects.
But since the method can be overriden like for String class. equals() method can be used to compare the values of two objects.
String str1 = "MyName";
String str2 = "MyName";
String str3 = str2;
if(str1 == str2){
System.out.println("Objects are equal")
}else{
System.out.println("Objects are not equal")
}
if(str1.equals(str2)){
System.out.println("Objects are equal")
}else{
System.out.println("Objects are not equal")
}
Output:
Objects are not equal
Objects are equal
String str2 = "MyName";
String str3 = str2;
if(str2 == str3){
System.out.println("Objects are equal")
}else{
System.out.println("Objects are not equal")
}
if(str3.equals(str2)){
System.out.println("Objects are equal")
}else{
System.out.println("Objects are not equal")
}
OutPut:
Objects are equal
Objects are equal
How to make sure that Childclass method actually overrides the method of the superclass?
Ans) The @Override annotation can be added to the javadoc for the new method. If you accidently miss an argument or capitalize the method name wrong, the compiler will generate a compile-time error.
How to find the size of an object?
Ans)The heap size of an object can be found using -
Runtime.totalMemory()-Runtime.freeMemory() .
Can an abstract class have a static method?
Ans) Yes an abstract class have a static method and it can be accessed by any other class(even not a concrete class).
Can an abstract class have a constructor?
Ans) Yes an abstract class have a default and parameterized constructors.
Why static methods cannot access non static variables or methods?
Ans) A static method cannot access non static variables or methods because static methods doesnt need the object to be accessed. So if a static method has non static variables or non static methods which has instantiated variables they will no be intialized since the object is not created and this could result in an error.
What is difference between stringbuffer and stringbuilder?
Ans) The only difference between StringBuffer and StringBuilder is that StringBuilder is unsynchronized whereas StringBuffer is synchronized. So when the application needs to be run only in a single thread then it is better to use StringBuilder. StringBuilder is more efficient than StringBuffer.
Criteria to choose among StringBuffer and StringBuilder
1)If your text can change and will only be accessed from a single thread, use a StringBuilder 2)because StringBuilder is unsynchronized.
If your text can changes, and will be accessed from multiple threads, use a StringBuffer because StringBuffer is synchronous.
Consider a scenario in which the admin want to sure that if some one has written System.exit() at some part of application then before system shutdown all the resources should be released. How is it possible?
Ans) This is possible using Runtime.getRuntime().addShutdownHook(Thread hook).
Straight from Java Spec:
This method registers a new virtual-machine shutdown hook.
The Java virtual machine shuts down in response to two kinds of events:
1. The program exits normally, when the last non-daemon thread exits or when the exit (equivalently, System.exit) method is invoked, or
2. The virtual machine is terminated in response to a user interrupt, such as typing ^C, or a system-wide event, such as user logoff or system shutdown.
A shutdown hook is simply an initialized but unstarted thread. When the virtual machine begins its shutdown sequence it will start all registered shutdown hooks in some unspecified order and let them run concurrently. When all the hooks have finished it will then run all uninvoked finalizers if finalization-on-exit has been enabled. Finally, the virtual machine will halt. Note that daemon threads will continue to run during the shutdown sequence, as will non-daemon threads if shutdown was initiated by invoking the exit method.
Once the shutdown sequence has begun it can be stopped only by invoking the halt method, which forcibly terminates the virtual machine.
What is the difference between final, finally and finalize() in Java?
Ans)
final - constant declaration. A final variable act as constant, a final class is immutable and a final method cannot be ovrriden.
finally - handles exception. The finally block is optional and provides a mechanism to clean up regardless of what happens within the try block (except System.exit(0) call). Use the finally block to close files or to release other system resources like database connections, statements etc.
finalize() - method helps in garbage collection. A method that is invoked before an object is discarded by the garbage collector, allowing it to clean up its state. Should not be used to release non-memory resources like file handles, sockets, database connections etc because Java has only a finite number of these resources and you do not know when the garbage collection is going to kick in to release these non-memory resources through the finalize() method.
How does Java allocate stack and heap memory?
Ans) Each time an object is created in Java it goes into the area of memory known as heap. The primitive variables like int and double are allocated in the stack, if they are local method variables and in the heap if they are member variables (i.e. fields of a class). In Java methods local variables are pushed into stack when a method is invoked and stack pointer is decremented when a method call is completed.
In a multi-threaded application each thread will have its own stack but will share the same heap. This is why care should be taken in your code to avoid any concurrent access issues in the heap space. The stack is threadsafe (each thread will have its own stack) but the heap is not threadsafe unless guarded with synchronisation through your code.
Explain re-entrant, recursive and idempotent methods/functions?
Ans) A method in stack is re-entrant allowing multiple concurrent invocations that do not interfere with each other.
A function is recursive if it calls itself. Given enough stack space, recursive method calls are perfectly valid in Java though it is tough to debug. Recursive functions are useful in removing iterations from many sorts of algorithms. Allrecursive functions are re-entrant but not all re-entrant functions are recursive. Idempotent methods are methods, which are written in such a way that repeated calls to the same method with the same arguments yield same results. For example clustered EJBs, which are written with idempotent methods, can automatically recover from a server failure as long as it can reach another server.
Can a private variable or method of a class can be accessed?
Ans) Yes its possible using reflection.
What is difference between static block and the init block?
Or
Difference between
Static {} and {}
Ans) The static block is loaded when the class is loaded by the JVM for the 1st time only whereas init {} block is loaded every time class is loaded. Also first the static block is loaded then the init block.
public class LoadingBlocks {
static{
System.out.println("Inside static");
}
{
System.out.println("Inside init");
}
public static void main(String args[]){
new LoadingBlocks();
new LoadingBlocks();
new LoadingBlocks();
}
}
Output:
Inside static
Inside init
Inside init
Inside init
Why inner class can access only final variable?
Ans) Local classes can most definitely reference instance variables. The reason they cannot reference non final local variables is because the local class instance can remain in memory after the method returns. When the method returns the local variables go out of scope, so a copy of them is needed. If the variables weren't final then the copy of the variable in the method could change, while the copy in the local class didn't, so they'd be out of synch.
Anonymous inner classes require final variables because of the way they are implemented in Java. An anonymous inner class (AIC) uses local variables by creating a private instance field which holds a copy of the value of the local variable. The inner class isn't actually using the local variable, but a copy. It should be fairly obvious at this point that a "Bad Thing"â„¢ can happen if either the original value or the copied value changes; there will be some unexpected data synchronization problems. In order to prevent this kind of problem, Java requires you to mark local variables that will be used by the AIC as final (i.e., unchangeable). This guarantees that the inner class' copies of local variables will always match the actual values.
What is fully abstract class?
Ans) An abstract class which has all methods as abstract and all fields are public static final.
What is dynamic binding and static binding?
Ans) Method invocation
The Java programming language provides two basic kinds of methods: instance methods and class (or static) methods. The difference are:
1. Instance methods require an instance before they can be invoked, whereas class methods do not.
2. Instance methods use dynamic (late) binding, whereas class methods use static (early) binding.
When the Java virtual machine invokes a class method, it selects the method to invoke based on the type of the object reference, which is always known at compile-time. On the other hand, when the virtual machine invokes an instance method, it selects the method to invoke based on the actual class of the object, which may only be known at run time.
What is Java Reflection?
Ans)Reflection is commonly used by programs which require the ability to examine or modify the runtime behavior of applications running in the Java virtual machine.
Drawbacks of Reflection: Reflection is powerful, but should not be used indiscriminately. If it is possible to perform an operation without using reflection, then it is preferable to avoid using it. The following concerns should be kept in mind when accessing code via reflection.
Performance Overhead: Because reflection involves types that are dynamically resolved, certain Java virtual machine optimizations can not be performed. Consequently, reflective operations have slower performance than their non-reflective counterparts, and should be avoided in sections of code which are called frequently in performance-sensitive applications.
Security Restrictions: Reflection requires a runtime permission which may not be present when running under a security manager. This is in an important consideration for code which has to run in a restricted security context, such as in an Applet.
Exposure of Internals: Since reflection allows code to perform operations that would be illegal in non-reflective code, such as accessing private fields and methods, the use of reflection can result in unexpected side-effects, which may render code dysfunctional and may destroy portability. Reflective code breaks abstractions and therefore may change behavior with upgrades of the platform.
When an obj is passed through a function , one can set the properties but cannot set a new memory location?
Ans) It is because when u pass an object the address value is passed and stored in some new address . like if address 1234 is passed , it is stored in 4567 location. So if u change in the value of an object it will take the address from 4567 and do 1234.setXXX(). If u set the object to null it will set 4567=null.
OOPS Concept
What is polymorphism?
Ans) Polymorphism gives us the ultimate flexibility in extensibility. The abiltiy to define more than one function with the same name is called Polymorphism. In java,c++ there are two type of polymorphism: compile time polymorphism (overloading) and runtime polymorphism (overriding).
When you override methods, JVM determines the proper methods to call at the program’s run time, not at the compile time. Overriding occurs when a class method has the same name and signature as a method in parent class.
Overloading occurs when several methods have same names with
Overloading is determined at the compile time.
Different method signature and different number or type of parameters.
Same method signature but different number of parameters.
Same method signature and same number of parameters but of different type
Example of Overloading
int add(int a,int b)
float add(float a,int b)
float add(int a ,float b)
void add(float a)
int add(int a)
void add(int a) //error conflict with the method int add(int a)
Example: Overloading
Class BookDetails{
String title;
String publisher;
float price;
setBook(String title){
}
setBook(String title, String publisher){
}
setBook(String title, String publisher,float price){
}
}
Example: Overriding
class BookDetails{
String title;
setBook(String title){ }
}
class ScienceBook extends BookDetails{
setBook(String title){} //overriding
setBook(String title, String publisher,float price){ } //overloading
}
What is inheritance?
Ans) Inheritance is the property which allows a Child class to inherit some properties from its parent class. In Java this is achieved by using extends keyword. Only properties with access modifier public and protected can be accessed in child class.
public class Parent{
public String parentName;
public int parentage;
public String familyName;
}
public class Child extends Parent{
public String childName;
public int childAge;
public void printMyName(){
System.out.println(“ My name is “+ chidName+” “ +familyName)
}
}
In above example the child has inherit its family name from the parent class just by inheriting the class.
What is multiple inheritance and does java support?
Ans) If a child class inherits the property from multiple classes is known as multiple inheritance.
Java does not allow to extend multiple classes but to overcome this problem it allows to implement multiple Interfaces.
What is abstraction?
Ans) Abstraction is way of converting real world objects in terms of class. For example creating a class Vehicle and injecting properties into it. E.g
public class Vehicle {
public String colour;
public String model;
}
What is encapsulation?
Ans) The encapsulation is achieved by combining the methods and attribute into a class. The class acts like a container encapsulating the properties. The users are exposed mainly public methods.The idea behind is to hide how thinigs work and just exposing the requests a user can do.
What is Association?
Ans) Association is a relationship between two classes. In this relationship the object of one instance perform an action on behalf of the other class. The typical behaviour can be invoking the method of other class and using the member of the other class.
public class MyMainClass{
public void init(){
new OtherClass.init();
}
}
What is Aggregation?
Ans) Aggregation has a relationship between two classes. In this relationship the object of one class is a member of the other class. Aggregation always insists for a direction.
public class MyMainClass{
OtherClass otherClassObj = new OtherClass();
}
What is Composition?
Ans) Composition is a special type of aggregation relationship with a difference that its the compulsion for the OtherClass object (in previous example) to exist for the existence of MyMainClass.
Collections Interview Questions
What is difference between ArrayList and vector?
Ans: )
1) Synchronization - ArrayList is not thread-safe whereas Vector is thread-safe. In Vector class each method like add(), get(int i) is surrounded with a synchronized block and thus making Vector class thread-safe.
2) Data growth - Internally, both the ArrayList and Vector hold onto their contents using an Array. When an element is inserted into an ArrayList or a Vector, the object will need to expand its internal array if it runs out of room. A Vector defaults to doubling the size of its array, while the ArrayList increases its array size by 50 percent.
How can Arraylist be synchronized without using Vector?
Ans) Arraylist can be synchronized using:
Collection.synchronizedList(List list)
Other collections can be synchronized:
Collection.synchronizedMap(Map map)
Collection.synchronizedCollection(Collection c)
If an Employee class is present and its objects are added in an arrayList. Now I want the list to be sorted on the basis of the employeeID of Employee class. What are the steps?
Ans) 1) Implement Comparable interface for the Employee class and override the compareTo(Object obj) method in which compare the employeeID
2) Now call Collections.sort() method and pass list as an argument.
Now consider that Employee class is a jar file.
1) Since Comparable interface cannot be implemented, create Comparator and override the compare(Object obj, Object obj1) method .
2) Call Collections.sort() on the list and pass comparator as an argument.
What is difference between HashMap and HashTable?
Ans) Both collections implements Map. Both collections store value as key-value pairs. The key differences between the two are
1. Hashmap is not synchronized in nature but hshtable is.
2. Another difference is that iterator in the HashMap is fail-safe while the enumerator for the Hashtable isn't.
Fail-safe - “if the Hashtable is structurally modified at any time after the iterator is created, in any way except through the iterator's own remove method, the iterator will throw a ConcurrentModificationException�
3. HashMap permits null values and only one null key, while Hashtable doesn't allow key or value as null.
What are the classes implementing List interface?
Ans)
There are three classes that implement List interface:
1) ArrayList : It is a resizable array implementation. The size of the ArrayList can be increased dynamically also operations like add,remove and get can be formed once the object is created. It also ensures that the data is retrieved in the manner it was stored. The ArrayList is not thread-safe.
2) Vector: It is thread-safe implementation of ArrayList. The methods are wrapped around a synchronized block.
3) LinkedList: the LinkedList also implements Queue interface and provide FIFO(First In First Out) operation for add operation. It is faster if than ArrayList if it performs insertion and deletion of elements from the middle of a list.
Which all classes implement Set interface?
Ans) A Set is a collection that contains no duplicate elements. More formally, sets contain no pair of elements e1 and e2 such that e1.equals(e2), and at most one null element. HashSet,SortedSet and TreeSet are the commnly used class which implements Set interface.
SortedSet - It is an interface which extends Set. A the name suggest , the interface allows the data to be iterated in the ascending order or sorted on the basis of Comparator or Comparable interface. All elements inserted into the interface must implement Comparable or Comparator interface.
TreeSet - It is the implementation of SortedSet interface.This implementation provides guaranteed log(n) time cost for the basic operations (add, remove and contains). The class is not synchronized.
HashSet: This class implements the Set interface, backed by a hash table (actually a HashMap instance). It makes no guarantees as to the iteration order of the set; in particular, it does not guarantee that the order will remain constant over time. This class permits the null element. This class offers constant time performance for the basic operations (add, remove, contains and size), assuming the hash function disperses the elements properly among the buckets
What is difference between List and a Set?
Ans)
1) List can contain duplicate values but Set doesnt allow. Set allows only to unique elements.
2) List allows retrieval of data to be in same order in the way it is inserted but Set doesnt ensures the sequence in which data can be retrieved.(Except HashSet)
What is difference between Arrays and ArrayList ?
Ans) Arrays are created of fix size whereas ArrayList is of not fix size. It means that once array is declared as :
int [] intArray= new int[6];
intArray[7] // will give ArraysOutOfBoundException.
Also the size of array cannot be incremented or decremented. But with arrayList the size is variable.
Once the array is created elements cannot be added or deleted from it. But with ArrayList the elements can be added and deleted at runtime.
List list = new ArrayList();
list.add(1);
list.add(3);
list.remove(0) // will remove the element from the 1st location.
ArrayList is one dimensional but array can be multidimensional.
int[][][] intArray= new int[3][2][1]; // 3 dimensional array
To create an array the size should be known or initalized to some value. If not initialized carefully there could me memory wastage. But arrayList is all about dynamic creation and there is no wastage of memory.
When to use ArrayList or LinkedList ?
Ans) Adding new elements is pretty fast for either type of list. For the ArrayList, doing random lookup using "get" is fast, but for LinkedList, it's slow. It's slow because there's no efficient way to index into the middle of a linked list. When removing elements, using ArrayList is slow. This is because all remaining elements in the underlying array of Object instances must be shifted down for each remove operation. But here LinkedList is fast, because deletion can be done simply by changing a couple of links. So an ArrayList works best for cases where you're doing random access on the list, and a LinkedList works better if you're doing a lot of editing in the middle of the list.
Consider a scenario. If an ArrayList has to be iterate to read data only, what are the possible ways and which is the fastest?
Ans) It can be done in two ways, using for loop or using iterator of ArrayList. The first option is faster than using iterator. Because value stored in arraylist is indexed access. So while accessing the value is accessed directly as per the index.
Now another question with respect to above question is if accessing through iterator is slow then why do we need it and when to use it.
Ans) For loop does not allow the updation in the array(add or remove operation) inside the loop whereas Iterator does. Also Iterator can be used where there is no clue what type of collections will be used because all collections have iterator.
Which design pattern Iterator follows?
Ans) It follows Iterator design pattern. Iterator Pattern is a type of behavioral pattern. The Iterator pattern is one, which allows you to navigate through a collection of data using a common interface without knowing about the underlying implementation. Iterator should be implemented as an interface. This allows the user to implement it anyway its easier for him/her to return data. The benefits of Iterator are about their strength to provide a common interface for iterating through collections without bothering about underlying implementation.
Example of Iteration design pattern - Enumeration The class java.util.Enumeration is an example of the Iterator pattern. It represents and abstract means of iterating over a collection of elements in some sequential order without the client having to know the representation of the collection being iterated over. It can be used to provide a uniform interface for traversing collections of all kinds.
Is it better to have a HashMap with large number of records or n number of small hashMaps?
Ans) It depends on the different scenario one is working on:
1) If the objects in the hashMap are same then there is no point in having different hashmap as the traverse time in a hashmap is invariant to the size of the Map.
2) If the objects are of different type like one of Person class , other of Animal class etc then also one can have single hashmap but different hashmap would score over it as it would have better readability.
Why is it preferred to declare: List list = new ArrayList(); instead of ArrayList = new ArrayList();
Ans) It is preferred because:
If later on code needs to be changed from ArrayList to Vector then only at the declaration place we can do that.
The most important one – If a function is declared such that it takes list. E.g void showDetails(List list);
When the parameter is declared as List to the function it can be called by passing any subclass of List like ArrayList,Vector,LinkedList making the function more flexible
What is difference between iterator access and index access?
Ans) Index based access allow access of the element directly on the basis of index. The cursor of the datastructure can directly goto the 'n' location and get the element. It doesnot traverse through n-1 elements.
In Iterator based access, the cursor has to traverse through each element to get the desired element.So to reach the 'n'th element it need to traverse through n-1 elements.
Insertion,updation or deletion will be faster for iterator based access if the operations are performed on elements present in between the datastructure.
Insertion,updation or deletion will be faster for index based access if the operations are performed on elements present at last of the datastructure.
Traversal or search in index based datastructure is faster.
ArrayList is index access and LinkedList is iterator access.
How to sort list in reverse order?
Ans) To sort the elements of the List in the reverse natural order of the strings, get a reverse Comparator from the Collections class with reverseOrder(). Then, pass the reverse Comparator to the sort() method.
List list = new ArrayList();
Comparator comp = Collections.reverseOrder();
Collections.sort(list, comp)
Can a null element added to a Treeset or HashSet?
Ans) A null element can be added only if the set contains one element because when a second element is added then as per set defination a check is made to check duplicate value and comparison with null element will throw NullPointerException.
HashSet is based on hashMap and can contain null element.
How to sort list of strings - case insensitive?
Ans) using Collections.sort(list, String.CASE_INSENSITIVE_ORDER);
How to make a List (ArrayList,Vector,LinkedList) read only?
Ans) A list implemenation can be made read only using Collections.unmodifiableList(list). This method returns a new list. If a user tries to perform add operation on the new list; UnSupportedOperationException is thrown.
What is ConcurrentHashMap?
Ans) A concurrentHashMap is thread-safe implementation of Map interface. In this class put and remove method are synchronized but not get method. This class is different from Hashtable in terms of locking; it means that hashtable use object level lock but this class uses bucket level lock thus having better performance.
Which is faster to iterate LinkedHashSet or LinkedList?
Ans) LinkedList.
Which data structure HashSet implements
Ans) HashSet implements hashmap internally to store the data. The data passed to hashset is stored as key in hashmap with null as value.
Arrange in the order of speed - HashMap,HashTable, Collections.synchronizedMap,concurrentHashmap
Ans) HashMap is fastest, ConcurrentHashMap,Collections.synchronizedMap,HashTable.
What is identityHashMap?
Ans) The IdentityHashMap uses == for equality checking instead of equals(). This can be used for both performance reasons, if you know that two different elements will never be equals and for preventing spoofing, where an object tries to imitate another.
What is WeakHashMap?
Ans) A hashtable-based Map implementation with weak keys. An entry in a WeakHashMap will automatically be removed when its key is no longer in ordinary use. More precisely, the presence of a mapping for a given key will not prevent the key from being discarded by the garbage collector, that is, made finalizable, finalized, and then reclaimed. When a key has been discarded its entry is effectively removed from the map, so this class behaves somewhat differently than other Map implementations.
Exceptions Interview Questions
What is an Exception?
Ans) The exception is said to be thrown whenever an exceptional event occurs in java which signals that something is not correct with the code written and may give unexpected result. An exceptional event is a occurrence of condition which alters the normal program flow. Exceptional handler is the code that does something about the exception.
Exceptions are defined in which java package?
Ans)All the exceptions are subclasses of java.lang.Exception
How are the exceptions handled in java?
Ans)When an exception occurs the execution of the program is transferred to an appropriate exception handler.The try-catch-finally block is used to handle the exception.
The code in which the exception may occur is enclosed in a try block, also called as a guarded region.
The catch clause matches a specific exception to a block of code which handles that exception.
And the clean up code which needs to be executed no matter the exception occurs or not is put inside the finally block
Explain the exception hierarchy in java.
Ans) The hierarchy is as follows:
Throwable is a parent class off all Exception classes. They are two types of Exceptions: Checked exceptions and UncheckedExceptions. Both type of exceptions extends Exception class.
What is Runtime Exception or unchecked exception?
Ans) Runtime exceptions represent problems that are the result of a programming problem. Such problems include arithmetic exceptions, such as dividing by zero; pointer exceptions, such as trying to access an object through a null reference; and indexing exceptions, such as attempting to access an array element through an index that is too large or too small. Runtime exceptions need not be explicitly caught in try catch block as it can occur anywhere in a program, and in a typical one they can be very numerous. Having to add runtime exceptions in every method declaration would reduce a program's clarity. Thus, the compiler does not require that you catch or specify runtime exceptions (although you can). The solution to rectify is to correct the programming logic where the exception has occurred or provide a check.
What is checked exception?
Ans) Checked exception are the exceptions which forces the programmer to catch them explicitly in try-catch block. It is a subClass of Exception. Example: IOException.
What is difference between Error and Exception?
Ans) An error is an irrecoverable condition occurring at runtime. Such as OutOfMemory error. These JVM errors and you can not repair them at runtime.Though error can be caught in catch block but the execution of application will come to a halt and is not recoverable.
While exceptions are conditions that occur because of bad input etc. e.g. FileNotFoundException will be thrown if the specified file does not exist. Or a NullPointerException will take place if you try using a null reference. In most of the cases it is possible to recover from an exception (probably by giving user a feedback for entering proper values etc.)
What is difference between ClassNotFoundException and NoClassDefFoundError?
Ans) A ClassNotFoundException is thrown when the reported class is not found by the ClassLoader in the CLASSPATH. It could also mean that the class in question is trying to be loaded from another class which was loaded in a parent classloader and hence the class from the child classloader is not visible.
Consider if NoClassDefFoundError occurs which is something like
java.lang.NoClassDefFoundError
src/com/TestClass
does not mean that the TestClass class is not in the CLASSPATH. It means that the class TestClass was found by the ClassLoader however when trying to load the class, it ran into an error reading the class definition. This typically happens when the class in question has static blocks or members which use a Class that's not found by the ClassLoader. So to find the culprit, view the source of the class in question (TestClass in this case) and look for code using static blocks or static members.
What is throw keyword?
Ans) Throw keyword is used to throw the exception manually. It is mainly used when the program fails to satisfy the given condition and it wants to warn the application.The exception thrown should be subclass of Throwable.
public void parent(){
try{
child();
}catch(MyCustomException e){ }
}
public void child{
String iAmMandatory=null;
if(iAmMandatory == null){
throw (new MyCustomException("Throwing exception using throw keyword");
}
}
What is use of throws keyword?
Ans) If the function is not capable of handling the exception then it can ask the calling method to handle it by simply putting the throws clauseat the function declaration.
public void parent(){
try{
child();
}catch(MyCustomException e){ }
}
public void child throws MyCustomException{
//put some logic so that the exception occurs.
}
What are the possible combination to write try, catch finally block?
Ans)
1) try{
//lines of code that may throw an exception
}catch(Exception e){
//lines of code to handle the exception thrown in try block
}finally{
//the clean code which is executed always no matter the exception occurs or not.
}
2 try{
}finally{}
3 try{
}catch(Exception e){
//lines of code to handle the exception thrown in try block
}
The catch blocks must always follow the try block. If there are more than one catch blocks they all must follow each other without any block in between. The finally block must follow the catch block if one is present or if the catch block is absent the finally block must follow the try block.
How to create custom Exception?
Ans) To create you own exception extend the Exception class or any of its subclasses.
e.g.
1 class New1Exception extends Exception { } // this will create Checked Exception
2 class NewException extends IOExcpetion { } // this will create Checked exception
3 class NewException extends NullPonterExcpetion { } // this will create UnChecked exception
When to make a custom checked Exception or custom unchecked Exception?
Ans) If an application can reasonably be expected to recover from an exception, make it a checked exception. If an application cannot do anything to recover from the exception, make it an unchecked exception.
What is StackOverflowError?
Ans) The StackOverFlowError is an Error Object thorwn by the Runtime System when it Encounters that your application/code has ran out of the memory. It may occur in case of recursive methods or a large amount of data is fetched from the server and stored in some object. This error is generated by JVM.
e.g. void swap(){
swap();
}
Why did the designers decide to force a method to specify all uncaught checked exceptions that can be thrown within its scope?
Ans) Any Exception that can be thrown by a method is part of the method's public programming interface. Those who call a method must know about the exceptions that a method can throw so that they can decide what to do about them. These exceptions are as much a part of that method's programming interface as its parameters and return value.
Once the control switches to the catch block does it return back to the try block to execute the balance code?
Ans) No. Once the control jumps to the catch block it never returns to the try block but it goes to finally block(if present).
Where is the clean up code like release of resources is put in try-catch-finally block and why?
Ans) The code is put in a finally block because irrespective of try or catch block execution the control will flow to finally block. Typically finally block contains release of connections, closing of result set etc.
Is it valid to have a try block without catch or finally?
Ans) NO. This will result in a compilation error. The try block must be followed by a catch or a finally block. It is legal to omit the either catch or the finally block but not both.
e.g. The following code is illegal.
try{
int i =0;
}
int a = 2;
System.out.println(“a = “+a);
Is it valid to place some code in between try the catch/finally block that follows it?
Ans) No. There should not be any line of code present between the try and the catch/finally block. e.g. The following code is wrong.
try{}
String str = “ABC”;
System.out.println(“str = “+str);
catch(Exception e){}
What happens if the exception is never caught and throws down the method stack?
Ans) If the exception is not caught by any of the method in the method’s stack till you get to the main() method, the main method throws that exception and the JVM halts its execution.
How do you get the descriptive information about the Exception occurred during the program execution?
Ans) All the exceptions inherit a method printStackTrace() from the Throwable class. This method prints the stack trace from where the exception occurred. It prints the most recently entered method first and continues down, printing the name of each method as it works its way down the call stack from the top.
Can you catch more than one exceptions in a single catch block?
Ans)Yes. If the exception class specified in the catch clause has subclasses, any exception object that is a subclass of the specified Exception class will be caught by that single catch block.
E.g..
try {
// Some code here that can throw an IOException
}
catch (IOException e) {
e.printStackTrace();
}
The catch block above will catch IOException and all its subclasses e.g. FileNotFoundException etc.
Why is not considered as a good practice to write a single catchall handler to catch all the exceptions?
Ans) You can write a single catch block to handle all the exceptions thrown during the program execution as follows :
try {
// code that can throw exception of any possible type
}catch (Exception e) {
e.printStackTrace();
}
If you use the Superclass Exception in the catch block then you will not get the valuable information about each of the exception thrown during the execution, though you can find out the class of the exception occurred. Also it will reduce the readability of the code as the programmer will not understand what is the exact reason for putting the try-catch block.
What is exception matching?
Ans) Exception matching is the process by which the the jvm finds out the matching catch block for the exception thrown from the list of catch blocks. When an exception is thrown, Java will try to find by looking at the available catch clauses in the top down manner. If it doesn't find one, it will search for a handler for a supertype of the exception. If it does not find a catch clause that matches a supertype for the exception, then the exception is propagated down the call stack. This process is called exception matching.
What happens if the handlers for the most specific exceptions is placed above the more general exceptions handler?
Ans) Compilation fails. The catch block for handling the most specific exceptions must always be placed above the catch block written to handle the more general exceptions.
e.g. The code below will not compile.
1 try {
// code that can throw IOException or its subtypes
} catch (IOException e) {
// handles IOExceptions and its subtypes
} catch (FileNotFoundException ex) {
// handle FileNotFoundException only
}
The code below will compile successfully :-
try {
// code that can throw IOException or its subtypes
} catch (FileNotFoundException ex) {
// handles IOExceptions and its subtypes
} catch (IOException e){
// handle FileNotFoundException only
}
Does the order of the catch blocks matter if the Exceptions caught by them are not subtype or supertype of each other?
Ans) No. If the exceptions are siblings in the Exception class’s hierarchy i.e. If one Exception class is not a subtype or supertype of the other, then the order in which their handlers(catch clauses) are placed does not matter.
What happens if a method does not throw an checked Exception directly but calls a method that does? What does 'Ducking' the exception mean?
Ans) If a method does not throw an checked Exception directly but calls a method that throws an exception then the calling method must handle the throw exception or declare the exception in its throws clause. If the calling method does not handle and declares the exception, the exceptions is passed to the next method in the method stack. This is called as ducking the exception down the method stack.
e.g. The code below will not compile as the getCar() method has not declared the CarNotFoundException which is thrown by the getColor () method.
void getCar() {
getColor();
}
void getColor () {
throw new CarNotFoundException();
}
Fix for the above code is
void getCar() throws CarNotFoundException {
getColor();
}
void getColor () {
throw new CarNotFoundException();
}
Is an empty catch block legal?
Ans) Yes you can leave the catch block without writing any actual code to handle the exception caught.
e.g. The code below is legal but not appropriate, as in this case you will nt get any information about the exception thrown.
try{
//code that may throw the FileNotFoundException
}catch(FileNotFound eFnf){
//no code to handle the FileNotFound exception
}
Can a catch block throw the exception caught by itself?
Ans) Yes. This is called rethrowing of the exception by catch block.
e.g. the catch block below catches the FileNotFound exception and rethrows it again.
void checkEx() throws FileNotFoundException {
try{
//code that may throw the FileNotFoundException
}catch(FileNotFound eFnf){
throw FileNotFound();
}
}
Inner Class Interview Questions
What is an inner class?
Ans) Inner class is a class defined inside other class and act like a member of the enclosing class.
What are the different types of inner classes?
Ans) There are two main types of inner classes –
Static member class
Inner class
Member class
Anonymous class
Local class
What is static member class?
Ans) A static member class behaves much like an ordinary top-level class, except that it can access the static members of the class that contains it. The static nested class can be accessed as the other static members of the enclosing class without having an instance of the outer class. The static class can contain non-static and static members and methods.
public class InnerClass {
static class StaticInner {
static int i = 9;
int no = 6;
private void method() {}
public void method1() {}
static void method2() {}
final void method3() {}
}
}
The static inner class can be accessed from Outer Class in the following manner:
InnerClass.StaticInner staticObj= new InnerClass. StaticInner ();
No outer class instance is required to instantiate the nested static class because the static class is a static member of the enclosing class.
What are non static inner classes?
Ans) The different type of static inner classes are: Non - static inner classes – classes associated with the object of the enclosing class. Member class - Classes declared outside a function (hence a "member") and not declared "static".
The member class can be declared as public, private, protected, final and abstract. E.g.
public class InnerClass {
class MemberClass {
public void method1() { }
}
}
Method local class – The inner class declared inside the method is called method local inner class. Method local inner class can only be declared as final or abstract. Method local class can only access global variables or method local variables if declared as final
public class InnerClass {
int i = 9;
public void method1() {
final int k = 6;
class MethodLocal {
MethodLocal() {
System.out.println(k + i);
}
}
}
}
Anonymous inner class - These are local classes which are automatically declared and instantiated in the middle of an expression. Also, like local classes, anonymous classes cannot be public, private, protected, or static. They can specify arguments to the constructor of the superclass, but cannot otherwise have a constructor. They can implement only one interface or extend a class.
Anonymous class cannot define any static fields, methods, or classes, except for static final constants.
Also, like local classes, anonymous classes cannot be public, private, protected, or static
Some examples:
public class MyFrame extends JFrame {
JButton btn = new JButton();
MyFrame() {
btn.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e) {
}
});
}
}
Anonymous class used with comparator
List l = new ArrayList();
l.add(new Parent(2));
l.add(new Parent(3));
Collections.sort(l, new Comparator() {
public int compare(Object o1, Object o2) {
Parent prt1 = (Parent) o1;
Parent prt2 = (Parent) o2;
if (prt1.getAge() > prt2.getAge()) {
return -1;
}else if(prt1.getAge()
A. An abstract class may contain code in method bodies, which is not allowed in an interface. With abstract classes, you have to inherit your class from it and Java does not allow multiple inheritance. On the other hand, you can implement multiple interfaces in your class.
. Why would you use a synchronized block vs. synchronized method?
A. Synchronized blocks place locks for shorter periods than synchronized methods.
Explain the usage of the keyword transient?
A. This keyword indicates that the value of this member variable does not have to be serialized with the object. When the class will be de-serialized, this variable will be initialized with a default value of its data type (i.e. zero for integers).
How can you force garbage collection?
A. You can't force GC, but could request it by calling System.gc(). JVM does not guarantee that GC will be started immediately.
How do you know if an explicit object casting is needed?
A. If you assign a superclass object to a variable of a subclass's data type, you need to do explicit casting. For example:
Object a; Customer b; b = (Customer) a;
When you assign a subclass to a variable having a supeclass type, the casting is performed automatically.
CIO, CTO & Developer Resources
What's the difference between the methods sleep() and wait()
A. The code sleep(1000); puts thread aside for exactly one second. The code wait(1000), causes a wait of up to one second. A thread could stop waiting earlier if it receives the notify() or notifyAll() call. The method wait() is defined in the class Object and the method sleep() is defined in the class Thread.
Can you write a Java class that could be used both as an applet as well as an application?
A. Yes. Add a main() method to the applet.
What's the difference between constructors and other methods?
A. Constructors must have the same name as the class and can not return a value. They are only called once while regular methods could be called many times.
Can you call one constructor from another if a class has multiple constructors
A. Yes. Use this() syntax.
Explain the usage of Java packages.
A. This is a way to organize files when a project consists of multiple modules. It also helps resolve naming conflicts when different packages have classes with the same names. Packages access level also allows you to protect data from being used by the non-authorized classes.
If a class is located in a package, what do you need to change in the OS environment to be able to use it?
A. You need to add a directory or a jar file that contains the package directories to the CLASSPATH environment variable. Let's say a class Employee belongs to a package com.xyz.hr; and is located in the file c:\dev\com\xyz\hr\Employee.java. In this case, you'd need to add c:\dev to the variable CLASSPATH. If this class contains the method main(), you could test it from a command prompt window as follows:
c:\>java com.xyz.hr.Employee
What's the difference between J2SDK 1.5 and J2SDK 5.0?
A.There's no difference, Sun Microsystems just re-branded this version.
What would you use to compare two String variables - the operator == or the method equals()?
A. I'd use the method equals() to compare the values of the Strings and the == to check if two variables point at the same instance of a String object.
Does it matter in what order catch statements for FileNotFoundException and IOExceptipon are written?
A. Yes, it does. The FileNoFoundException is inherited from the IOException. Exception's subclasses have to be caught first.
Can an inner class declared inside of a method access local variables of this method?
A. It's possible if these variables are final.
* Q17. What can go wrong if you replace && with & in the following code:
String a=null; if (a!=null && a.length()>10) {...}
A. A single ampersand here would lead to a NullPointerException.
. What's the main difference between a Vector and an ArrayList
A. Java Vector class is internally synchronized and ArrayList is not.
When should the method invokeLater()be used?
A. This method is used to ensure that Swing components are updated through the event-dispatching thread.
How can a subclass call a method or a constructor defined in a superclass?
A. Use the following syntax: super.myMethod(); To call a constructor of the superclass, just write super(); in the first line of the subclass's constructor.
What is the difference between the instanceof and getclass, these two are same or not ?
Answer: instanceof is a operator, not a function while getClass is a method of java.lang.Object class. Consider a condition where we use
if(o.getClass().getName().equals("java.lang.Math")){ }
This method only checks if the classname we have passed is equal to java.lang.Math. The class java.lang.Math is loaded by the bootstrap ClassLoader. This class is an abstract class.This class loader is responsible for loading classes. Every Class object contains a reference to the ClassLoader that defines. getClass() method returns the runtime class of an object. It fetches the java instance of the given fully qualified type name. The code we have written is not necessary, because we should not compare getClass.getName(). The reason behind it is that if the two different class loaders load the same class but for the JVM, it will consider both classes as different classes so, we can't compare their names. It can only gives the implementing class but can't compare a interface, but instanceof operator can.
The instanceof operator compares an object to a specified type. We can use it to test if an object is an instance of a class, an instance of a subclass, or an instance of a class that implements a particular interface. We should try to use instanceof operator in place of getClass() method. Remember instanceof opeator and getClass are not same. Try this example, it will help you to better understand the difference between the two.
Interface one{
}
Class Two implements one {
}
Class Three implements one {
}
public class Test {
public static void main(String args[]) {
one test1 = new Two();
one test2 = new Three();
System.out.println(test1 instanceof one); //true
System.out.println(test2 instanceof one); //true
System.out.println(Test.getClass().equals(test2.getClass())); //false
}
}
Does it matter in what order catch statements for FileNotFoundException and IOExceptipon are written?
Answer: Yes, it does. The FileNoFoundException is inherited from the IOException. Exception's subclasses have to be caught first.
What is a Thread?
Ans) In Java, "thread" means two different things:
An instance of class java.lang.Thread.
A thread of execution.
An instance of Thread is just…an object. Like any other object in Java, it has variables and methods, and lives and dies on the heap. But a thread of execution is an individual process (a "lightweight" process) that has its own call stack. In Java, there is one thread per call stack—or, to think of it in reverse, one call stack per thread. Even if you don't create any new threads in your program, threads are back there running.
The main() method, that starts the whole ball rolling, runs in one thread, called (surprisingly) the main thread. If you looked at the main call stack (and you can, any time you get a stack trace from something that happens after main begins, but not within another thread), you'd see that main() is the first method on the stack— the method at the bottom. But as soon as you create a new thread, a new stack materializes and methods called from that thread run in a call stack that's separate from the main() call stack.
What is difference between thread and process?
Ans) Differences between threads and processes are:-
1. Threads share the address space of the process that created it; processes have their own address.
2. Threads have direct access to the data segment of its process; processes have their own copy of the data segment of the parent process.
3. Threads can directly communicate with other threads of its process; processes must use interprocess communication to communicate with sibling processes.
4. Threads have almost no overhead; processes have considerable overhead.
5. New threads are easily created; new processes require duplication of the parent process.
6. Threads can exercise considerable control over threads of the same process; processes can only exercise control over child processes.
7. Changes to the main thread (cancellation, priority change, etc.) may affect the behavior of the other threads of the process; changes to the parent process do not affect child processes.
What are the advantages or usage of threads?
Ans)
Threads support concurrent operations. For example,
• Multiple requests by a client on a server can be handled as an individual client thread.
• Long computations or high-latency disk and network operations can be handled in the background without disturbing foreground computations or screen updates.
Threads often result in simpler programs.
• In sequential programming, updating multiple displays normally requires a big while-loop that performs small parts of each display update. Unfortunately, this loop basically simulates an operating system scheduler. In Java, each view can be assigned a thread to provide continuous updates.
• Programs that need to respond to user-initiated events can set up service routines to handle the events without having to insert code in the main routine to look for these events.
Threads provide a high degree of control.
• Imagine launching a complex computation that occasionally takes longer than is satisfactory. A "watchdog" thread can be activated that will "kill" the computation if it becomes costly, perhaps in favor of an alternate, approximate solution. Note that sequential programs must muddy the computation with termination code, whereas, a Java program can use thread control to non-intrusively supervise any operation.
Threaded applications exploit parallelism.
• A computer with multiple CPUs can literally execute multiple threads on different functional units without having to simulating multi-tasking ("time sharing").
• On some computers, one CPU handles the display while another handles computations or database accesses, thus, providing extremely fast user interface response times.
What are the two ways of creating thread?
Ans) There are two ways to create a new thread.
1)Extend the Thread class and override the run() method in your class. Create an instance of the subclass and invoke the start() method on it, which will create a new thread of execution. e.g.
public class NewThread extends Thread{
public void run(){
// the code that has to be executed in a separate new thread goes here
}
public static void main(String [] args){
NewThread c = new NewThread();
c.start();
}
}
2)Implements the Runnable interface.The class will have to implement the run() method in the Runnable interface. Create an instance of this class. Pass the reference of this instance to the Thread constructor a new thread of execution will be created. e.g. class
public class NewThread implements Runnable{
public void run(){
// the code that has to be executed in a separate new thread goes here
}
public static void main(String [] args){
NewThread c = new NewThread();
Thread t = new Thread(c);
t.start();
}
}
What are the different states of a thread's lifecycle?
Ans) The different states of threads are as follows:
1) New – When a thread is instantiated it is in New state until the start() method is called on the thread instance. In this state the thread is not considered to be alive.
2) Runnable – The thread enters into this state after the start method is called in the thread instance. The thread may enter into the Runnable state from Running state. In this state the thread is considered to be alive.
3) Running – When the thread scheduler picks up the thread from the Runnable thread’s pool, the thread starts running and the thread is said to be in Running state.
4) Waiting/Blocked/Sleeping – In these states the thread is said to be alive but not runnable. The thread switches to this state because of reasons like wait method called or sleep method has been called on the running thread or thread might be waiting for some i/o resource so blocked. 5) Dead – When the thread finishes its execution i.e. the run() method execution completes, it is said to be in dead state. A dead state can not be started again. If a start() method is invoked on a dead thread a runtime exception will occur.
What is use of synchronized keyword?
Ans) synchronized keyword can be applied to static/non-static methods or a block of code. Only one thread at a time can access synchronized methods and if there are multiple threads trying to access the same method then other threads have to wait for the execution of method by one thread. Synchronized keyword provides a lock on the object and thus prevents race condition. E.g.
public void synchronized method(){}
public void synchronized staticmethod(){}
public void myMethod(){
synchronized (this){ // synchronized keyword on block of code
}
}
What is the difference when the synchronized keyword is applied to a static method or to a non static method?
Ans) When a synch non static method is called a lock is obtained on the object. When a synch static method is called a lock is obtained on the class and not on the object. The lock on the object and the lock on the class don’t interfere with each other. It means, a thread accessing a synch non static method, then the other thread can access the synch static method at the same time but can’t access the synch non static method.
What is a volatile keyword?
Ans) In general each thread has its own copy of variable, such that one thread is not concerned with the value of same variable in the other thread. But sometime this may not be the case. Consider a scenario in which the count variable is holding the number of times a method is called for a given class irrespective of any thread calling, in this case irrespective of thread access the count has to be increased so the count variable is declared as volatile. The copy of volatile variable is stored in the main memory, so every time a thread access the variable even for reading purpose the local copy is updated each time from the main memory. The volatile variable also have performance issues.
What is the difference between yield() and sleep()?
Ans) yield() allows the current the thread to release its lock from the object and scheduler gives the lock of the object to the other thread with same priority.
sleep() allows the thread to go to sleep state for x milliseconds. When a thread goes into sleep state it doesn’t release the lock.
What is the difference between wait() and sleep()?
Ans)
1) wait() is a method of Object class. sleep() is a method of Object class.
2) sleep() allows the thread to go to sleep state for x milliseconds. When a thread goes into sleep state it doesn’t release the lock. wait() allows thread to release the lock and goes to suspended state. The thread is only active when a notify() or notifAll() method is called for the same object.
What is difference between notify() and notfiyAll()?
Ans) notify( ) wakes up the first thread that called wait( ) on the same object.
notifyAll( ) wakes up all the threads that called wait( ) on the same object. The
highest priority thread will run first.
What happens if a start method is not invoked and the run method is directly invoked?
Ans) If a thread has been instantiated but not started its is said to be in new state. Unless until a start() method is invoked on the instance of the thread, it will not said to be alive. If you do not call a start() method on the newly created thread instance thread is not considered to be alive. If the start() method is not invoked and the run() method is directly called on the Thread instance, the code inside the run() method will not run in a separate new thread but it will start running in the existing thread.
What happens when start() is called?
Ans) A new thread of execution with a new call stack starts. The state of thread changes from new to runnable. When the thread gets chance to execute its target run() method starts to run.
If code running is a thread creates a new thread what will be the initial priority of the newly created thread?
Ans) When a code running in a thread creates a new thread object , the priority of the new thread is set equal to the priority of the thread which has created it.
When jvm starts up, which thread will be started up first?
Ans) When jvm starts up the thread executing main method is started.
What are the daemon threads?
Ans) Daemon thread are service provider threads run in the background,these not used to run the application code generally.When all user threads(non-daemon threads) complete their execution the jvm exit the application whatever may be the state of the daemon threads. Jvm does not wait for the daemon threads to complete their execution if all user threads have completed their execution.
To create Daemon thread set the daemon value of Thread using setDaemon(boolean value) method. By default all the threads created by user are user thread. To check whether a thread is a Daemon thread or a user thread use isDaemon() method.
Example of the Daemon thread is the Garbage Collector run by jvm to reclaim the unused memory by the application. The Garbage collector code runs in a Daemon thread which terminates as all the user threads are done with their execution.
What all constructors are present in the Thread class?
Ans) Thread()
Thread(Runnable target)
Thread(Runnable target, String name)
Thread(String name)
Can the variables or classes be Synchronized?
Ans) No. Only methods can be synchronized.
Q19) How many locks does an object have?
Ans) Each object has only one lock.
Can a class have both Synchronized and non-synchronized methods?
Ans) Yes a class can have both synchronized and non-synchronized methods.
Java Keywords
What are different types of access modifiers in Java?
Ans) There are four different types of modifiers:
Modifer
Accessible in the same package
Accessible in different package
Private
No
No
Protected
Yes
Yes, only if the class extends the main class
Default
Yes
No
Public
Yes
Yes
What is the use of final keyword?
Ans) The final keyword can be assigned to
Class level variable
method
class
Objects
If a final is assigned to a variable, the variable behaves as a constant. It means that the value of variable once set cannot be changed.
final int i=1;
i =5; // error
If a final is assigned to a method then it cannot be overridden in its child class.
class Parent {
final void print(){
System.out.println(“Inside”);
}
}
class Child extends Parent{
public final void print(){ // error cannot override final method
System.out.println(“Inside”);
}
}
If a class is made as final, then no other class can extend it and make it as parent class. E.g. String Class.
Final objects are instantiated only once. i.e
final Map map = new HashMap();
map.put(“key”,”value”);
map = new HashMap(); // error
What is use of synchronized keyword?
Ans) This keyword is used to prevent concurrency. Synchronized keyword can be applied to static/non-static methods or a block of code. Only one thread at a time can access synchronized methods and if there are multiple threads trying to access the same method then other threads have to wait for the execution of method by one thread. Synchronized keyword provides a lock on the object and thus prevents race condition. E.g.
public void synchronized method(){}
public void synchronized staticmethod(){}
public void myMethod(){
synchronized (this){ // synchronized keyword on block of code
}
}
What is volatile keyword?
Ans) In general each thread has its own copy of variable, such that one thread is not concerned with the value of same variable in the other thread. But sometime this may not be the case. Consider a scenario in which the count variable is holding the number of times a method is called for a given class irrespective of any thread calling, in this case irrespective of thread access the count has to be increased. In this case the count variable is declared as volatile. The copy of volatile variable is stored in the main memory, so every time a thread access the variable even for reading purpose the local copy is updated each time from the main memory. The volatile variable also have performance issues.
What is a transient variable?
Ans) If some of the properties of a class are not required to be serialized then the varaibles are marked as transient. When an object is deserialized the transient variables retains the default value depending on the type of variable declared and hence lost its original value.
What is a strictfp modifier?
Ans) Strictfp is used with variable only . It is used to restrict floating point calculations ( fp ) to ensure portability ( platform Independent ). When this modifier is specified, the JVM adheres to the Java specifications ( IEEE-754 floating-point specification ) and returns the consistent value independent of the platform. That is, if you want the answers from your code (which uses floating point values) to be consistent in all platforms, then you need to specify the strictfp modifier.
What is a static variable?
Ans) Static keyword can be used with the variables and methods but not with the class but there are static class. Anything declared as static is related to class and not objects.
Static variable : Multiples objects of a class shares the same instance of a static variable.Consider the example:
public class Counter{
private static int count=0;
private int nonStaticcount=0;
public void incrementCounter(){
count++;
nonStaticcount++;
}
public int getCount(){
return count;
}
public int getNonStaticcount(){
return nonStaticcount;
}
public static void main(String args[]){
Counter countObj1 = new Counter();
Counter countObj2 = new Counter();
countObj1.incrementCounter();
countObj1.incrementCounter();
System.out.println("Static count for Obj1: "+countObj1.getCount());
System.out.println("NonStatic count for Obj1: "+countObj1.getNonStaticcount());
System.out.println("Static count for Obj2: "+countObj2.getCount())
System.out.println("NonStatic count for Obj2: "+countObj2.getNonStaticcount())
}
Output
Static count for Obj1: 2
NonStatic count for Obj1: 2
Static count for Obj2: 2
NonStatic count for Obj2: 0
In the above program obj1 and obj2 share the same instance of static variable count hence if the value is incremented by one object , the incremented value will be reflected across the other objects.
What is a static method?
Ans)A method defined as static is called static method. A static method can be accessed without creating the objects. Just by using the Class name the method can be accessed.
Static method can only access static variables and not local or global non-static variables. For eg:
public class Test{
public static void printMe(){
System.out.println("Hello World");
}
}
public class MainClass{
public static void main(String args[]){
Test.printMe()
}
}
OutPut:
Hello World
Also static method can call only static methods and not non static methods. But non-static methods can call static mehtods.
Why static methods cannot access non static variables or methods?
Ans) A static method cannot access non static variables or methods because static methods doesnt need the object to be accessed. So if a static method has non static variables or non static methods which has instantiated variables they will no be intialized since the object is not created and this could result in an error.
What is static class ?
Ans) A class cannot be declared static. But a class can be said a static class if all the variables and methods of the class are static and the constructor is private. Making the constructor private will prevent the class to be instantiated. So the only possibility to access is using Class name only
What is throw keyword?
Ans) Throw keyword is used to throw the exception manually. It is mainly used when the program fails to satisfy the given condition and it wants to warn the application.The exception thrown should be subclass of Throwable.
public void parent(){
try{
child();
}catch(MyCustomException e){ }
}
public void child{
String iAmMandatory=null;
if(iAmMandatory == null){
throw (new MyCustomException("Throwing exception using throw keyword");
}
}
What is use of throws keyword?
Ans) If the function is not capable of handling the exception then it can ask the calling method to handle it by simply putting the throws clauseat the function declaration.
public void parent(){
try{
child();
}catch(MyCustomException e){ }
}
public void child throws MyCustomException{
//put some logic so that the exception occurs.
}
Key Concepts in Java
Why is main() method static?
Ans) To access the static method the object of the class is not needed. The method can be access directly with the help of ClassName. So when a program is started the jvm search for the class with main method and calls it without creating an object of the class.
What is the difference between static methods and instance methods?
Ans) instance method belongs to the instance of a class therefore it requires an instance before it can be invoked, whereas static method belongs to the class itself and not to any class instance so it doesn’t need an instance to be invoked.
Instance methods use dynamic (late) binding, whereas static methods use static (early) binding.
When the JVM invokes a class instance method, it selects the method to invoke based on the type of the object reference, which is always known at run-time. On the other hand, when the JVM invokes a static method, it selects the method to invoke based on the actual class of the object, which may only be known at compile time.
Can static block throw exception?
Ans) Yes, static block can throw only Runtime exception or can use a try-catch block to catch checked exception.
Typically scenario will be if JDBC connection is created in static block and it fails then exception can be caught, logged and application can exit. If System.exit() is not done, then application may continue and next time if the class is referred JVM will throw NoClassDefFounderror since the class was not loaded by the Classloader.
What is difference between abstract class and interface?
Ans) 1) A class is called abstract when it contains at least one abstract method. It can also contain n numbers of concrete method.Interface can contain only abstract( non implemented) methods.
2) The abstract class can have public,private,protect or default variables and also constants. In interface the variable is by default public final. In nutshell the interface doesnt have any variables it only has constants.
3) A class can extend only one abstract class but a class can implement multiple interfaces.
4) If an interface is implemented its compulsory to implement all of its methods but if an abstract class is extended its not compulsory to implement all methods. 5) The problem with an interface is, if you want to add a new feature (method) in its contract, then you MUST implement those method in all of the classes which implement that interface. However, in the case of an abstract class, the method can be simply implemented in the abstract class and the same can be called by its subclass.
Explain with example to describe when to use abstract class and interface?
Ans) Consider a scenario where all Cars will have 4 tyres and other features can be different.
In this case any subclass of Car has to have 4 tyres. This is a case where abstract class will be used and a default implementaion for tyres will be provided.
public abstract class Car{
public abstract String getCarName();
public final int getNoOfTyres(){
return 4;
}
}
Consider a scenario where Cars can have any number of tyres and other features can also be different. In this case interface will be created.
public interface Car{
public abstract String getCarName();
public abstract int getNoOfTyres();
}
Does java support multiple interitance? Why?
Ans) Java doesnt support multiple inheritance but it provide a way through which it can enact it.
Consider the scenario is C++
Class A{
public void add(){
// some text
}
}
Class B{
public void add(){
// some text
}
}
Class C extends A,B{
public static void main(String arg[]){
C objC = new C();
objC.add(); // problem, compiler gets confused and cant
decide to call Class A or B method.
}
This problem is called Diamond problem.
This problem in java is taken care with the use of interfaces
In Java similar problem would look like:
interface A{
add();
}
interface B{
add();
}
class C implements A,B{
add(){
// doesnt matter which interface it belong to
}
}
Can this keyword be assigned null value?
Ans) No
What are the different types of references in java?
Ans) Java has a more expressive system of reference than most other garbage-collected programming languages, which allows for special behavior for garbage collection. A normal reference in Java is known as a strong reference. The java.lang.ref package defines three other types of references—soft, weak, and phantom references. Each type of reference is designed for a specific use.
A SoftReference can be used to implement a cache. An object that is not reachable by a strong reference (that is, not strongly reachable), but is referenced by a soft reference is called softly reachable. A softly reachable object may be garbage collected at the discretion of the garbage collector. This generally means that softly reachable objects will only be garbage collected when free memory is low, but again, it is at the discretion of the garbage collector. Semantically, a soft reference means "keep this object unless the memory is needed."
A WeakReference is used to implement weak maps. An object that is not strongly or softly reachable, but is referenced by a weak reference is called weakly reachable. A weakly reachable object will be garbage collected during the next collection cycle. This behavior is used in the class java.util.WeakHashMap. A weak map allows the programmer to put key/value pairs in the map and not worry about the objects taking up memory when the key is no longer reachable anywhere else. Another possible application of weak references is the string intern pool. Semantically, a weak reference means "get rid of this object when nothing else references it."
A PhantomReference is used to reference objects that have been marked for garbage collection and have been finalized, but have not yet been reclaimed. An object that is not strongly, softly or weakly reachable, but is referenced by a phantom reference is called phantom reachable. This allows for more flexible cleanup than is possible with the finalization mechanism alone. Semantically, a phantom reference means "this object is no longer needed and has been finalized in preparation for being collected."
How to change the heap size of a JVM?
Ans) The old generation's default heap size can be overridden by using the -Xms and -Xmx switches to specify the initial and maximum sizes respectively:
java -Xms
For example:
java -Xms64m -Xmx128m program
What is difference between instanceof and isInstance(Object obj)?
Ans) Differences are as follows:
1) instanceof is a reserved word of Java, but isInstance(Object obj) is a method of java.lang.Class.
if (obj instanceof MyType) {
...
}else if (MyType.class.isInstance(obj)) {
...
}
2) instanceof is used of identify whether the object is type of a particular class or its subclass but isInstance(obj) is used to identify object of a particular class.
Java supports pass by value or pass by reference?
Ans) Java supports only pass by value. The arguments passed as a parameter to a method is mainly primitive data types or objects. For the data type the actual value is passed.
Java passes the references by value just like any other parameter. This means the references passed to the method are actually copies of the original references.Java copies and passes the reference by value, not the object. Thus, method manipulation will alter the objects, since the references point to the original objects.Consider the example:
public void tricky(Point arg1, Point arg2)
{
arg1.x = 100;
arg1.y = 100;
Point temp = arg1;
arg1 = arg2;
arg2 = temp;
}
public static void main(String [] args)
{
Point pnt1 = new Point(0,0);
Point pnt2 = new Point(0,0);
System.out.println("X: " + pnt1.x + " Y: " +pnt1.y);
System.out.println("X: " + pnt2.x + " Y: " +pnt2.y);
System.out.println(" ");
tricky(pnt1,pnt2);
System.out.println("X: " + pnt1.x + " Y:" + pnt1.y);
System.out.println("X: " + pnt2.x + " Y: " +pnt2.y);
}
OutPut:
X: 0 Y: 0
X: 0 Y: 0
X: 100 Y: 100
X: 0 Y: 0
The method successfully alters the value of pnt1, even though it is passed by value; however, a swap of pnt1 and pnt2 fails! This is the major source of confusion. In the main() method, pnt1 and pnt2 are nothing more than object references. When you pass pnt1 and pnt2 to the tricky() method, Java passes the references by value just like any other parameter. This means the references passed to the method are actually copies of the original references.
What is memory leak?
Ans) A memory leak is where an unreferenced object that will never be used again still hangs around in memory and doesnt get garbage collected.
What is the difference between equals() and ==?
Ans) == operator is used to compare the references of the objects.
public bollean equals(Object o) is the method provided by the Object class. The default implementation uses == operator to compare two objects.
But since the method can be overriden like for String class. equals() method can be used to compare the values of two objects.
String str1 = "MyName";
String str2 = "MyName";
String str3 = str2;
if(str1 == str2){
System.out.println("Objects are equal")
}else{
System.out.println("Objects are not equal")
}
if(str1.equals(str2)){
System.out.println("Objects are equal")
}else{
System.out.println("Objects are not equal")
}
Output:
Objects are not equal
Objects are equal
String str2 = "MyName";
String str3 = str2;
if(str2 == str3){
System.out.println("Objects are equal")
}else{
System.out.println("Objects are not equal")
}
if(str3.equals(str2)){
System.out.println("Objects are equal")
}else{
System.out.println("Objects are not equal")
}
OutPut:
Objects are equal
Objects are equal
How to make sure that Childclass method actually overrides the method of the superclass?
Ans) The @Override annotation can be added to the javadoc for the new method. If you accidently miss an argument or capitalize the method name wrong, the compiler will generate a compile-time error.
How to find the size of an object?
Ans)The heap size of an object can be found using -
Runtime.totalMemory()-Runtime.freeMemory() .
Can an abstract class have a static method?
Ans) Yes an abstract class have a static method and it can be accessed by any other class(even not a concrete class).
Can an abstract class have a constructor?
Ans) Yes an abstract class have a default and parameterized constructors.
Why static methods cannot access non static variables or methods?
Ans) A static method cannot access non static variables or methods because static methods doesnt need the object to be accessed. So if a static method has non static variables or non static methods which has instantiated variables they will no be intialized since the object is not created and this could result in an error.
What is difference between stringbuffer and stringbuilder?
Ans) The only difference between StringBuffer and StringBuilder is that StringBuilder is unsynchronized whereas StringBuffer is synchronized. So when the application needs to be run only in a single thread then it is better to use StringBuilder. StringBuilder is more efficient than StringBuffer.
Criteria to choose among StringBuffer and StringBuilder
1)If your text can change and will only be accessed from a single thread, use a StringBuilder 2)because StringBuilder is unsynchronized.
If your text can changes, and will be accessed from multiple threads, use a StringBuffer because StringBuffer is synchronous.
Consider a scenario in which the admin want to sure that if some one has written System.exit() at some part of application then before system shutdown all the resources should be released. How is it possible?
Ans) This is possible using Runtime.getRuntime().addShutdownHook(Thread hook).
Straight from Java Spec:
This method registers a new virtual-machine shutdown hook.
The Java virtual machine shuts down in response to two kinds of events:
1. The program exits normally, when the last non-daemon thread exits or when the exit (equivalently, System.exit) method is invoked, or
2. The virtual machine is terminated in response to a user interrupt, such as typing ^C, or a system-wide event, such as user logoff or system shutdown.
A shutdown hook is simply an initialized but unstarted thread. When the virtual machine begins its shutdown sequence it will start all registered shutdown hooks in some unspecified order and let them run concurrently. When all the hooks have finished it will then run all uninvoked finalizers if finalization-on-exit has been enabled. Finally, the virtual machine will halt. Note that daemon threads will continue to run during the shutdown sequence, as will non-daemon threads if shutdown was initiated by invoking the exit method.
Once the shutdown sequence has begun it can be stopped only by invoking the halt method, which forcibly terminates the virtual machine.
What is the difference between final, finally and finalize() in Java?
Ans)
final - constant declaration. A final variable act as constant, a final class is immutable and a final method cannot be ovrriden.
finally - handles exception. The finally block is optional and provides a mechanism to clean up regardless of what happens within the try block (except System.exit(0) call). Use the finally block to close files or to release other system resources like database connections, statements etc.
finalize() - method helps in garbage collection. A method that is invoked before an object is discarded by the garbage collector, allowing it to clean up its state. Should not be used to release non-memory resources like file handles, sockets, database connections etc because Java has only a finite number of these resources and you do not know when the garbage collection is going to kick in to release these non-memory resources through the finalize() method.
How does Java allocate stack and heap memory?
Ans) Each time an object is created in Java it goes into the area of memory known as heap. The primitive variables like int and double are allocated in the stack, if they are local method variables and in the heap if they are member variables (i.e. fields of a class). In Java methods local variables are pushed into stack when a method is invoked and stack pointer is decremented when a method call is completed.
In a multi-threaded application each thread will have its own stack but will share the same heap. This is why care should be taken in your code to avoid any concurrent access issues in the heap space. The stack is threadsafe (each thread will have its own stack) but the heap is not threadsafe unless guarded with synchronisation through your code.
Explain re-entrant, recursive and idempotent methods/functions?
Ans) A method in stack is re-entrant allowing multiple concurrent invocations that do not interfere with each other.
A function is recursive if it calls itself. Given enough stack space, recursive method calls are perfectly valid in Java though it is tough to debug. Recursive functions are useful in removing iterations from many sorts of algorithms. Allrecursive functions are re-entrant but not all re-entrant functions are recursive. Idempotent methods are methods, which are written in such a way that repeated calls to the same method with the same arguments yield same results. For example clustered EJBs, which are written with idempotent methods, can automatically recover from a server failure as long as it can reach another server.
Can a private variable or method of a class can be accessed?
Ans) Yes its possible using reflection.
What is difference between static block and the init block?
Or
Difference between
Static {} and {}
Ans) The static block is loaded when the class is loaded by the JVM for the 1st time only whereas init {} block is loaded every time class is loaded. Also first the static block is loaded then the init block.
public class LoadingBlocks {
static{
System.out.println("Inside static");
}
{
System.out.println("Inside init");
}
public static void main(String args[]){
new LoadingBlocks();
new LoadingBlocks();
new LoadingBlocks();
}
}
Output:
Inside static
Inside init
Inside init
Inside init
Why inner class can access only final variable?
Ans) Local classes can most definitely reference instance variables. The reason they cannot reference non final local variables is because the local class instance can remain in memory after the method returns. When the method returns the local variables go out of scope, so a copy of them is needed. If the variables weren't final then the copy of the variable in the method could change, while the copy in the local class didn't, so they'd be out of synch.
Anonymous inner classes require final variables because of the way they are implemented in Java. An anonymous inner class (AIC) uses local variables by creating a private instance field which holds a copy of the value of the local variable. The inner class isn't actually using the local variable, but a copy. It should be fairly obvious at this point that a "Bad Thing"â„¢ can happen if either the original value or the copied value changes; there will be some unexpected data synchronization problems. In order to prevent this kind of problem, Java requires you to mark local variables that will be used by the AIC as final (i.e., unchangeable). This guarantees that the inner class' copies of local variables will always match the actual values.
What is fully abstract class?
Ans) An abstract class which has all methods as abstract and all fields are public static final.
What is dynamic binding and static binding?
Ans) Method invocation
The Java programming language provides two basic kinds of methods: instance methods and class (or static) methods. The difference are:
1. Instance methods require an instance before they can be invoked, whereas class methods do not.
2. Instance methods use dynamic (late) binding, whereas class methods use static (early) binding.
When the Java virtual machine invokes a class method, it selects the method to invoke based on the type of the object reference, which is always known at compile-time. On the other hand, when the virtual machine invokes an instance method, it selects the method to invoke based on the actual class of the object, which may only be known at run time.
What is Java Reflection?
Ans)Reflection is commonly used by programs which require the ability to examine or modify the runtime behavior of applications running in the Java virtual machine.
Drawbacks of Reflection: Reflection is powerful, but should not be used indiscriminately. If it is possible to perform an operation without using reflection, then it is preferable to avoid using it. The following concerns should be kept in mind when accessing code via reflection.
Performance Overhead: Because reflection involves types that are dynamically resolved, certain Java virtual machine optimizations can not be performed. Consequently, reflective operations have slower performance than their non-reflective counterparts, and should be avoided in sections of code which are called frequently in performance-sensitive applications.
Security Restrictions: Reflection requires a runtime permission which may not be present when running under a security manager. This is in an important consideration for code which has to run in a restricted security context, such as in an Applet.
Exposure of Internals: Since reflection allows code to perform operations that would be illegal in non-reflective code, such as accessing private fields and methods, the use of reflection can result in unexpected side-effects, which may render code dysfunctional and may destroy portability. Reflective code breaks abstractions and therefore may change behavior with upgrades of the platform.
When an obj is passed through a function , one can set the properties but cannot set a new memory location?
Ans) It is because when u pass an object the address value is passed and stored in some new address . like if address 1234 is passed , it is stored in 4567 location. So if u change in the value of an object it will take the address from 4567 and do 1234.setXXX(). If u set the object to null it will set 4567=null.
OOPS Concept
What is polymorphism?
Ans) Polymorphism gives us the ultimate flexibility in extensibility. The abiltiy to define more than one function with the same name is called Polymorphism. In java,c++ there are two type of polymorphism: compile time polymorphism (overloading) and runtime polymorphism (overriding).
When you override methods, JVM determines the proper methods to call at the program’s run time, not at the compile time. Overriding occurs when a class method has the same name and signature as a method in parent class.
Overloading occurs when several methods have same names with
Overloading is determined at the compile time.
Different method signature and different number or type of parameters.
Same method signature but different number of parameters.
Same method signature and same number of parameters but of different type
Example of Overloading
int add(int a,int b)
float add(float a,int b)
float add(int a ,float b)
void add(float a)
int add(int a)
void add(int a) //error conflict with the method int add(int a)
Example: Overloading
Class BookDetails{
String title;
String publisher;
float price;
setBook(String title){
}
setBook(String title, String publisher){
}
setBook(String title, String publisher,float price){
}
}
Example: Overriding
class BookDetails{
String title;
setBook(String title){ }
}
class ScienceBook extends BookDetails{
setBook(String title){} //overriding
setBook(String title, String publisher,float price){ } //overloading
}
What is inheritance?
Ans) Inheritance is the property which allows a Child class to inherit some properties from its parent class. In Java this is achieved by using extends keyword. Only properties with access modifier public and protected can be accessed in child class.
public class Parent{
public String parentName;
public int parentage;
public String familyName;
}
public class Child extends Parent{
public String childName;
public int childAge;
public void printMyName(){
System.out.println(“ My name is “+ chidName+” “ +familyName)
}
}
In above example the child has inherit its family name from the parent class just by inheriting the class.
What is multiple inheritance and does java support?
Ans) If a child class inherits the property from multiple classes is known as multiple inheritance.
Java does not allow to extend multiple classes but to overcome this problem it allows to implement multiple Interfaces.
What is abstraction?
Ans) Abstraction is way of converting real world objects in terms of class. For example creating a class Vehicle and injecting properties into it. E.g
public class Vehicle {
public String colour;
public String model;
}
What is encapsulation?
Ans) The encapsulation is achieved by combining the methods and attribute into a class. The class acts like a container encapsulating the properties. The users are exposed mainly public methods.The idea behind is to hide how thinigs work and just exposing the requests a user can do.
What is Association?
Ans) Association is a relationship between two classes. In this relationship the object of one instance perform an action on behalf of the other class. The typical behaviour can be invoking the method of other class and using the member of the other class.
public class MyMainClass{
public void init(){
new OtherClass.init();
}
}
What is Aggregation?
Ans) Aggregation has a relationship between two classes. In this relationship the object of one class is a member of the other class. Aggregation always insists for a direction.
public class MyMainClass{
OtherClass otherClassObj = new OtherClass();
}
What is Composition?
Ans) Composition is a special type of aggregation relationship with a difference that its the compulsion for the OtherClass object (in previous example) to exist for the existence of MyMainClass.
Collections Interview Questions
What is difference between ArrayList and vector?
Ans: )
1) Synchronization - ArrayList is not thread-safe whereas Vector is thread-safe. In Vector class each method like add(), get(int i) is surrounded with a synchronized block and thus making Vector class thread-safe.
2) Data growth - Internally, both the ArrayList and Vector hold onto their contents using an Array. When an element is inserted into an ArrayList or a Vector, the object will need to expand its internal array if it runs out of room. A Vector defaults to doubling the size of its array, while the ArrayList increases its array size by 50 percent.
How can Arraylist be synchronized without using Vector?
Ans) Arraylist can be synchronized using:
Collection.synchronizedList(List list)
Other collections can be synchronized:
Collection.synchronizedMap(Map map)
Collection.synchronizedCollection(Collection c)
If an Employee class is present and its objects are added in an arrayList. Now I want the list to be sorted on the basis of the employeeID of Employee class. What are the steps?
Ans) 1) Implement Comparable interface for the Employee class and override the compareTo(Object obj) method in which compare the employeeID
2) Now call Collections.sort() method and pass list as an argument.
Now consider that Employee class is a jar file.
1) Since Comparable interface cannot be implemented, create Comparator and override the compare(Object obj, Object obj1) method .
2) Call Collections.sort() on the list and pass comparator as an argument.
What is difference between HashMap and HashTable?
Ans) Both collections implements Map. Both collections store value as key-value pairs. The key differences between the two are
1. Hashmap is not synchronized in nature but hshtable is.
2. Another difference is that iterator in the HashMap is fail-safe while the enumerator for the Hashtable isn't.
Fail-safe - “if the Hashtable is structurally modified at any time after the iterator is created, in any way except through the iterator's own remove method, the iterator will throw a ConcurrentModificationException�
3. HashMap permits null values and only one null key, while Hashtable doesn't allow key or value as null.
What are the classes implementing List interface?
Ans)
There are three classes that implement List interface:
1) ArrayList : It is a resizable array implementation. The size of the ArrayList can be increased dynamically also operations like add,remove and get can be formed once the object is created. It also ensures that the data is retrieved in the manner it was stored. The ArrayList is not thread-safe.
2) Vector: It is thread-safe implementation of ArrayList. The methods are wrapped around a synchronized block.
3) LinkedList: the LinkedList also implements Queue interface and provide FIFO(First In First Out) operation for add operation. It is faster if than ArrayList if it performs insertion and deletion of elements from the middle of a list.
Which all classes implement Set interface?
Ans) A Set is a collection that contains no duplicate elements. More formally, sets contain no pair of elements e1 and e2 such that e1.equals(e2), and at most one null element. HashSet,SortedSet and TreeSet are the commnly used class which implements Set interface.
SortedSet - It is an interface which extends Set. A the name suggest , the interface allows the data to be iterated in the ascending order or sorted on the basis of Comparator or Comparable interface. All elements inserted into the interface must implement Comparable or Comparator interface.
TreeSet - It is the implementation of SortedSet interface.This implementation provides guaranteed log(n) time cost for the basic operations (add, remove and contains). The class is not synchronized.
HashSet: This class implements the Set interface, backed by a hash table (actually a HashMap instance). It makes no guarantees as to the iteration order of the set; in particular, it does not guarantee that the order will remain constant over time. This class permits the null element. This class offers constant time performance for the basic operations (add, remove, contains and size), assuming the hash function disperses the elements properly among the buckets
What is difference between List and a Set?
Ans)
1) List can contain duplicate values but Set doesnt allow. Set allows only to unique elements.
2) List allows retrieval of data to be in same order in the way it is inserted but Set doesnt ensures the sequence in which data can be retrieved.(Except HashSet)
What is difference between Arrays and ArrayList ?
Ans) Arrays are created of fix size whereas ArrayList is of not fix size. It means that once array is declared as :
int [] intArray= new int[6];
intArray[7] // will give ArraysOutOfBoundException.
Also the size of array cannot be incremented or decremented. But with arrayList the size is variable.
Once the array is created elements cannot be added or deleted from it. But with ArrayList the elements can be added and deleted at runtime.
List list = new ArrayList();
list.add(1);
list.add(3);
list.remove(0) // will remove the element from the 1st location.
ArrayList is one dimensional but array can be multidimensional.
int[][][] intArray= new int[3][2][1]; // 3 dimensional array
To create an array the size should be known or initalized to some value. If not initialized carefully there could me memory wastage. But arrayList is all about dynamic creation and there is no wastage of memory.
When to use ArrayList or LinkedList ?
Ans) Adding new elements is pretty fast for either type of list. For the ArrayList, doing random lookup using "get" is fast, but for LinkedList, it's slow. It's slow because there's no efficient way to index into the middle of a linked list. When removing elements, using ArrayList is slow. This is because all remaining elements in the underlying array of Object instances must be shifted down for each remove operation. But here LinkedList is fast, because deletion can be done simply by changing a couple of links. So an ArrayList works best for cases where you're doing random access on the list, and a LinkedList works better if you're doing a lot of editing in the middle of the list.
Consider a scenario. If an ArrayList has to be iterate to read data only, what are the possible ways and which is the fastest?
Ans) It can be done in two ways, using for loop or using iterator of ArrayList. The first option is faster than using iterator. Because value stored in arraylist is indexed access. So while accessing the value is accessed directly as per the index.
Now another question with respect to above question is if accessing through iterator is slow then why do we need it and when to use it.
Ans) For loop does not allow the updation in the array(add or remove operation) inside the loop whereas Iterator does. Also Iterator can be used where there is no clue what type of collections will be used because all collections have iterator.
Which design pattern Iterator follows?
Ans) It follows Iterator design pattern. Iterator Pattern is a type of behavioral pattern. The Iterator pattern is one, which allows you to navigate through a collection of data using a common interface without knowing about the underlying implementation. Iterator should be implemented as an interface. This allows the user to implement it anyway its easier for him/her to return data. The benefits of Iterator are about their strength to provide a common interface for iterating through collections without bothering about underlying implementation.
Example of Iteration design pattern - Enumeration The class java.util.Enumeration is an example of the Iterator pattern. It represents and abstract means of iterating over a collection of elements in some sequential order without the client having to know the representation of the collection being iterated over. It can be used to provide a uniform interface for traversing collections of all kinds.
Is it better to have a HashMap with large number of records or n number of small hashMaps?
Ans) It depends on the different scenario one is working on:
1) If the objects in the hashMap are same then there is no point in having different hashmap as the traverse time in a hashmap is invariant to the size of the Map.
2) If the objects are of different type like one of Person class , other of Animal class etc then also one can have single hashmap but different hashmap would score over it as it would have better readability.
Why is it preferred to declare: List
Ans) It is preferred because:
If later on code needs to be changed from ArrayList to Vector then only at the declaration place we can do that.
The most important one – If a function is declared such that it takes list. E.g void showDetails(List list);
When the parameter is declared as List to the function it can be called by passing any subclass of List like ArrayList,Vector,LinkedList making the function more flexible
What is difference between iterator access and index access?
Ans) Index based access allow access of the element directly on the basis of index. The cursor of the datastructure can directly goto the 'n' location and get the element. It doesnot traverse through n-1 elements.
In Iterator based access, the cursor has to traverse through each element to get the desired element.So to reach the 'n'th element it need to traverse through n-1 elements.
Insertion,updation or deletion will be faster for iterator based access if the operations are performed on elements present in between the datastructure.
Insertion,updation or deletion will be faster for index based access if the operations are performed on elements present at last of the datastructure.
Traversal or search in index based datastructure is faster.
ArrayList is index access and LinkedList is iterator access.
How to sort list in reverse order?
Ans) To sort the elements of the List in the reverse natural order of the strings, get a reverse Comparator from the Collections class with reverseOrder(). Then, pass the reverse Comparator to the sort() method.
List list = new ArrayList();
Comparator comp = Collections.reverseOrder();
Collections.sort(list, comp)
Can a null element added to a Treeset or HashSet?
Ans) A null element can be added only if the set contains one element because when a second element is added then as per set defination a check is made to check duplicate value and comparison with null element will throw NullPointerException.
HashSet is based on hashMap and can contain null element.
How to sort list of strings - case insensitive?
Ans) using Collections.sort(list, String.CASE_INSENSITIVE_ORDER);
How to make a List (ArrayList,Vector,LinkedList) read only?
Ans) A list implemenation can be made read only using Collections.unmodifiableList(list). This method returns a new list. If a user tries to perform add operation on the new list; UnSupportedOperationException is thrown.
What is ConcurrentHashMap?
Ans) A concurrentHashMap is thread-safe implementation of Map interface. In this class put and remove method are synchronized but not get method. This class is different from Hashtable in terms of locking; it means that hashtable use object level lock but this class uses bucket level lock thus having better performance.
Which is faster to iterate LinkedHashSet or LinkedList?
Ans) LinkedList.
Which data structure HashSet implements
Ans) HashSet implements hashmap internally to store the data. The data passed to hashset is stored as key in hashmap with null as value.
Arrange in the order of speed - HashMap,HashTable, Collections.synchronizedMap,concurrentHashmap
Ans) HashMap is fastest, ConcurrentHashMap,Collections.synchronizedMap,HashTable.
What is identityHashMap?
Ans) The IdentityHashMap uses == for equality checking instead of equals(). This can be used for both performance reasons, if you know that two different elements will never be equals and for preventing spoofing, where an object tries to imitate another.
What is WeakHashMap?
Ans) A hashtable-based Map implementation with weak keys. An entry in a WeakHashMap will automatically be removed when its key is no longer in ordinary use. More precisely, the presence of a mapping for a given key will not prevent the key from being discarded by the garbage collector, that is, made finalizable, finalized, and then reclaimed. When a key has been discarded its entry is effectively removed from the map, so this class behaves somewhat differently than other Map implementations.
Exceptions Interview Questions
What is an Exception?
Ans) The exception is said to be thrown whenever an exceptional event occurs in java which signals that something is not correct with the code written and may give unexpected result. An exceptional event is a occurrence of condition which alters the normal program flow. Exceptional handler is the code that does something about the exception.
Exceptions are defined in which java package?
Ans)All the exceptions are subclasses of java.lang.Exception
How are the exceptions handled in java?
Ans)When an exception occurs the execution of the program is transferred to an appropriate exception handler.The try-catch-finally block is used to handle the exception.
The code in which the exception may occur is enclosed in a try block, also called as a guarded region.
The catch clause matches a specific exception to a block of code which handles that exception.
And the clean up code which needs to be executed no matter the exception occurs or not is put inside the finally block
Explain the exception hierarchy in java.
Ans) The hierarchy is as follows:
Throwable is a parent class off all Exception classes. They are two types of Exceptions: Checked exceptions and UncheckedExceptions. Both type of exceptions extends Exception class.
What is Runtime Exception or unchecked exception?
Ans) Runtime exceptions represent problems that are the result of a programming problem. Such problems include arithmetic exceptions, such as dividing by zero; pointer exceptions, such as trying to access an object through a null reference; and indexing exceptions, such as attempting to access an array element through an index that is too large or too small. Runtime exceptions need not be explicitly caught in try catch block as it can occur anywhere in a program, and in a typical one they can be very numerous. Having to add runtime exceptions in every method declaration would reduce a program's clarity. Thus, the compiler does not require that you catch or specify runtime exceptions (although you can). The solution to rectify is to correct the programming logic where the exception has occurred or provide a check.
What is checked exception?
Ans) Checked exception are the exceptions which forces the programmer to catch them explicitly in try-catch block. It is a subClass of Exception. Example: IOException.
What is difference between Error and Exception?
Ans) An error is an irrecoverable condition occurring at runtime. Such as OutOfMemory error. These JVM errors and you can not repair them at runtime.Though error can be caught in catch block but the execution of application will come to a halt and is not recoverable.
While exceptions are conditions that occur because of bad input etc. e.g. FileNotFoundException will be thrown if the specified file does not exist. Or a NullPointerException will take place if you try using a null reference. In most of the cases it is possible to recover from an exception (probably by giving user a feedback for entering proper values etc.)
What is difference between ClassNotFoundException and NoClassDefFoundError?
Ans) A ClassNotFoundException is thrown when the reported class is not found by the ClassLoader in the CLASSPATH. It could also mean that the class in question is trying to be loaded from another class which was loaded in a parent classloader and hence the class from the child classloader is not visible.
Consider if NoClassDefFoundError occurs which is something like
java.lang.NoClassDefFoundError
src/com/TestClass
does not mean that the TestClass class is not in the CLASSPATH. It means that the class TestClass was found by the ClassLoader however when trying to load the class, it ran into an error reading the class definition. This typically happens when the class in question has static blocks or members which use a Class that's not found by the ClassLoader. So to find the culprit, view the source of the class in question (TestClass in this case) and look for code using static blocks or static members.
What is throw keyword?
Ans) Throw keyword is used to throw the exception manually. It is mainly used when the program fails to satisfy the given condition and it wants to warn the application.The exception thrown should be subclass of Throwable.
public void parent(){
try{
child();
}catch(MyCustomException e){ }
}
public void child{
String iAmMandatory=null;
if(iAmMandatory == null){
throw (new MyCustomException("Throwing exception using throw keyword");
}
}
What is use of throws keyword?
Ans) If the function is not capable of handling the exception then it can ask the calling method to handle it by simply putting the throws clauseat the function declaration.
public void parent(){
try{
child();
}catch(MyCustomException e){ }
}
public void child throws MyCustomException{
//put some logic so that the exception occurs.
}
What are the possible combination to write try, catch finally block?
Ans)
1) try{
//lines of code that may throw an exception
}catch(Exception e){
//lines of code to handle the exception thrown in try block
}finally{
//the clean code which is executed always no matter the exception occurs or not.
}
2 try{
}finally{}
3 try{
}catch(Exception e){
//lines of code to handle the exception thrown in try block
}
The catch blocks must always follow the try block. If there are more than one catch blocks they all must follow each other without any block in between. The finally block must follow the catch block if one is present or if the catch block is absent the finally block must follow the try block.
How to create custom Exception?
Ans) To create you own exception extend the Exception class or any of its subclasses.
e.g.
1 class New1Exception extends Exception { } // this will create Checked Exception
2 class NewException extends IOExcpetion { } // this will create Checked exception
3 class NewException extends NullPonterExcpetion { } // this will create UnChecked exception
When to make a custom checked Exception or custom unchecked Exception?
Ans) If an application can reasonably be expected to recover from an exception, make it a checked exception. If an application cannot do anything to recover from the exception, make it an unchecked exception.
What is StackOverflowError?
Ans) The StackOverFlowError is an Error Object thorwn by the Runtime System when it Encounters that your application/code has ran out of the memory. It may occur in case of recursive methods or a large amount of data is fetched from the server and stored in some object. This error is generated by JVM.
e.g. void swap(){
swap();
}
Why did the designers decide to force a method to specify all uncaught checked exceptions that can be thrown within its scope?
Ans) Any Exception that can be thrown by a method is part of the method's public programming interface. Those who call a method must know about the exceptions that a method can throw so that they can decide what to do about them. These exceptions are as much a part of that method's programming interface as its parameters and return value.
Once the control switches to the catch block does it return back to the try block to execute the balance code?
Ans) No. Once the control jumps to the catch block it never returns to the try block but it goes to finally block(if present).
Where is the clean up code like release of resources is put in try-catch-finally block and why?
Ans) The code is put in a finally block because irrespective of try or catch block execution the control will flow to finally block. Typically finally block contains release of connections, closing of result set etc.
Is it valid to have a try block without catch or finally?
Ans) NO. This will result in a compilation error. The try block must be followed by a catch or a finally block. It is legal to omit the either catch or the finally block but not both.
e.g. The following code is illegal.
try{
int i =0;
}
int a = 2;
System.out.println(“a = “+a);
Is it valid to place some code in between try the catch/finally block that follows it?
Ans) No. There should not be any line of code present between the try and the catch/finally block. e.g. The following code is wrong.
try{}
String str = “ABC”;
System.out.println(“str = “+str);
catch(Exception e){}
What happens if the exception is never caught and throws down the method stack?
Ans) If the exception is not caught by any of the method in the method’s stack till you get to the main() method, the main method throws that exception and the JVM halts its execution.
How do you get the descriptive information about the Exception occurred during the program execution?
Ans) All the exceptions inherit a method printStackTrace() from the Throwable class. This method prints the stack trace from where the exception occurred. It prints the most recently entered method first and continues down, printing the name of each method as it works its way down the call stack from the top.
Can you catch more than one exceptions in a single catch block?
Ans)Yes. If the exception class specified in the catch clause has subclasses, any exception object that is a subclass of the specified Exception class will be caught by that single catch block.
E.g..
try {
// Some code here that can throw an IOException
}
catch (IOException e) {
e.printStackTrace();
}
The catch block above will catch IOException and all its subclasses e.g. FileNotFoundException etc.
Why is not considered as a good practice to write a single catchall handler to catch all the exceptions?
Ans) You can write a single catch block to handle all the exceptions thrown during the program execution as follows :
try {
// code that can throw exception of any possible type
}catch (Exception e) {
e.printStackTrace();
}
If you use the Superclass Exception in the catch block then you will not get the valuable information about each of the exception thrown during the execution, though you can find out the class of the exception occurred. Also it will reduce the readability of the code as the programmer will not understand what is the exact reason for putting the try-catch block.
What is exception matching?
Ans) Exception matching is the process by which the the jvm finds out the matching catch block for the exception thrown from the list of catch blocks. When an exception is thrown, Java will try to find by looking at the available catch clauses in the top down manner. If it doesn't find one, it will search for a handler for a supertype of the exception. If it does not find a catch clause that matches a supertype for the exception, then the exception is propagated down the call stack. This process is called exception matching.
What happens if the handlers for the most specific exceptions is placed above the more general exceptions handler?
Ans) Compilation fails. The catch block for handling the most specific exceptions must always be placed above the catch block written to handle the more general exceptions.
e.g. The code below will not compile.
1 try {
// code that can throw IOException or its subtypes
} catch (IOException e) {
// handles IOExceptions and its subtypes
} catch (FileNotFoundException ex) {
// handle FileNotFoundException only
}
The code below will compile successfully :-
try {
// code that can throw IOException or its subtypes
} catch (FileNotFoundException ex) {
// handles IOExceptions and its subtypes
} catch (IOException e){
// handle FileNotFoundException only
}
Does the order of the catch blocks matter if the Exceptions caught by them are not subtype or supertype of each other?
Ans) No. If the exceptions are siblings in the Exception class’s hierarchy i.e. If one Exception class is not a subtype or supertype of the other, then the order in which their handlers(catch clauses) are placed does not matter.
What happens if a method does not throw an checked Exception directly but calls a method that does? What does 'Ducking' the exception mean?
Ans) If a method does not throw an checked Exception directly but calls a method that throws an exception then the calling method must handle the throw exception or declare the exception in its throws clause. If the calling method does not handle and declares the exception, the exceptions is passed to the next method in the method stack. This is called as ducking the exception down the method stack.
e.g. The code below will not compile as the getCar() method has not declared the CarNotFoundException which is thrown by the getColor () method.
void getCar() {
getColor();
}
void getColor () {
throw new CarNotFoundException();
}
Fix for the above code is
void getCar() throws CarNotFoundException {
getColor();
}
void getColor () {
throw new CarNotFoundException();
}
Is an empty catch block legal?
Ans) Yes you can leave the catch block without writing any actual code to handle the exception caught.
e.g. The code below is legal but not appropriate, as in this case you will nt get any information about the exception thrown.
try{
//code that may throw the FileNotFoundException
}catch(FileNotFound eFnf){
//no code to handle the FileNotFound exception
}
Can a catch block throw the exception caught by itself?
Ans) Yes. This is called rethrowing of the exception by catch block.
e.g. the catch block below catches the FileNotFound exception and rethrows it again.
void checkEx() throws FileNotFoundException {
try{
//code that may throw the FileNotFoundException
}catch(FileNotFound eFnf){
throw FileNotFound();
}
}
Inner Class Interview Questions
What is an inner class?
Ans) Inner class is a class defined inside other class and act like a member of the enclosing class.
What are the different types of inner classes?
Ans) There are two main types of inner classes –
Static member class
Inner class
Member class
Anonymous class
Local class
What is static member class?
Ans) A static member class behaves much like an ordinary top-level class, except that it can access the static members of the class that contains it. The static nested class can be accessed as the other static members of the enclosing class without having an instance of the outer class. The static class can contain non-static and static members and methods.
public class InnerClass {
static class StaticInner {
static int i = 9;
int no = 6;
private void method() {}
public void method1() {}
static void method2() {}
final void method3() {}
}
}
The static inner class can be accessed from Outer Class in the following manner:
InnerClass.StaticInner staticObj= new InnerClass. StaticInner ();
No outer class instance is required to instantiate the nested static class because the static class is a static member of the enclosing class.
What are non static inner classes?
Ans) The different type of static inner classes are: Non - static inner classes – classes associated with the object of the enclosing class. Member class - Classes declared outside a function (hence a "member") and not declared "static".
The member class can be declared as public, private, protected, final and abstract. E.g.
public class InnerClass {
class MemberClass {
public void method1() { }
}
}
Method local class – The inner class declared inside the method is called method local inner class. Method local inner class can only be declared as final or abstract. Method local class can only access global variables or method local variables if declared as final
public class InnerClass {
int i = 9;
public void method1() {
final int k = 6;
class MethodLocal {
MethodLocal() {
System.out.println(k + i);
}
}
}
}
Anonymous inner class - These are local classes which are automatically declared and instantiated in the middle of an expression. Also, like local classes, anonymous classes cannot be public, private, protected, or static. They can specify arguments to the constructor of the superclass, but cannot otherwise have a constructor. They can implement only one interface or extend a class.
Anonymous class cannot define any static fields, methods, or classes, except for static final constants.
Also, like local classes, anonymous classes cannot be public, private, protected, or static
Some examples:
public class MyFrame extends JFrame {
JButton btn = new JButton();
MyFrame() {
btn.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e) {
}
});
}
}
Anonymous class used with comparator
List
l.add(new Parent(2));
l.add(new Parent(3));
Collections.sort(l, new Comparator() {
public int compare(Object o1, Object o2) {
Parent prt1 = (Parent) o1;
Parent prt2 = (Parent) o2;
if (prt1.getAge() > prt2.getAge()) {
return -1;
}else if(prt1.getAge()
|
Does a static nested class have access to the enclosing class' non-static methods or instance variables?
Ans) No .
|
What are the advantages of Inner classes?
Ans) The embedding of inner class into the outer class in the case when the inner class is to be used only by one class i.e. the outer class makes the package more streamlined. Nesting the inner class code where it is used (inside the outer class) makes the code more readable and maintainable.
The inner class shares a special relationship with the outer class i.e. the inner class has access to all members of the outer class and still have its own type is the main advantages of Inner class. Advantage of inner class is that they can be hidden from the other classes in the same package and still have the access to all the members (private also) of the enclosing class. So the outer class members which are going to be used by the inner class can be made private and the inner class members can be hidden from the classes in the same package. This increases the level of encapsulation.
If a class A is written requires another class B for its own use, there are two ways to do this. One way is to write a separate class B or to write an inner class B inside class A. Advantage of writing the inner class B in the class A is you can avoid having a separate class. Inner classes are best used in the event handling mechanism and to implement the helper classes. The advantage of using inner class for event handling mechanism is that the use of if/else to select the component to be handled can be avoided. If inner classes are used each component gets its own event handler and each event handler implicitly knows the component it is working for. e.g.
Button btn1 = new Button("Submit"); Btn.addActionListener(new ActionListener(){/br> } );
The advantage of using static nested class is that to instantiate a static nested class you need not create an instance of the enclosing class which reduces the number of objects the application creates at runtime.
|
What are disadvantages of using inner classes?
Ans) 1. Using inner class increases the total number of classes being used by the application. For all the classes created by JVM and loaded in the memory, jvm has to perform some tasks like creating the object of type class. Jvm may have to perform some routine tasks for these extra classes created which may result slower performance if the application is using more number of inner classes. 2. Inner classes get limited support of ide/tools as compared to the top level classes, so working with the inner classes is sometimes annoying for the developer.
|
What are different types of anonymous classes?
Ans 1) Plain old anonymous class type one–
e.g. class superClass{ void doSomething() { System.out.println(“Doing something in the Super class”); } } class hasAnonymous{ superClass anon = new superClass(){ void doSomething() { System.out.println(“Doing something in the Anonymous class”); } }; Here anon is the reference which is of type superClass which is the class extended by the anonymous class i.e. superclass of the anonymous class. The method doSomething() is the super class method overridden by the anonymous class. 2) Plain old anonymous class type two – interface Eatable{ public void prepareSweets(); } class serveMeal{ Eatable food = new Eatable(){ public void prepareSweets(){ //come implementation code goes here } }; } food is reference variable of type Eatable interface which refers to the anonymous class which is the implementer of the interface Eatable. The anonymous implementer class of the interface Eatable implements its method prepareSweets() inside it. 3) Argument defined anonymous class – e.g. interface Vehicle { void getNoOfWheels(); } class Car { void getType(Vehical v) { } } class BeautifulCars { void getTheBeautifilCar() { Car c = new Car (); c.getType (new Vehicle () { public void getNoOfWheels () { System.out.println("It has four wheels"); } }); } } Anonymous class is defined as the argument of the method getTheBeautifilCar(), this anonymous class is the implementer of the interface Vehicle. The method of class Car getTheBeautifilCar() expects the argument as an object of type Vehicle. So first we create an object of Car referenced by the variable ‘c’. On this object of Car we call the method getTheBeautifilCar() and in the argument we create an anonymous class in place which is the implementer of interface Vehicle hence of type Vehicle. |
If you compile a file containing inner class how many .class files are created and what are all of them accessible in usual way?
Ans) If a inner class enclosed with an outer class is compiled then one .class file for each inner class an a .class file for the outer class is created. e.g.
class EnclosingOuter { class Inner{ } } If you compile the above code with command % javac EnclosingOuter.java Two files EnclosingOuter.class EnclosingOuter$Inner.class will be created. Though a separate inner class file is generated, the inner class file is not accessible in the usual way like, % java EnclosingOuter$Inner |
How to access the inner class from code within the outer class?
Ans) The inner class is instantiated only through the outer class instance.
class EnclosingOuter { private int noInnerClass = 1; public void getNoOfInnerClasses(){ Inner in = new Inner(); System.out.println(“No Of Inner classes is : “+ in.getNoOfClassesFromOuter()); } class Inner{ public int getNoOfClassesFromOuter(){ return noInnerClass; } } Here the method getNoOfInnerClasses() is called on the outer class’s instance through this outer class instance the inner class instance in is created. |
How to create an inner class instance from outside the outer class instance code?
Ans) To create an instance of the inner class you must have the instance of its enclosing class.
e.g. class EnclosingOuter { class Inner{ } } To create the instance of inner class from class other than the enclosing class. 1) class OtherThanOuter{ EnclosingOuter out = new EnclosingOuter(); EnclosingOuter.Inner in = out.new Inner(); } 2) class OtherThanOuter{ EnclosingOuter.Inner out = new EnclosingOuter.Inner (); } |
How to refer to the outer this i.e. outer class’s current instance from inside the inner class?
Ans) The outer this reference i.e. the outer class’ current instance’ reference can be refered using ‘OuterClassName.this’. E.g
class EnclosingOuter { class Inner{ System.out.println(“Inner class reference is “ + this); // inner class instance
|
Which modifiers can be applied to the inner class?
Ans) Following are modifiers that can be applied to the inner:
public private abstract final protected strictfp static – turns the inner class into static nested class. |
Can the method local inner class object access method’s local variables?
Ans) No, a method local inner class object can not access the method local variable.
Reason: The local variables are not guaranteed to live as long as the local inner class object. The method local variable live on stack and exist only till the method lives, their scope is limited only code inside the method they are declared in. But the local inner class object created within the method lives on heap and it may exist even after the method ends if in case the reference of this local inner class is passed into some other code and is stored in an instance variable. So we can not be sure that the local variables will live till the method local inner class object lives, therefore the method local inner class object can not access the method local variable. To access the method local variables, the variable has to be declared as final. |
Can a method local inner class access the local final variables?Why?
Ans) Yes. Because the final variables are stored on heap and they live as long as the method local inner class object may live.
|
Which modifiers can be applied to the method local inner class?
Ans) Only abstract or final keyword isallowed.
|
Can a local class declared inside a static method have access to the instance members of the outer class?
Ans) No. There is no this reference available in the static method .The static method class can not have access to any members of the outer class other than static members.
|
Can a method which is not in the definition of the superclass of an anonymous class be invoked on that anonymous class reference?
Ans) No. Compilation will fail.As the reference variable type of the anonymous class will be of superclass which will not know of any method defined inside the anonymous class the compilation will fail.
e.g. class SuperClass{ void doSomething() { System.out.println("In the Super class"); } } class hasAnonymous{ SuperClass anon = new SuperClass(){ void doSomething() { System.out.println("In the Anonymous class"); } void doStuff() { System.out.println("An Anonymous class method not present in superClass"); } }; public void doIt(){ anon.doSomething(); // legal superClass has this method anon.doStuff(); // Not legal } } The above code does not compile as the superClass does not know about the anonymous class method doStuff(). |
Can an anonymous class define method of its own?
Ans) Yes. But there will be no way by which the methods defined in the anonymous class which are not present in its superclass be invoked. As only those methods which are defined in the suprclass which the anonymous class extends be invoked defining the methods in the anonymous class will be of no use.
|
Can an anonymous class implement multiple interfaces directly?
Ans) No. An anonymous class can implement only one interface. If the anonymous class is extending a class then it becomes the implementer of all the interfaces implemented by its superclass automatically.
|
Can an anonymous class implement an interface and also extend a class at the same time?
Ans) No. An anonymous class can either extend a class or implement a single interface. If the anonymous class is extending a class then it becomes the implementer of all the interfaces implemented by its superclass automatically.
|
Serialization Interview Questions |
What is Serialization?
Ans) Serializable is a marker interface. When an object has to be transferred over a network ( typically through rmi or EJB) or persist the state of an object to a file, the object Class needs to implement Serializable interface. Implementing this interface will allow the object converted into bytestream and transfer over a network.
|
What is use of serialVersionUID?
Ans) During object serialization, the default Java serialization mechanism writes the metadata about the object, which includes the class name, field names and types, and superclass. This class definition is stored as a part of the serialized object. This stored metadata enables the deserialization process to reconstitute the objects and map the stream data into the class attributes with the appropriate type
Everytime an object is serialized the java serialization mechanism automatically computes a hash value. ObjectStreamClass'scomputeSerialVersionUID() method passes the class name, sorted member names, modifiers, and interfaces to the secure hash algorithm (SHA), which returns a hash value.The serialVersionUID is also called suid. So when the serilaize object is retrieved , the JVM first evaluates the suid of the serialized class and compares the suid value with the one of the object. If the suid values match then the object is said to be compatible with the class and hence it is de-serialized. If not InvalidClassExceptionexception is thrown. Changes to a serializable class can be compatible or incompatible. Following is the list of changes which are compatible:
List of incompatible changes:
So, if no suid is present , inspite of making compatible changes, jvm generates new suid thus resulting in an exception if prior release version object is used .
The only way to get rid of the exception is to recompile and deploy the application again. If we explicitly metion the suid using the statement: then if any of the metioned compatible changes are made the class need not to be recompiled. But for incompatible changes there is no other way than to compile again. |
What is the need of Serialization?
Ans) The serialization is used :-
|
Other than Serialization what are the different approach to make object Serializable?
Ans) Besides the Serializable interface, at least three alternate approaches can serialize Java objects:
1)For object serialization, instead of implementing the Serializable interface, a developer can implement the Externalizable interface, which extends Serializable. By implementing Externalizable, a developer is responsible for implementing the writeExternal() and readExternal() methods. As a result, a developer has sole control over reading and writing the serialized objects. 2)XML serialization is an often-used approach for data interchange. This approach lags runtime performance when compared with Java serialization, both in terms of the size of the object and the processing time. With a speedier XML parser, the performance gap with respect to the processing time narrows. Nonetheless, XML serialization provides a more malleable solution when faced with changes in the serializable object. 3)Finally, consider a "roll-your-own" serialization approach. You can write an object's content directly via either the ObjectOutputStream or the DataOutputStream. While this approach is more involved in its initial implementation, it offers the greatest flexibility and extensibility. In addition, this approach provides a performance advantage over Java serialization. |
Do we need to implement any method of Serializable interface to make an object serializable?
Ans) No. Serializable is a Marker Interface. It does not have any methods.
|
What happens if the object to be serialized includes the references to other serializable objects?
Ans) If the object to be serialized includes the references to other objects whose class implements serializable then all those object’s state also will be saved as the part of the serialized state of the object in question. The whole object graph of the object to be serialized will be saved during serialization automatically provided all the objects included in the object’s graph are serializable.
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What happens if an object is serializable but it includes a reference to a non-serializable object?
Ans- If you try to serialize an object of a class which implements serializable, but the object includes a reference to an non-serializable class then a ‘NotSerializableException’ will be thrown at runtime.
e.g.
public class NonSerial {
//This is a non-serializable class }
public class MyClass implements Serializable{
private static final long serialVersionUID = 1L; private NonSerial nonSerial; MyClass(NonSerial nonSerial){ this.nonSerial = nonSerial; } public static void main(String [] args) { NonSerial nonSer = new NonSerial(); MyClass c = new MyClass(nonSer); try { FileOutputStream fs = new FileOutputStream("test1.ser"); ObjectOutputStream os = new ObjectOutputStream(fs); os.writeObject(c); os.close(); } catch (Exception e) { e.printStackTrace(); } try { FileInputStream fis = new FileInputStream("test1.ser"); ObjectInputStream ois = new ObjectInputStream(fis); c = (MyClass) ois.readObject(); ois.close(); } catch (Exception e) { e.printStackTrace(); } } }
On execution of above code following exception will be thrown –
java.io.NotSerializableException: NonSerial at java.io.ObjectOutputStream.writeObject0(ObjectOutputStream.java) |
Are the static variables saved as the part of serialization?
Ans) No. The static variables belong to the class and not to an object they are not the part of the state of the object so they are not saved as the part of serialized object.
|
What is a transient variable?
Ans) These variables are not included in the process of serialization and are not the part of the object’s serialized state.
|
What will be the value of transient variable after de-serialization?
Ans) It’s default value.
e.g. if the transient variable in question is an int, it’s value after deserialization will be zero.
public class TestTransientVal implements Serializable{
private static final long serialVersionUID = -22L; private String name; transient private int age; TestTransientVal(int age, String name) { this.age = age; this.name = name; } public static void main(String [] args) { TestTransientVal c = new TestTransientVal(1,"ONE"); System.out.println("Before serialization: - " + c.name + " "+ c.age); try { FileOutputStream fs = new FileOutputStream("testTransients.ser"); ObjectOutputStream os = new ObjectOutputStream(fs); os.writeObject(c); os.close(); } catch (Exception e) { e.printStackTrace(); } try { FileInputStream fis = new FileInputStream("testTransients.ser"); ObjectInputStream ois = new ObjectInputStream(fis); c = (TestTransientVal) ois.readObject(); ois.close(); } catch (Exception e) { e.printStackTrace(); } System.out.println("After de-serialization:- " + c.name + " "+ c.age); }
}
Result of executing above piece of code –
Before serialization: - Value of non-transient variable ONE Value of transient variable 1 After de-serialization:- Value of non-transient variable ONE Value of transient variable 0
Explanation –
The transient variable is not saved as the part of the state of the serailized variable, it’s value after de-serialization is it’s default value. |
Does the order in which the value of the transient variables and the state of the object using the defaultWriteObject() method are saved during serialization matter?
Ans) Yes. As while restoring the object’s state the transient variables and the serializable variables that are stored must be restored in the same order in which they were saved.
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How can one customize the Serialization process? or What is the purpose of implementing the writeObject() and readObject() method?
Ans) When you want to store the transient variables state as a part of the serialized object at the time of serialization the class must implement the following methods –
private void wrtiteObject(ObjectOutputStream outStream)
{ //code to save the transient variables state as a part of serialized object }
private void readObject(ObjectInputStream inStream)
{ //code to read the transient variables state and assign it to the de-serialized object }
e.g.
public class TestCustomizedSerialization implements Serializable{
private static final long serialVersionUID =-22L; private String noOfSerVar; transient private int noOfTranVar; TestCustomizedSerialization(int noOfTranVar, String noOfSerVar) { this.noOfTranVar = noOfTranVar; this.noOfSerVar = noOfSerVar; } private void writeObject(ObjectOutputStream os) { try { os.defaultWriteObject(); os.writeInt(noOfTranVar); } catch (Exception e) { e.printStackTrace(); } } private void readObject(ObjectInputStream is) { try { is.defaultReadObject(); int noOfTransients = (is.readInt()); } catch (Exception e) { e.printStackTrace(); } } public int getNoOfTranVar() { return noOfTranVar; } }
The value of transient variable ‘noOfTranVar’ is saved as part of the serialized object manually by implementing writeObject() and restored by implementing readObject().
The normal serializable variables are saved and restored by calling defaultWriteObject() and defaultReadObject()respectively. These methods perform the normal serialization and de-sirialization process for the object to be saved or restored respectively. |
If a class is serializable but its superclass in not , what will be the state of the instance variables inherited from super class after deserialization?
Ans) The values of the instance variables inherited from superclass will be reset to the values they were given during the original construction of the object as the non-serializable super-class constructor will run.
E.g.
public class ParentNonSerializable {
int noOfWheels; ParentNonSerializable(){ this.noOfWheels = 4; } }
public class ChildSerializable extends ParentNonSerializable implements Serializable {
private static final long serialVersionUID = 1L; String color;
ChildSerializable() {
this.noOfWheels = 8; this.color = "blue"; } }
public class SubSerialSuperNotSerial {
public static void main(String [] args) { ChildSerializable c = new ChildSerializable(); System.out.println("Before : - " + c.noOfWheels + " "+ c.color); try { FileOutputStream fs = new FileOutputStream("superNotSerail.ser"); ObjectOutputStream os = new ObjectOutputStream(fs); os.writeObject(c); os.close(); } catch (Exception e) { e.printStackTrace(); } try { FileInputStream fis = new FileInputStream("superNotSerail.ser"); ObjectInputStream ois = new ObjectInputStream(fis); c = (ChildSerializable) ois.readObject(); ois.close(); } catch (Exception e) { e.printStackTrace(); } System.out.println("After :- " + c.noOfWheels + " "+ c.color); }
}
Result on executing above code –
Before : - 8 blue After :- 4 blue
The instance variable ‘noOfWheels’ is inherited from superclass which is not serializable. Therefore while restoring it the non-serializable superclass constructor runs and its value is set to 8 and is not same as the value saved during serialization which is 4.
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To serialize an array or a collection all the members of it must be serializable. True /False?
Ans) true.
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