Java 21 Interview Preparation: 8 Topics Every Senior Java Developer Should Know

 Java 21 Interview Preparation: 8 Topics Every Senior Java Developer Should Know


Java 21 introduced important features that interviewers may explore through coding questions, internal workings, and real-world scenarios.

If you have 8+ years of Java experience, prepare these topics:

1. Virtual Threads

* Platform threads vs virtual threads
* How does Project Loom work?
* When should you use virtual threads?
* What are thread pinning and its implications?

2. Structured Concurrency (Preview in Java 21)

* What problem does structured concurrency solve?
* How do you manage multiple concurrent tasks?
* How should task cancellation and failures be handled?

3. Pattern Matching for switch

* How is it different from traditional switch?
* How do type patterns and guarded patterns work?
* What is exhaustive pattern matching?

4. Record Patterns

* How do record patterns simplify data extraction?
* How do nested record patterns work?
* How can they be combined with pattern matching for switch?

5. Sequenced Collections

* What are SequencedCollection, SequencedSet, and SequencedMap?
* How do getFirst(), getLast(), and reversed() work?
* What problem do these interfaces solve?

6. Generational ZGC (Experimental in Java 21)

* What is the difference between generational and non-generational ZGC?
* How can generational garbage collection help with allocation-heavy workloads?
* What trade-offs matter when tuning garbage collection?

7. Key Encapsulation Mechanism (KEM) API (Preview in Java 21)

* What is a key encapsulation mechanism?
* How does it differ from traditional key agreement?
* Where does it fit into secure key establishment?

8. Performance & Migration

* How would you migrate an application from Java 17 to Java 21?
* How would you benchmark virtual threads?
* How would you investigate increased latency or memory usage after migration?

My recommendation: Start with Virtual Threads, Pattern Matching, Record Patterns, and Sequenced Collections. Then explore preview features and JVM performance.

One important interview tip: Java 21 features aren’t all final features. Structured Concurrency and Generational ZGC were preview and experimental, respectively, in Java 21. Always distinguish the Java 21 release status from what became final in later releases.

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