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SKILL.md
npx skills add gentleman-programming/gentleman-skills --skill java-21
Java 21 language and runtime patterns for modern, safe code. Trigger: When writing Java 21 code using records, sealed types, or virtual threads.
npx skills add gentleman-programming/gentleman-skills --skill java-21
Load this skill when:
Use records for DTOs and value objects, validate in compact constructors.
Use sealed interfaces/classes and switch pattern matching for exhaustiveness.
Use virtual threads to handle blocking I/O without large thread pools.
| Task | Pattern |
|---|---|
| Immutable DTOs | Use records with validation |
| Closed hierarchies | Use sealed interfaces + switch |
| Blocking I/O scale | Use virtual threads executor |
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();
};
}
}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
}
}// BAD: mutable DTO
public class UserDto {
public String name;
public String email;
}// BAD: expensive and unbounded
new Thread(() -> blockingFetch("https://api" )).start();