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 --- ## Type System (Modern Java) **Records for data** [4][9] — Immutable carriers with auto-generated `equals`/`hashCode`/ `toString`. Compact constructors validate before fields are sealed. ```java // 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] ```java 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...