java

name	java
description	Use when: write or review modern, idiomatic Java with immutability, clear types, and clean structure.

Goal: modern Java that is immutable by default, null-safe, and testable.

Use for:

• new classes, services, or APIs in Java
• reviewing structure, immutability, and error handling
• modernizing verbose or legacy patterns

Workflow:

1. Model immutable data with records and final fields.
2. Use Optional for absence; avoid returning null.
3. Prefer the Streams API for transformations over manual loops.
4. Throw specific exceptions; never swallow them silently.
5. Manage resources with try-with-resources.
6. Verify with JUnit, a static analyzer, and the formatter.

Idioms:

• records for immutable value types
• Optional return types instead of null
• switch expressions and pattern matching
• var for obvious local types
• dependency injection over static singletons

Rules:

• do not return or pass null; use Optional or explicit checks
• close resources with try-with-resources, not manual finally
• keep classes focused; favor composition over inheritance
• program to interfaces for testability