A community-driven Java interview preparation guide covering Core Java, OOP, Collections, Concurrency, Modern Java, JVM Internals, JDBC, Testing, Design Patterns, and more.
If you're preparing for a Java software engineering internship, graduate role, junior developer position, or Java-focused technical interview, this repository is designed to be a structured study reference.
The original guide contains 150 interview questions and answers across 20 topic areas.
| Topic | Questions |
|---|---|
| 1. Core Java Basics | 8 |
| 2. OOP Concepts | 11 |
| 3. Data Types, Variables & Operators | 7 |
| 4. Strings | 5 |
| 5. Exception Handling | 9 |
| 6. Collections Framework | 14 |
| 7. Generics | 5 |
| 8. Multithreading & Concurrency | 16 |
| 9. Java 8+ Features | 12 |
| 10. JVM Internals & Memory Management | 8 |
| 11. Garbage Collection | 6 |
| 12. I/O and NIO | 4 |
| 13. Serialization | 4 |
| 14. Reflection & Annotations | 4 |
| 15. Design Patterns in Java | 6 |
| 16. JDBC & Database Access | 5 |
| 17. Modules (JPMS) | 2 |
| 18. Modern Java Features (9–21+) | 7 |
| 19. Testing (JUnit & Mockito) | 5 |
| 20. Best Practices & Common Pitfalls | 12 |
Total: 150 questions
Core Java → OOP → Data Types → Strings → Exception Handling → Collections
Generics → Concurrency → Java 8+ → I/O & NIO → Serialization → JDBC → Design Patterns
JVM Internals → Garbage Collection → Reflection & Annotations → JPMS → Modern Java → Testing → Best Practices
For each question:
- Try to answer it before revealing the explanation.
- Compare your answer with the guide.
- Write a small example when possible.
- Explain the concept in your own words.
- Practice related coding problems.
- Core Java Basics — 8 questions
- OOP Concepts — 11 questions
- Data Types, Variables & Operators — 7 questions
- Strings — 5 questions
- Exception Handling — 9 questions
- Collections Framework — 14 questions
- Generics — 5 questions
- Multithreading & Concurrency — 16 questions
- Java 8+ Features — 12 questions
- JVM Internals & Memory Management — 8 questions
- Garbage Collection — 6 questions
- I/O and NIO — 4 questions
- Serialization — 4 questions
- Reflection & Annotations — 4 questions
- Design Patterns in Java — 6 questions
- JDBC & Database Access — 5 questions
- Modules (JPMS) — 2 questions
- Modern Java Features (9–21+) — 7 questions
- Testing (JUnit & Mockito) — 5 questions
- Best Practices & Common Pitfalls — 12 questions
Found a mistake? Have a clearer explanation? Want to add a question, quiz, coding problem, or resource?
See CONTRIBUTING.md.
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See ROADMAP.md for planned additions.
This project is licensed under the MIT License. See LICENSE.