Core Java Interview Questions part-3

Q) Can I have multiple main methods in the same class?

A) No the program fails to compile. The compiler says that the main method is already defined in the class.

Q) Constructor

            The automatic initialization is performed through the constructor, constructor has same name has class name. Constructor has no return type not even void. We can pass the parameters to the constructor. this() is used to invoke a constructor of the same class. Super () is used to invoke a super class constructor. Constructor is called immediately after the object is created before the new operator completes.

à Constructor can use the access modifiers public, protected, private or have no access modifier

à Constructor can not use the modifiers abstract, static, final, native, synchronized or strictfp

à Constructor can be overloaded, we cannot override.

à You cannot use this() and Super() in the same constructor.

Class A(

A(){

     System.out.println(“hello”);

}}

Class B extends A {

B(){

     System.out.println(“friend”);

}}

Class print {

Public static void main (String args []){

B b = new B();

}

o/p:- hello friend

Q) Diff Constructor & Method

Constructor	Method
Use to instance of a class	Grouping java statement
No return type	Void (or) valid return type
Same name as class name	As a name except the class method name, begin with lower case.
“This” refer to another constructor in the same class	Refers to instance of class
“Super” to invoke the super class constructor	Execute an overridden method in the super class
“Inheritance” cannot be inherited	Can be inherited
We can “overload” but we cannot “overridden”	Can be inherited
Will automatically invoke when an object is created	Method has called explicitly

Q) Garbage collection

G.C is also called automatic memory management as JVM automatically removes the unused variables/objects (value is null) from the memory. User program cann't directly free the object from memory, instead it is the job of the garbage collector to automatically free the objects that are no longer referenced by a program. Every class inherits finalize() method from java.lang.Object, the finalize() method is called by garbage collector when it determines no more references to the object exists. In Java, it is good idea to explicitly assign null into a variable when no more in use, calling System.gc() and Runtime.gc(), JVM tries to recycle the unused objects, but there is no guarantee when all the objects will garbage collected. Garbage collection is a low-priority thread.

G.C is a low priority thread in java, G.C cannot be forced explicitly. JVM may do garbage collection if it is running short of memory. The call System.gc() does NOT force the garbage collection but only suggests that the JVM may make an effort to do garbage collection.

Q) How an object becomes eligible for Garbage Collection?

A) An object is eligible for garbage collection when no object refers to it, An object also becomes eligible when its reference is set to null. The objects referred by method variables or local variables are eligible for garbage collection when they go out of scope.

Integer i = new Integer(7);
i = null;

Q) Final, Finally, Finalize

Final: - When we declare a sub class a final the compiler will give error as “cannot subclass final class” Final to prevent inheritance and method overriding. Once to declare a variable as final it cannot occupy memory per instance basis.

     à  Final class cannot have static methods   

     à  Final class cannot have abstract methods (Because of final class never allows any class to inherit it)

     à  Final class can have a final method.

Finally: - Finally create a block of code that will be executed after try catch block has completed. Finally block will execute whether or not an exception is thrown. If an exception is thrown, the finally block will execute even if no catch statement match the exception. Any time a method is about to return to the caller from inside try/catch block, via an uncaught exception or an explicit return statement, the finally clause is also execute.

                       

    Using System.exit() in try block will not allow finally code to execute

Finalize: - some times an object need to perform some actions when it is going to destroy, if an object holding some non-java resource such as file handle (or) window character font, these resources are freed before the object is going to destroy.

Q) Can we declare abstract method in final class?

A) It indicates an error to declare abstract method in final class. Because of final class never allows any class to inherit it.

Q) Can we declare final method in abstract class?

A) If a method is defined as final then we can’t provide the reimplementation for that final method in it’s derived classes i.e overriding is not possible for that method. We can declare final method in abstract class suppose of it is abstract too, then there is no used to declare like that.

Q) Superclass & Subclass

            A super class is a class that is inherited whereas subclass is a class that does the inheriting

Q) How will u implement 1) polymorphism 2) multiple inheritance 3) multilevel inheritance in java?

A) Polymorphism           – overloading and overriding

    Multiple inheritances  – interfaces.

    Multilevel inheritance – extending class.