Prev Next

Java / Java 21 Interview Questions

What is Java 21 and why is it a significant release?

Java 21, released in September 2023, is a Long-Term Support (LTS) release of the Java platform, making it the next major production-grade baseline after Java 17. It is the first LTS release to deliver Virtual Threads as a stable feature (Project Loom), Record Patterns, Pattern Matching for switch, and Sequenced Collections — all finalised and ready for production use.

The LTS designation means Oracle and other JDK vendors commit to extended support windows (typically 5–8 years depending on the vendor), making Java 21 the version most enterprises will target for migration from Java 11 or 17.

Key Java 21 JEPs (Finalised)
JEPFeatureCategory
444Virtual ThreadsConcurrency (Loom)
441Pattern Matching for switchLanguage
440Record PatternsLanguage
431Sequenced CollectionsCore Libraries
439Generational ZGCGC
445Unnamed Classes and Instance Main Methods (Preview)Language
453Structured Concurrency (Preview)Concurrency
446Scoped Values (Preview)Concurrency

Java 21 also continues the deprecation of thread primitives like Thread.stop(), and permanently removes the Security Manager, Applet API, and older RMI features that were deprecated in prior releases.

What concurrency feature was finalised and became production-ready in Java 21?
What type of release is Java 21?

Invest now in Acorns!!! 🚀 Join Acorns and get your $5 bonus!

Invest now in Acorns!!! 🚀
Join Acorns and get your $5 bonus!

Earn passively and while sleeping

Acorns is a micro-investing app that automatically invests your "spare change" from daily purchases into diversified, expert-built portfolios of ETFs. It is designed for beginners, allowing you to start investing with as little as $5. The service automates saving and investing. Disclosure: I may receive a referral bonus.

Invest now!!! Get Free equity stock (US, UK only)!

Use Robinhood app to invest in stocks. It is safe and secure. Use the Referral link to claim your free stock when you sign up!.

The Robinhood app makes it easy to trade stocks, crypto and more.

Webull! Receive free stock by signing up using the link: Webull signup.

More Related questions...

What is Java 21 and why is it a significant release? What are Virtual Threads in Java 21 and how do they differ from Platform Threads? How does Pattern Matching for switch work in Java 21? What are Record Patterns in Java 21 and how do they enable deconstruction? What are Sequenced Collections in Java 21? What are sealed classes and interfaces in Java and why are they important for pattern matching? What are Java Records and what do they automatically generate? What are Text Blocks in Java and how do you use them? What is Structured Concurrency in Java 21 and what problem does it solve? What are Scoped Values in Java 21 and how do they differ from ThreadLocal? What is Generational ZGC in Java 21 and why does it improve upon the original ZGC? What important String methods were added from Java 11 through Java 21? What is 'var' in Java and what are its limitations? What are switch expressions in Java and how do they differ from switch statements? How does pattern matching for instanceof work in Java 16+? What Stream API improvements were introduced in Java 9 through Java 21? How has Optional been improved and how should it be used correctly? How does CompletableFuture work in Java and how does it relate to virtual threads? What does the 'volatile' keyword guarantee in Java's memory model? What are the immutable collection factory methods introduced in Java 9? What are functional interfaces in Java and how are lambdas related to them? What are the most important Collectors and how do you write custom ones? What are Unnamed Classes and Instance Main Methods in Java 21 (Preview)? What improvements were made to NullPointerException messages in Java 14? How does type erasure affect instanceof checks with generics in Java? What are the differences between 'synchronized' and ReentrantLock in Java? What are the key classes in the java.time package and when do you use each? What garbage collectors are available in Java 21 and how do you choose between them? What is the Java Platform Module System (JPMS) and when should you use it? What are String Templates in Java 21 (Preview) and how do they improve string interpolation? What are the contracts for equals(), hashCode(), and Comparable in Java? What are the best practices for exception handling in Java? Why is immutability important in Java and how do you implement it correctly? What are default and static methods in Java interfaces? How does HashMap work internally in Java? How do virtual threads compare to reactive programming (Project Reactor / RxJava)? What is the Java 11 HttpClient and how do you use it for HTTP requests? What capabilities do Java enums have beyond simple named constants? What are the java.util.concurrent.atomic classes and how do they work? How does Java Reflection work and what are its performance implications? What are Unnamed Patterns and Variables in Java 21 (Preview) and how do they reduce boilerplate? When should you use StructuredTaskScope instead of CompletableFuture in Java 21? What major APIs were removed between Java 17 and Java 21? What are the most important JVM flags for tuning Java 21 application performance? What are the key steps and pitfalls when migrating an application to Java 21? How do Spring 7 and Spring Boot 4 optimize thread usage compared to older blocking models?
Show more question and Answers...

Spring

Comments & Discussions

Interviews Questions

About Javapedia.net

Javapedia.net is for Java and J2EE developers, technologist and college students who prepare of interview. Also this site includes many practical examples.

This site is developed using J2EE technologies by Steve Antony, a senior Developer/lead at one of the logistics based company.

contact: javatutorials2016[at]gmail[dot]com

Kindly consider donating for maintaining this website. Thanks.

Copyright © 2026, javapedia.net, all rights reserved. privacy policy.

DMCA.com Protection Status