java-immutability-modelinglisted

# Skill: java-immutability-modeling ## Activation Contract Use this skill when modeling Java data with records, value objects, immutable classes, defensive copies, collection ownership, equality, and state validation. Do **not** use this skill for persistence mapping only, serialization-only configuration, generic DTO naming, or non-Java data modeling. ## Responsibility This skill teaches safe Java state modeling. It does not call other skills, impose immutability everywhere, or choose framework annotations unless context requires them. ## Required Context - Java version. - Whether the type is domain model, value object, DTO, event, command, or persistence entity. - Mutability requirements and ownership of collections. - Equality/hashCode expectations. ## Context Budget - Keep this `SKILL.md` focused on modeling decisions. - Use `references/java-immutability.md` for records and defensive-copy details. ## Hard Rules - Prefer immutable data for values, messages, events, and DTOs unless mutation is required. - Validate invariants at construction time. - Make defensive copies of mutable inputs and outputs when ownership is not transferred. - Do not expose mutable internals accidentally. - Use records for transparent immutable aggregates when they match the domain and Java version. - Avoid records for entities that require identity, lazy mutation, or framework lifecycle constraints unless explicitly supported. ## Decision Gates | Condition | Action | |---|---| | Data