java-21

## When to Use Load this skill when: - Writing Java 21 application or library code - Designing immutable DTOs or value objects - Modeling closed hierarchies with sealed types - Using virtual threads for blocking I/O ## Critical Patterns ### Pattern 1: Records for immutable data Use records for DTOs and value objects, validate in compact constructors. ### Pattern 2: Sealed types + pattern matching Use sealed interfaces/classes and switch pattern matching for exhaustiveness. ### Pattern 3: Virtual threads for I/O Use virtual threads to handle blocking I/O without large thread pools. ## Code Examples ### Example 1: Record with validation ```java package com.acme.user; public record Email(String value) { public Email { if (value == null || !value.contains("@")) { throw new IllegalArgumentException("Invalid email"); } } } ``` ### Example 2: Sealed hierarchy + switch pattern matching ```java package com.acme.payment; public sealed interface Payment permits Card, BankTransfer { } public record Card(String last4) implements Payment { } public record BankTransfer(String iban) implements Payment { } public final class PaymentPrinter { public String describe(Payment payment) { return switch (payment) { case Card card -> "card-" + card.last4(); case BankTransfer bank -> "iban-" + bank.iban(); }; } } ``` ### Example 3: Virtual threads for blocking calls ```java package com.acme.io; import java.util.concurrent.Executors; public final class Fetcher { public void fetchAll(java.util.List<String> urls) throws Exception { try (var executor = Executors.newVirtualThreadPerTaskExecutor()) { for (String url : urls) { executor.submit(() -> blockingFetch(url)); } } } private void blockingFetch(String url) { // perform blocking I/O here } } ``` ## Anti-Patterns ### Don't: Use mutable data carriers ```java // BAD: mutable DTO public class UserDto { public String name; public String email; } ``` ### Don't: Spin up raw platform threads per request ```java // BAD: expensive and unbounded new Thread(() -> blockingFetch("https://api" )).start(); ``` ## Quick Reference | Task | Pattern | |------|---------| | Immutable DTOs | Use records with validation | | Closed hierarchies | Use sealed interfaces + switch | | Blocking I/O scale | Use virtual threads executor | ## Resources - Java 21 Release Notes: https://www.oracle.com/java/technologies/javase/21-relnote-issues.html - JEP Index: https://openjdk.org/jeps/0