Java Interview Questions And Answers


Java Interview Questions And Answers

Basic Java Interview Questions


Q1. Explain JDK, JRE and JVM?





It stands for Java Development Kit.

It stands for Java Runtime Environment.

It stands for Java Virtual Machine.

It is the tool necessary to compile, document and package Java programs.

JRE refers to a runtime environment in which Java bytecode can be executed.

It is an abstract machine. It is a specification that provides a run-time environment in which Java bytecode can be executed.

It contains JRE + development tools.

It’s an implementation of the JVM which physically exists.

JVM follows three notations: Specification, Implementation, and Runtime Instance.

Q2. Explain public static void main(String args[]) in Java.

main() in Java is the entry point for any Java program. It is always written as public static void main(String[] args).

· public: Public is an access modifier, which is used to specify who can access this method. Public means that this Method will be accessible by any Class.

· static: It is a keyword in java which identifies it is class-based. main() is made static in Java so that it can be accessed without creating the instance of a Class. In case, main is not made static then the compiler will throw an error as main() is called by the JVM before any objects are made and only static methods can be directly invoked via the class. 

· void: It is the return type of the method. Void defines the method which will not return any value.

· main: It is the name of the method which is searched by JVM as a starting point for an application with a particular signature only. It is the method where the main execution occurs.

· String args[]: It is the parameter passed to the main method.

Q3. Why Java is platform independent?

Java is called platform independent because of its byte codes which can run on any system irrespective of its underlying operating system.

Q4. Why Java is not 100% Object-oriented?

Java is not 100% Object-oriented because it makes use of eight primitive data types such as boolean, byte, char, int, float, double, long, short which are not objects.

Q5. What are wrapper classes in Java?

Wrapper classes convert the Java primitives into the reference types (objects). Every primitive data type has a class dedicated to it. These are known as wrapper classes because they “wrap” the primitive data type into an object of that class. Refer to the below image which displays different primitive type, wrapper class and constructor argument.

Q6. What are constructors in Java?

In Java, constructor refers to a block of code which is used to initialize an object. It must have the same name as that of the class. Also, it has no return type and it is automatically called when an object is created.

There are two types of constructors:

1.     Default Constructor: In Java, a default constructor is the one which does not take any inputs. In other words, default constructors are the no argument constructors which will be created by default in case you no other constructor is defined by the user. Its main purpose is to initialize the instance variables with the default values. Also, it is majorly used for object creation. 

2.     Parameterized Constructor: The parameterized constructor in Java, is the constructor which is capable of initializing the instance variables with the provided values. In other words, the constructors which take the arguments are called parameterized constructors.

Q7. What is singleton class in Java and how can we make a class singleton?

Singleton class is a class whose only one instance can be created at any given time, in one JVM. A class can be made singleton by making its constructor private.

Q8. What is the difference between Array list and vector in Java?



Array List is not synchronized.

 Vector is synchronized.

Array List is fast as it’s non-synchronized.

Vector is slow as it is thread safe.

If an element is inserted into the Array List, it increases its Array size by 50%.

Vector defaults to doubling size of its array.

Array List does not define the increment size.

Vector defines the increment size.

Array List can only use Iterator for traversing an Array List.

Vector can use both Enumeration and Iterator for traversing.

Q9. What is the difference between equals() and == in Java?

Equals() method is defined in Object class in Java and used for checking equality of two objects defined by business logic.

“==” or equality operator in Java is a binary operator provided by Java programming language and used to compare primitives and objects. public boolean equals(Object o) is the method provided by the Object class. The default implementation uses == operator to compare two objects. For example: method can be overridden like String class. equals() method is used to compare the values of two objects.

Q10. What are the differences between Heap and Stack Memory in Java?

The major difference between Heap and Stack memory are:





Stack memory is used only by one thread of execution.

Heap memory is used by all the parts of the application.


Stack memory can’t be accessed by other threads.

Objects stored in the heap are globally accessible.

Memory Management

Follows LIFO manner to free memory.

Memory management is based on the generation associated with each object.


Exists until the end of execution of the thread.

Heap memory lives from the start till the end of application execution.


Stack memory only contains local primitive and reference variables to objects in heap space.

Whenever an object is created, it’s always stored in the Heap space.

Q11. What is a package in Java? List down various advantages of packages.

Packages in Java, are the collection of related classes and interfaces which are bundled together. By using packages, developers can easily modularize the code and optimize its reuse. Also, the code within the packages can be imported by other classes and reused. Below I have listed down a few of its advantages:

·        Packages help in avoiding name clashes

·        They provide easier access control on the code

·        Packages can also contain hidden classes which are not visible to the outer classes and only used within the package

·        Creates a proper hierarchical structure which makes it easier to locate the related classes

Q12. Why pointers are not used in Java?

Java doesn’t use pointers because they are unsafe and increases the complexity of the program. Since, Java is known for its simplicity of code, adding the concept of pointers will be contradicting. Moreover, since JVM is responsible for implicit memory allocation, thus in order to avoid direct access to memory by the user, pointers are discouraged in Java.

Q13. What is JIT compiler in Java?

JIT stands for Just-In-Time compiler in Java. It is a program that helps in converting the Java bytecode into instructions that are sent directly to the processor. By default, the JIT compiler is enabled in Java and is activated whenever a Java method is invoked. The JIT compiler then compiles the bytecode of the invoked method into native machine code, compiling it “just in time” to execute. Once the method has been compiled, the JVM summons the compiled code of that method directly rather than interpreting it. This is why it is often responsible for the performance optimization of Java applications at the run time.

Q14. What are access modifiers in Java?

In Java, access modifiers are special keywords which are used to restrict the access of a class, constructor, data member and method in another class. Java supports four types of access modifiers:

1. Default

2. Private

3. Protected

4. Public






Same class





Same Package subclass





Same Package non-subclass





Different package subclass





Different package non-subclass





Q15. Define a Java Class.

A class in Java is a blueprint which includes all your data. A class contains fields (variables) and methods to describe the behavior of an object. Let’s have a look at the syntax of a class.

Q16. What is an object in Java and how is it created?

An object is a real-world entity that has a state and behavior. An object has three characteristics:

1. State

2. Behavior

3. Identity

An object is created using the ‘new’ keyword. For example:

ClassName obj = new ClassName();

Q17. What is Object Oriented Programming?

Object-oriented programming or popularly known as OOPs is a programming model or approach where the programs are organized around objects rather than logic and functions. In other words, OOP mainly focuses on the objects that are required to be manipulated instead of logic. This approach is ideal for the programs large and complex codes and needs to be actively updated or maintained.

Q18. What are the main concepts of OOPs in Java?

Object-Oriented Programming or OOPs is a programming style that is associated with concepts like:

1. Inheritance: Inheritance is a process where one class acquires the properties of another.

2. Encapsulation: Encapsulation in Java is a mechanism of wrapping up the data and code together as a single unit.

3. Abstraction: Abstraction is the methodology of hiding the implementation details from the user and only providing the functionality to the users. 

4. Polymorphism: Polymorphism is the ability of a variable, function or object to take multiple forms.

Q19. What is the difference between a local variable and an instance variable?

In Java, a local variable is typically used inside a method, constructor, or a block and has only local scope. Thus, this variable can be used only within the scope of a block. The best benefit of having a local variable is that other methods in the class won’t be even aware of that variable.






if(x > 100)


String test = "Edureka";


Whereas, an instance variable in Java, is a variable which is bounded to its object itself. These variables are declared within a class, but outside a method. Every object of that class will create it’s own copy of the variable while using it. Thus, any changes made to the variable won’t reflect in any other instances of that class and will be bound to that particular instance only

Q20. Differentiate between the constructors and methods in Java?



1. Used to represent the behavior of an object

1. Used to initialize the state of an object

2. Must have a return type

2. Do not have any return type

3. Needs to be invoked explicitly

3. Is invoked implicitly

4. No default method is provided by the compiler

4. A default constructor is provided by the compiler if the class has none

5. Method name may or may not be same as class name

5. Constructor name must always be the same as the class name

In case you are facing any challenges with these Java interview questions, please comment on your problems in the section below.

Q21. What is final keyword in Java?

final is a special keyword in Java that is used as a non-access modifier. A final variable can be used in different contexts such as:

·        final variable

When the final keyword is used with a variable then its value can’t be changed once assigned. In case the no value has been assigned to the final variable then using only the class constructor a value can be assigned to it.

·        final method

When a method is declared final then it can’t be overridden by the inheriting class.

·        final class

When a class is declared as final in Java, it can’t be extended by any subclass class but it can extend other class.

Q22. What is the difference between break and continue statements?



1. Can be used in switch and loop (for, while, do while) statements

1. Can be only used with loop statements

2. It causes the switch or loop statements to terminate the moment it is executed

2. It doesn’t terminate the loop but causes the loop to jump to the next iteration

3. It terminates the innermost enclosing loop or switch immediately

3. A continue within a loop nested with a switch will cause the next loop iteration to execute

Example break:









for (int i = 0; i < 5; i++)


if (i == 3)






Example continue:









for (int i = 0; i < 5; i++)


if(i == 2)





Q23.What is an infinite loop in Java? Explain with an example.

An infinite loop is an instruction sequence in Java that loops endlessly when a functional exit isn’t met. This type of loop can be the result of a programming error or may also be a deliberate action based on the application behavior. An infinite loop will terminate automatically once the application exits.