design-pattern-applylisted

# Design Pattern Application A skill that **judges the value of applying a pattern** and prescribes it, grounded in GoF *Design Patterns*, *Head First Design Patterns*, and Spring DI principles. For grounding facts see this skill's `reference/kb/` (creational · structural · behavioral patterns, DI/Strategy/Factory); for principles see `reference/principles.md`. The index is `reference/kb/INDEX.md` (if you can't find the path, search with Glob). > **Core**: A pattern is not a goal but a prescription. Apply it **only when the problem (an axis > of change/extension) actually exists**, and state the extension benefit against the cost of > indirection. If a simpler prescription (Extract Function, sealed `when`) suffices, don't add the > pattern (YAGNI). ## Judgment order (proceed exactly like this) 1. **Confirm the axis of change** — identify "what changes often" from past change history / upcoming cases. If none, hold off. 2. **Examine simpler alternatives first** — is Extract Function / sealed `when` / polymorphism enough? 3. **Pattern matching** — if a real axis of change matches the signals below, prescribe that pattern. 4. **State the trade-off** — more classes · reduced traceability vs. extensibility. Does the benefit justify the cost? 5. **Apply in small steps** — introduce interface → Move implementation → delegate from call sites, testing at every step. ## Signal → pattern | Signal | Pattern | |---|---| | Policy/algorithm scattered across branches wit