Java Interview Questions (Part 1)

Front 1

How many types of memory areas are allocated by JVM?

Back 1

Class(Method) Area

Heap

Stack

Program Counter Register

Native Method Stack

Front 2

What is JIT compiler?

Back 2

Just-In-Time(JIT) compiler:It is used to improve the performance. JIT compiles parts of the byte code that have similar functionality at the same time, and hence reduces the amount of time needed for compilation.Here the term “compiler” refers to a translator from the instruction set of a Java virtual machine (JVM) to the instruction set of a specific CPU.

Front 3

What is platform?

Back 3

A platform is basically the hardware or software environment in which a program runs. There are two types of platforms software-based and hardware-based. Java provides software-based platform.

Front 4

What is the main difference between Java platform and other platforms?

Back 4

The Java platform differs from most other platforms in the sense that it's a software-based platform that runs on top of other hardware-based platforms.It has two components:

Runtime Environment

API(Application Programming Interface)

Front 5

What gives Java its 'write once and run anywhere' nature?

Back 5

The bytecode. Java is compiled to be a byte code which is the intermediate language between source code and machine code. This byte code is not platform specific and hence can be fed to any platform.

Front 6

What is classloader?

Back 6

The class-loader is a subsystem of JVM that is used to load classes and interfaces.There are many types of class-loaders e.g. Bootstrap class-loader, Extension class-loader, System class-loader, Plugin class-loader etc.

Front 7

Is Empty .java file name a valid source file name?

Back 7

Yes, save your java file by .java only, compile it by javac .java and run by java yourclassname Let's take a simple example:

Front 8

Is delete,next,main,exit or null keyword in java?

Back 8

No.

Front 9

If I don't provide any arguments on the command line, then the String array of Main method will be empty or null?

Back 9

It is empty. But not null.

Front 10

What if I write static public void instead of public static void?

Back 10

Program compiles and runs properly.

Front 11

What is the default value of the local variables?

Back 11

The local variables are not initialized to any default value, neither primitives nor object references.

Front 12

What is difference between object oriented programming language and object based programming language?

Back 12

Object based programming languages follow all the features of OOPs except Inheritance. Examples of object based programming languages are JavaScript, VBScript etc.

Front 13

What will be the initial value of an object reference which is defined as an instance variable?

Back 13

The object references are all initialized to null in Java.

Front 14

What is constructor?

Back 14

Constructor is just like a method that is used to initialize the state of an object. It is invoked at the time of object creation.

Front 15

What is the purpose of default constructor?

Back 15

The default constructor provides the default values to the objects. The java compiler creates a default constructor only if there is no constructor in the class.

Front 16

Does constructor return any value?

Back 16

yes, that is current instance (You cannot use return type yet it returns a value)

Front 17

Is constructor inherited?

Back 17

No, constructor is not inherited.

Front 18

Can you make a constructor final?

Back 18

No, constructor can't be final.

Front 19

What is static variable?

Back 19

static variable is used to refer the common property of all objects (that is not unique for each object) e.g. company name of employees,college name of students etc.

static variable gets memory only once in class area at the time of class loading.

Front 20

What is static method?

Back 20

A static method belongs to the class rather than object of a class.

A static method can be invoked without the need for creating an instance of a class.

static method can access static data member and can change the value of it.

Front 21

Why main method is static?

Back 21

because object is not required to call static method if It were non-static method,jvm creates object first then call main() method that will lead to the problem of extra memory allocation.

Front 22

What is static block?

Back 22

Is used to initialize the static data member.

It is executed before main method at the time of class-loading.

Front 23

Can we execute a program without main() method?

Back 23

Yes, one of the way is static block.

Front 24

What if the static modifier is removed from the signature of the main method?

Back 24

Program compiles. But at runtime throws an error "NoSuchMethodError".

Front 25

What is difference between static (class) method and instance method?

Back 25

static or class method

1)A method i.e. declared as static is known as static method.

2)Object is not required to call static method.

3)Non-static (instance) members cannot be accessed in static context (static method, static block and static nested class) directly.

4)For example: public static int cube(int n){ return n*n*n;}

instance method

A method i.e. not declared as static is known as instance method.

Object is required to call instance methods.

static and non-static variables both can be accessed in instance methods.

For example: public void msg(){...}.

Front 26

What is this in java?

Back 26

It is a keyword that that refers to the current object.

Front 27

What is Inheritance?

Back 27

Inheritance is a mechanism in which one object acquires all the properties and behaviour of another object of another class. It represents IS-A relationship. It is used for Code Resusability and Method Overriding.

Front 28

Which class is the superclass for every class.

Back 28

Object class.

Front 29

Why multiple inheritance is not supported in java?

Back 29

To reduce the complexity and simplify the language, multiple inheritance is not supported in java in case of class

Front 30

What is composition?

Back 30

Holding the reference of the other class within some other class is known as composition.

Front 31

What is difference between aggregation and composition?

Back 31

Aggregation represents weak relationship whereas composition represents strong relationship. For example: bike has an indicator (aggregation) but bike has an engine (compostion).

Front 32

Why Java does not support pointers?

Back 32

Pointer is a variable that refers to the memory address. They are not used in java because they are unsafe(unsecured) and complex to understand.

Front 33

What is super in java?

Back 33

It is a keyword that refers to the immediate parent class object.

Front 34

Can you use this() and super() both in a constructor?

Back 34

No. Because super() or this() must be the first statement.

Front 35

What is object cloning?

Back 35

The object cloning is used to create the exact copy of an object.

Front 36

What is method overloading?

Back 36

If a class have multiple methods by same name but different parameters, it is known as Method Overloading. It increases the readability of the program.

Front 37

Why method overloading is not possible by changing the return type in java?

Back 37

Becauseof ambiguity

Front 38

Can we overload main() method?

Back 38

Yes, You can have many main() methods in a class by overloading the main method.

Front 39

What is method overriding:

Back 39

If a subclass provides a specific implementation of a method that is already provided by its parent class, it is known as Method Overriding. It is used for runtime polymorphism and to provide the specific implementation of the method.

Front 40

Can we override static method?

Back 40

No, you can't override the static method because they are the part of class not object.

Front 41

Why we cannot override static method?

Back 41

It is because the static method is the part of class and it is bound with class whereas instance method is bound with object and static gets memory in class area and instance gets memory in heap.

Front 42

Can we override the overloaded method?

Back 42

Yes.

Front 43

Difference between method Overloading and Overriding.

Back 43

Method Overloading

Method overloading increases the readability of the program.

method overlaoding is occurs within the class.

In this case, parameter must be different.

Method Overriding

Method overriding provides the specific implementation of the method that is already provided by its super class.

Method overriding occurs in two classes that have IS-A relationship.

In this case, parameter must be same.

Front 44

Can you have virtual functions in Java?

Back 44

Yes, all functions in Java are virtual by default.

Front 45

What is covariant return type?

Back 45

Now, since java5, it is possible to override any method by changing the return type if the return type of the subclass overriding method is subclass type. It is known as covariant return type.

Front 46

What is final variable?

Back 46

If you make any variable as final, you cannot change the value of final variable(It will be constant)

Front 47

What is final method?

Back 47

Final methods can't be overriden

Front 48

What is final class?

Back 48

Final class can't be inherited.

Front 49

What is blank final variable?

Back 49

A final variable, not initalized at the time of declaration, is known as blank final variable.

Front 50

Can we intialize blank final variable?

Back 50

Yes, only in constructor if it is non-static. If it is static blank final variable, it can be initialized only in the static block.

Front 51

Can you declare the main method as final?

Back 51

Yes, such as, public static final void main(String[] args){}.