Question 1

What are the key features introduced in Java 21 LTS?

Accepted Answer

Java 21 LTS introduces Virtual Threads for lightweight concurrency, Record Patterns and Pattern Matching for switch for cleaner code, Sequenced Collections for predictable ordering, and the Vector API for SIMD operations. It also includes String Templates (preview) and Scoped Values as a safer alternative to ThreadLocal.

Question 2

What are virtual threads in Java 21?

Accepted Answer

Virtual threads are lightweight threads managed by the JVM rather than the operating system, enabling millions of concurrent threads with minimal resource overhead. They solve the scalability limitations of traditional platform threads in high-throughput server applications by being much cheaper to create and maintain.

Question 3

How do virtual threads differ from platform threads?

Accepted Answer

Platform threads are thin wrappers around OS threads with high memory overhead (1-2 MB each), while virtual threads are JVM-managed and lightweight (few KB each). Virtual threads share a small pool of carrier threads, enabling millions of concurrent threads without proportional resource consumption.

Question 4

What are record patterns in Java 21?

Accepted Answer

Record patterns allow you to destructure record instances directly in pattern matching contexts. Instead of calling accessor methods, you can extract record components in a single declarative operation, making code more concise and readable when working with complex data structures.

Question 5

What is pattern matching for switch in Java 21?

Accepted Answer

Pattern matching for switch extends switch statements and expressions to allow patterns instead of just constants in case labels. This enables type testing, deconstruction, and more expressive conditional logic directly within switch constructs, eliminating cascading if-else statements with instanceof checks.

Question 6

What are guarded patterns in Java 21 switch statements?

Accepted Answer

Guarded patterns allow you to add boolean conditions to case labels using the when keyword. They enable more refined matching by combining type patterns with additional predicates, letting you specify both type and value constraints in a single case label without nested if statements.

Question 7

What are sequenced collections in Java 21?

Accepted Answer

Sequenced collections are new interfaces (SequencedCollection, SequencedSet, SequencedMap) that provide a uniform API for accessing elements in collections with defined encounter order. They introduce methods like getFirst(), getLast(), and reversed() to consistently work with both ends of ordered collections.

Question 8

What are string templates in Java 21?

Accepted Answer

String templates are a preview feature that provide a safer and more readable way to compose strings from literal text and embedded expressions. They solve problems of error-prone string concatenation and security vulnerabilities like SQL injection by making string composition more explicit and validatable.

Question 9

How do you use the STR template processor in Java 21?

Accepted Answer

The STR template processor is used by prefixing a string template with STR. followed by the template expression. Expressions are embedded using \{expression} syntax, and STR evaluates them and interpolates the results into the string at runtime.

Question 10

What is structured concurrency in Java 21?

Accepted Answer

Structured concurrency is a preview feature that treats multiple concurrent tasks as a single unit of work, ensuring all subtasks complete before the main task finishes. It simplifies error handling and cancellation by treating concurrent operations as a structured block with a clear lifecycle.

Question 11

How do you use StructuredTaskScope in Java 21?

Accepted Answer

You use StructuredTaskScope by creating a scope with try-with-resources, forking subtasks using the fork() method, calling join() to wait for completion, and then retrieving results from the returned Subtask objects. The scope automatically manages task lifecycle and ensures all subtasks complete before closing.

Question 12

What are scoped values in Java 21?

Accepted Answer

Scoped values are a preview feature that provide a safer and more efficient alternative to ThreadLocal for sharing immutable data within and across threads. They allow data to be bound to a specific scope, making it accessible to all methods called within that scope without explicit parameter passing.

Question 13

How do scoped values differ from ThreadLocal?

Accepted Answer

Scoped values are immutable, rebindable within nested scopes, and automatically cleaned up when their scope ends, whereas ThreadLocal variables are mutable and require explicit cleanup. Scoped values also have significantly better performance characteristics, especially with virtual threads.

Question 14

What is Generational ZGC in Java 21?

Accepted Answer

Generational ZGC is an enhancement to the Z Garbage Collector that separates the heap into young and old generations while maintaining ZGC's low-latency characteristics. Unlike classic ZGC, it focuses collection efforts on young objects that typically die quickly, resulting in improved throughput and reduced memory overhead.

Question 15

How do you enable Generational ZGC in Java 21?

Accepted Answer

In Java 21, enable Generational ZGC using the -XX:+UseZGC -XX:+ZGenerational JVM flags. Since it's a preview feature in Java 21, you may also need -XX:+UnlockExperimentalVMOptions depending on your Java distribution.

Java 21 LTS Overview(4 questions)

What are the key features introduced in Java 21 LTS?

What is the difference between Java 21 and Java 17 LTS?

What preview features from Java 17-20 have been finalized in Java 21?

What are the main reasons to upgrade to Java 21?

Virtual Threads (Project Loom)(10 questions)

What are virtual threads and why were they introduced in Java 21?

How do virtual threads differ from platform (OS) threads?

How do you create a virtual thread using Thread.ofVirtual()?

What is Executors.newVirtualThreadPerTaskExecutor() and when would you use it?

What is thread pinning and how do you identify it?

Why should you avoid synchronized blocks with virtual threads?

What is a carrier thread in the context of virtual threads?

How do you migrate an existing thread pool-based application to virtual threads?

When should you NOT use virtual threads?

How do virtual threads handle blocking I/O operations?

Record Patterns(4 questions)

What are record patterns in Java 21?

How do you deconstruct a record using pattern matching?

What is nested record pattern matching?

How do you use record patterns in switch expressions?

Pattern Matching for switch(5 questions)

What is pattern matching for switch in Java 21?

What are guarded patterns (when clauses) in switch?

How does exhaustiveness checking work with switch patterns?

How do you handle null cases in pattern matching switch?

How do pattern matching switch statements work with sealed classes?

Sequenced Collections(5 questions)

What are sequenced collections introduced in Java 21?

What is the SequencedCollection interface?

How do you use getFirst() and getLast() methods?

What is the reversed() method and how does it work?

Which existing collections implement SequencedCollection?

String Templates (Preview)(4 questions)

What are string templates and what problem do they solve?

How do you use the STR template processor?

How do string templates prevent injection attacks?

What is the difference between string templates and string concatenation?

Generational ZGC(4 questions)

What is Generational ZGC and how does it differ from classic ZGC?

How do you enable Generational ZGC in Java 21?

What are the performance benefits of Generational ZGC?

When should you choose Generational ZGC over G1GC?

Structured Concurrency (Preview)(5 questions)

What is structured concurrency in Java 21?

How do you use StructuredTaskScope?

What is ShutdownOnSuccess policy?

What is ShutdownOnFailure policy?

What is the relationship between structured concurrency and virtual threads?

Scoped Values (Preview)(4 questions)

What are scoped values in Java 21?

How do scoped values differ from ThreadLocal?

How do scoped values work with virtual threads?

What is the performance advantage of scoped values?

Foreign Function and Memory API(4 questions)

What is the Foreign Function and Memory API?

How do you allocate native memory using Arena?

How do you call native functions using Linker?

What are the different Arena types (confined, shared, auto, global)?

Best Practices and Migration(4 questions)

What are the best practices for writing idiomatic Java 21 code?

What is the recommended approach for concurrent programming in Java 21?

What are common migration pitfalls from Java 11/17 to Java 21?

What are the best practices for using virtual threads in web applications?