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Top 30 Java Interview Questions and Answers

26/Sep/2021 | 5 minutes to read


Here is a List of essential Java Interview Questions and Answers for Freshers and mid level of Experienced Professionals. All answers for these Java questions are explained in a simple and easiest way. These basic, advanced and latest Java questions will help you to clear your next Job interview.

Java Interview Questions and Answers

These interview questions are targeted for Java Developers. You must know the answers of these frequently asked Java interview questions to clear the interview. We have separate document for Core Java, Java coding, Java collections and Java data structures interview questions.

1. Why is Java not a pure object-oriented language?

An Object-Oriented Programming language should have four basic principles as below.
  • Abstraction
  • Encapsulation
  • Inheritance
  • Polymorphism
So Java, C# are object-oriented languages but if anyone says that these are not 100 percent or purely object-oriented then there should be something else to satisfy him. A pure object oriented language should have these additional fundamentals as below.
  • 5. All predefined types should be objects.
  • 6. All user predefined types should be objects.
  • 7. All the operations should be performed by the exposed methods of an object.
But Java has certain primitive types like int, char, bool, float etc. which are not objects and using the static keyword you can perform operations without an object. So Java fails on points 5 and 7. Hence Java is not considered as pure object-oriented language but It is object-oriented language. For more visit Java is not purely object-oriented.

2. Why is String immutable in Java?

When we can not update or mutate the state of an object by any means then it's known as Immutable Object. When you assign a string object to a variable then it becomes immutable as you can not change its state.
If you declare two string variables "a" and "b" holding the same values "Hello Testing" then both variables point to the same object in memory or string pool. If you declare a third variable which also stores the same value "Hello Testing" then JVM will reference an existing object created for variable "a" instead of creating a new object in memory. So in this case, the state of an object remains unchanged, Hence String is immutable in Java.
String immutability provides some benefits as below.

  • Caching
  • Security
  • Synchronization
  • Performance
For more visit Why is String Immutable in Java?

2. What is the role of String Pool in Java?

3. Explain Instance variables and local variables in Java.

4. What do you understand from Association and Aggregation?

5. Differentiate Abstract class and Interface.

6. Explain Synchronized block in Java.

7. What is Singleton class?

8. Can we have singleton without private constructor?

9. What is the Executor interface?

10. Explain Thread, Process

11. Explain Docker Deployment Process.

12. What is the difference between Permgen and Metaspace in Java?

For more visit Permgen vs Metaspace.

13. Explain Stack Memory in Java. How does it work?

14. Explain Heap Space and How it works?

15. How to increase Heap size for a Java Program?

16. Explain the deadlock situation.

17. What is the need for custom exceptions in Java?

18. How does Serialization and De-serialization work in Java?

19. Explain Thread Context Switching Problems in Java.

20. How will you create an executable JAR with dependencies using Maven?

Some General Interview Questions for Java

1. How much will you rate yourself in Java?

When you attend an interview, Interviewer may ask you to rate yourself in a specific Technology like Java, So It's depend on your knowledge and work experience in Java.

2. What challenges did you face while working on Java?

This question may be specific to your technology and completely depends on your past work experience. So you need to just explain the challenges you faced related to Java in your Project.

3. What was your role in the last Project related to Java?

It's based on your role and responsibilities assigned to you and what functionality you implemented using Java in your project. This question is generally asked in every interview.

4. How much experience do you have in Java?

Here you can tell about your overall work experience on Java.

5. Have you done any Java Certification or Training?

It depends on the candidate whether you have done any Java training or certification. Certifications or training are not essential but good to have.


We have covered some frequently asked Java Interview Questions and Answers to help you for your Interview. All these Essential Java Interview Questions are targeted for mid level of experienced Professionals and freshers.
While attending any Java Interview if you face any difficulty to answer any question please write to us at Our IT Expert team will find the best answer and will update on the portal. In case we find any new Java questions, we will update the same here.