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Java Developer interview questions & mock practice

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What interview questions are asked for a Java Developer?

A Java Developer interview in 2026 runs across 4 rounds — core java round, multithreading & memory, spring / framework round, coding + design. Below are the most-asked Java Developer interview questions and a focused prep plan. Rehearse every answer out loud with the free timed practice below before the real one.

  • Core Java
  • OOP
  • Collections
  • Multithreading
  • Spring Boot
  • JPA / Hibernate
  • JVM

Free interview practice · Java Developer

Practise your Java Developer interview now — free

Step through the 12 most-asked Java Developer questions one at a time, under a timer, just like the real thing. Jot your answer, then reveal what a strong answer covers. No signup needed to practise.

Interview rounds

The Java Developer interview process

Core Java, OOP, collections, multithreading and Spring Boot — the Java interview that dominates services companies and enterprise backends in India.

1. Core Java round

OOP, collections, exceptions and the JVM.

2. Multithreading & memory

Threads, synchronisation, garbage collection.

3. Spring / framework round

Spring Boot, dependency injection, REST and JPA.

4. Coding + design

DSA in Java plus basic low-level design.

Most-asked questions

Most-asked Java Developer interview questions

12 of the questions Java Developer candidates are asked most often in India. Practise answering each one out loud in the timed practice above.

  1. 1. What is the difference between an abstract class and an interface in Java?
  2. 2. Explain the four pillars of OOP with Java examples.
  3. 3. What is the difference between ArrayList and LinkedList?
  4. 4. Explain how HashMap works internally in Java.
  5. 5. What is the difference between == and .equals() in Java?
  6. 6. What is the difference between String, StringBuilder and StringBuffer?
  7. 7. Explain the Java memory model and how garbage collection works.
  8. 8. What is the difference between checked and unchecked exceptions?
  9. 9. How do you create a thread, and what is the difference between Runnable and Thread?
  10. 10. What is the difference between synchronized and volatile?
  11. 11. What is dependency injection in Spring, and why is it useful?
  12. 12. Explain the difference between @Component, @Service and @Repository in Spring.
How to prepare

How to prepare for your Java Developer interview

Master collections cold: HashMap internals, ArrayList vs LinkedList, and when to use which.

Know OOP and the 'why' behind interfaces vs abstract classes — these come up in almost every Java interview.

Be solid on multithreading: synchronization, volatile, the ExecutorService, and common concurrency pitfalls.

Learn Spring Boot fundamentals: DI, annotations, REST controllers, JPA/Hibernate and exception handling.

Understand the JVM basics: heap vs stack, garbage collection, and what causes OutOfMemoryError.

Practise other roles

Interview prep guides

Java Developer interview — FAQs

What questions are asked in a Java Developer interview?

Common Java Developer interview questions include: What is the difference between an abstract class and an interface in Java? Explain the four pillars of OOP with Java examples. What is the difference between ArrayList and LinkedList? Explain how HashMap works internally in Java. Interviews usually run across 4 rounds — Core Java round, Multithreading & memory, Spring / framework round, Coding + design. Practise them with the free timed practice on this page.

How many rounds are in a Java Developer interview?

A typical Java Developer interview has 4 rounds: Core Java round (OOP, collections, exceptions and the JVM.); Multithreading & memory (Threads, synchronisation, garbage collection.); Spring / framework round (Spring Boot, dependency injection, REST and JPA.); Coding + design (DSA in Java plus basic low-level design.).

How do I prepare for a Java Developer interview?

To prepare for a Java Developer interview: Master collections cold: HashMap internals, ArrayList vs LinkedList, and when to use which. Know OOP and the 'why' behind interfaces vs abstract classes — these come up in almost every Java interview. Be solid on multithreading: synchronization, volatile, the ExecutorService, and common concurrency pitfalls. Then rehearse every question out loud with the timed practice on this page before the real thing.

What skills do I need for a Java Developer role?

Core Java Developer skills tested in interviews include Core Java, OOP, Collections, Multithreading, Spring Boot, JPA / Hibernate, JVM. OnJob shows you exactly which of these skills stand between you and a 100% match on every live Java Developer job.

Is the Java Developer mock interview practice free?

Yes. The questions, the answer guidance and the timed practice on this page are free and need no signup. With a free OnJob account you can also take timed role tests and free certification tests. Personalised AI role and skill tests are part of the paid plans.

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