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

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

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

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

// BAD: mutable DTO public class UserDto { public String name; public String email; }

Don't: Spin up raw platform threads per request

// 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