java

Java Patterns

Reference for modern Java patterns, type system, and best practices.

Iron Law

FAVOR COMPOSITION OVER INHERITANCE

Delegation and interfaces over class hierarchies. Inheritance creates tight coupling, breaks encapsulation, and makes refactoring dangerous. Use interfaces for polymorphism, records for data, and sealed classes for restricted hierarchies. Extend only when the "is-a" relationship is genuinely invariant. — Effective Java, Item 18 [1, Item 18]

When This Skill Activates

• Working with Java codebases (records, sealed classes, Optional, streams)
• Implementing concurrent code or API design
• Structuring Java packages and dependencies

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Type System (Modern Java)

Records for data [4][9] — Immutable carriers with auto-generated equals/hashCode/ toString. Compact constructors validate before fields are sealed.

// BAD: Mutable POJO. GOOD: Immutable record [1, Item 17][4]
public record User(String name, String email, Instant createdAt) {
    public User {
        Objects.requireNonNull(name); Objects.requireNonNull(email);
    }
}

Sealed classes [5][9] — Close the subtype set, enabling exhaustive pattern matching. [6]

public sealed interface Result<T> permits Success, Failure {
    record Success<T>(T value) implements Result<T> {}
    record Failure<T>(String error) implements Result<T> {}
}
// Exhaustive — no default needed [6]
switch (result) {
    case Success<User> s -> handleSuccess(s.value());
    case Failure<User> f -> handleError(f.error());
}

Optional for absent values [1, Item 22][25] — Never return null. Use functional chaining (map/orElse), not isPresent()/get().

public Optional<User> findById(String id) { return Optional.ofNullable(userMap.get(id)); }
optional.map(User::name).orElse("Anonymous");

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Error Handling

All AutoCloseable resources MUST use try-with-resources. [1] Custom unchecked exceptions carry structured fields for precise handling. [3]