Top 30+ Best Java Interview Questions 2021[UPDATED]

Java Interview Questions
Designed by: James Gosling
First appeared: May 23, 1995; 26 years ago
Latest release: Java SE 16 (16 March 2021)
Stable release: Java SE 16.0.2 / 20 July 2021; 34 days ago
Paradigm: Multi-paradigm: generic, object-oriented (class-based), functional, imperative, reflective

Java Interview Questions

In the software development domain, Java is a perennial contributor. This makes it important to learn & master Java skills and to practice the skills learned in real-time. This programming language is object-oriented & class-based. Moreover, it’s a very popular programming language in the world.

If you are preparing to have an interview on Java, you are on the right platform. It will be right if you go through this couple of questions and answer. This will give you an idea about what to encounter and the proper way of answering it out.

What is Java?

Java is a programming language that’s object-oriented & class-based plus it’s a very common programming language around the world.

A Hello World message in Java:

Hello World message in Java

State why Java is a platform-independent language?

Java language is developed in a way that won’t depend on software or hardware. This is because a compiler compiles code & then converts it into platform-independent byte-code that can be operated on numerous systems.

A condition to operate that byte-code is for a machine to have a JRE which is a runtime environment installed.

Explain why Java isn’t a pure object-oriented language?

Java programing language supports primitive data kinds including byte, char, Boolean, short, float, int, long, & double. Thus, it isn’t a pure object-oriented language.

What are the instance variable & local variable?

Instance variables are variables that are accessible via all class methods. They’re declared outside methods & inside class. The variables describe the features of an object & remain bound to it at every cost.

Every object of the class will have a copy of variables for usage. When a modification is done to these variables, just that instance will have been impacted by it, & every other class instance continues to be unaffected.

Local variables are those variables that are within some block, constructor, or function & can be accessed just inside them. Usage of these variables is limited to block scope. When a local variable is affirmed in a method, other class methods don’t know the local variable.

How can one Install Java?

Installing Java via command prompt allows it to generate essential log files in troubleshooting the problem. When installing Java, you need to

  • Visit & click on the Free Java-Download button.
  • Then click on Save button & save Java software at the Desktop
  • After that, verify the Java software that’s saved on the desktop.
  • Then open Windows-Command Prompt window.
  • With Windows XP Click at the Start key then Run and Type in the cmd
  • With Windows Vista & Windows 7, you need at Start key to Type in cmd at the Start-Search field.
  • cd <Java-download directory>
  • IRun installer & follow the onscreen directives.

How does Java enable high-performance?

Java utilizes a Just-In-Time compiler that allows high performance. It’s utilized in converting instructions to bytecodes.

State Java IDE’s?

Eclipse & NetBeans are JAVA IDE’s

What’s a Class?

Java codes are stated in Class. This has methods and variables. Methods are places where exact business logic needs to be done. This consists of a set of statements/instructions which satisfy a certain requirement. Variables are qualities that define a state of the class.

What’s an Object?

An object is an instance of the class. The object has behavior & state. When JVM reads the “new()” keyword it creates an instance of the class.

What’s Inheritance?

Inheritance means a single class could extend to a different class. This allows the codes to be reused from one class to a different class. The existing class is understood as Super-class whereas the derived class is understood as a sub-class.

What’s the platform?

The platform is a software or hardware environment where a piece of program is executed. There are 2 kinds of platforms hardware-based and software-based. Java offers a software-based platform.

What’s the meaning of data encapsulation?

This is an Object-Oriented-Programming concept of hiding information attributes & behaviors within a single unit.

This helps developers follow modularity as one develops software by guaranteeing that every object is independent of different objects by having its attributes, methods & functionalities.

It’s used for the security of private properties of objects & serves the function of information hiding.


What’s Polymorphism?

Polymorphism means several forms. A single object could refer to a sub-class or super-class depending on the reference kind that’s known as polymorphism.

What does Interface mean?

Numerous inheritances can’t be attained in java. In overcoming this issue Interface concept is brought about. Therefore, an interface means a template that has just method declarations & not method implementation.

What’s meant by Public & Private-access specifiers?

Methods & instance variables are recognized as members.

Public members are visible in a similar package and outside the package that’s for other packages. Public members of Class-A can be seen in Class B of a similar package and in class C of different packages.

Private members are visible in a similar class just & not for other classes at similar packages and classes in outside packages. Private members in class-A are visible just in the class. It’s invisible for classes- B and class C.

What’s a thread in relation to Java?

Thread enables a program to function efficiently via doing numerous things at a similar time. Furthermore, threads can be utilized to do complex tasks in the background without interrupting the major program. This can be formed by extending the Thread class & overriding the run() method.

What’s enumeration in relation to Java?

Enumeration is a list of named-constant. The enumeration in java defines a class type. Moreover, enumerations have constructors, instance & methods variables. It’s formed with the help of enum-keyword. Every enumeration constant is public, static & final via default. Enumeration outlines a class type & has constructors, one doesn’t instantiate an enumeration using new. Enumeration variables are utilized & declared in a similar way as one does primitive variables.

What’s an abstraction in relation to Java?

Objects are building blocks of Object-Oriented-Programming. Object consists of some properties & methods. They can be hidden from the outer world via access modifiers. Access can be provided only for needed functions & properties to other programs.

What’s a collection in java?

A collection is like a container that groups numerous items into a single unit. For instance, jar-of-chocolates, a list of names, etc. Moreover, collections are utilized in all programming languages & when Java came, it arrived with a few Collection-classes including Vector, Hashtable, Stack, and Array.

What’s API in relation to java?

Java API (application programming interface) is a list of every class which is part of JDK (Java development kit). This comprises every Java package, interface, and class along with methods, fields, & constructors. The pre-written classes offer a fantastic amount of functionality to a programmer.

a thread in relation to Java

Initializing array in java?

Initializing array in java

What’s the package in relation to java?

Java package is utilized to group connected classes. Thinking of it as a folder in a file directory. Packages are used to prevent name conflicts, & to write excellent maintainable code. Additionally, packages are shared into 2 categories:

  • User-defined Packages allow one to create packages
  • In-built Packages are packages from Java API

How does one check java-version?

Execute java-version on command-prompt/terminal

How to allow java in chrome?

  • In Java-Control Panel, click on the Security tab
  • Choose the option Enable Java content at this browser
  • Click at Apply & then OK for confirming the changes
  • Restart your browser to allow the changes

What’s a string in relation to java?

This is a sequence of characters, for instance, a string of five characters “Hello”. At java, a string means an immutable object that means it’s a constant & cannot be transformed the moment it’s created.

What’s singleton-class in relation to java?

Singleton design pattern is utilized to restrict class instantiation & ensures that just one instance of a class exists in JVM. A singleton class is a class that can have just one object at a given time in the JVM instance.

Explain garbage collection in relation to java?

Java garbage collection is an auto process. Programmers don’t require to explicitly mark objects which are to be deleted. Garbage-collection implementation lives in JVM. Every JVM allows one to implement garbage collection the way it pleases. The requirement that suits the JVM requirement. Though there are numerous JVMs, Oracle’s HotSpot is the most common. This provides strong & mature garbage set collection options.

Explain JVM in relation to java?

JVM (Java virtual machine) is a virtual machine that allows a computer to operate Java programs and programs written in different languages which are compiled to Java bytecode. JVM is comprehensive by the specification which formally defines what’s needed in JVM implementation.

What’s byte-code in relation to java?

Bytecode is a compiled Java program format. When a Java program is converted to bytecode, it’s transferred across the network & executed by Java-Virtual-Machine (JVM). Bytecode files contain .class extension.

How can one uninstall java?

  • Click at Start
  • Choose Settings
  • Choose System
  • Click on Apps and features
  • Choose a program to uninstall & then click on Uninstall-button
  • Respond to prompts to uninstall fully


Those are some of the key questions that you are likely to encounter when it comes to JAVA interview questions. With these questions, you will be aware of some of the questions to encounter for better preparation.

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What are Java data types?

Data types are divided into two groups:
Primitive data types – includes byte , short , int , long , float , double , boolean and char.
Non-primitive data types – such as String , Arrays and Classes (you will learn more about these in a later chapter)

What type of language is Java?

object-oriented language

What is keywords in Java?

One of 50 reserved terms that have a special function and a set definition in the Java programming language. 

What is OOPS in Java?

OOP stands for Object-Oriented Programming

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