Top 60+ Java 21 Interview Questions and Answers

Java 21 is a Long-Term Support (LTS) release packed with powerful features like virtual threads, record patterns, pattern matching in switch, and string templates. This blog provides a rich set of Java 21 interview questions curated for both freshers and experienced developers.

⚙️ Core Java 21 Concepts

1. What is new in Java 21?

Key features include virtual threads (Project Loom), pattern matching for switch, record patterns, string templates (preview), sequenced collections, scoped values, unnamed patterns and variables, and more.

2. What are virtual threads?

Virtual threads are lightweight threads managed by the JVM. They reduce the overhead of traditional threads and allow millions of concurrent tasks.

Thread.startVirtualThread(() -> System.out.println("Hello from virtual thread"));
3. Difference between virtual thread and platform thread?
  • Platform threads are mapped to OS threads; virtual threads are not.
  • Virtual threads scale better and use fewer system resources.
4. What is the purpose of ScopedValue?

ScopedValue is a safer, faster alternative to ThreadLocal for passing immutable context data in concurrent programming.

5. How do record patterns simplify code?
record Person(String name, int age) {}

if (obj instanceof Person(String name, int age)) {
    System.out.println(name + " is " + age);
}
6. What are sequenced collections?

A new collection interface that provides consistent access to elements in both forward and reverse directions using getFirst(), getLast(), and reversed().

7. Explain pattern matching in switch with example.
switch (obj) {
    case String s -> System.out.println("String: " + s);
    case Integer i -> System.out.println("Integer: " + i);
}
8. How to use string templates in Java 21?
import static java.util.FormatProcessor.FMT;
String name = "Java";
System.out.println(FMT."Hello, \{name}!");
Note: String templates are a preview feature in Java 21.
9. What is unnamed pattern or variable?
if (obj instanceof Person(String _, int age)) {
    System.out.println("Age: " + age);
}

Use underscore for unused components in a pattern.

🧠 Advanced and Scenario-based Questions

10. Can we replace ThreadLocal with ScopedValue?

Yes, in most cases. ScopedValue is immutable and designed to work well with virtual threads.

11. What happens if you start too many platform threads?

You’ll run into OS limitations or performance degradation. Virtual threads solve this issue.

12. Is Java 21 backward compatible with Java 17?

Yes, most Java 17 code will work, but preview/incubating features need compilation flags.

13. Can we use yield with virtual threads?

Yes, Thread.yield() can be used to hint the scheduler in concurrent systems.

14. How do you test code using preview features?
javac --enable-preview --release 21 MyApp.java
java --enable-preview MyApp
15. Is Java 21 production-ready?

Yes. Being an LTS release, it's ideal for production with long-term support.

📦 Collections, Threads & Functional Questions

16. What is the difference between List.of() and new ArrayList()?

List.of() returns an immutable list. new ArrayList() is mutable.

17. What does list.reversed() return in Java 21?

A reversed view of the list using SequencedCollection.

18. Can you use streams with virtual threads?

Yes, but it's often better to use structured concurrency with virtual threads for blocking tasks.

19. How do you debug virtual threads?

Use new JDK tooling like JFR (Java Flight Recorder) and updated thread dumps.

20. Are virtual threads suitable for CPU-bound tasks?

Not particularly. They shine with I/O-bound, high-concurrency workloads.

📋 Miscellaneous Interview Questions

- 21. Can record patterns be nested? - 22. Is Java 21 supported in Spring Boot 3.x? - 23. Can you run 1 million virtual threads on 4 CPU cores? - 24. Explain the lifecycle of a virtual thread. - 25. Are virtual threads garbage collected? - 26. How are string templates better than string concatenation? - 27. How do you enable preview features in Maven or Gradle? - 28. What replaced the deprecated Security Manager? - 29. What are the new APIs in java.time for Java 21? - 30. What is the status of Project Panama and Project Valhalla? - 31. How can you use record patterns with lists or maps? - 32. What’s the difference between var and unnamed variables? - 33. Explain structured concurrency in Java 21. - 34. What are some JVM improvements in Java 21? - 35. What are incubator modules? - 36. How does Java 21 improve observability? - 37. Are preview features stable in production? - 38. What are caveats with using string templates? - 39. How can Java 21 be used with GraalVM? - 40. Why is Java moving towards preview-first features?

💡 Java 21 vs Java 17 Comparison Questions

- 41. What are the major differences between Java 21 and 17? - 42. Why should teams upgrade to Java 21? - 43. What LTS support does Java 21 offer? - 44. Does Java 21 offer performance improvements over Java 17? - 45. What projects in Java 21 were in preview in Java 17?

👨‍💻 Code Output & Prediction Questions

46. What is the output of the following?
Thread t = Thread.startVirtualThread(() -> System.out.println("Hi"));
t.join();
47. Will the following compile?
record Employee(String name, int id) {}
Object o = new Employee("Alice", 101);
if (o instanceof Employee(String name, int _)) {
    System.out.println(name);
}
48. Predict output:
List<String> list = List.of("a", "b", "c");
System.out.println(list.reversed().get(0));

✅ Conclusion

Java 21 is a game-changing release for developers. Whether you're preparing for interviews or adopting Java 21 in your project, understanding these features will make you more effective and productive.

Stay tuned: Upcoming blog → Java 21 Real-world Project Ideas!