gof-decorator

One-line summary

Wrap an object in another object that shares its interface and adds responsibility — at runtime, in any combination, without modifying the original.

When to use this skill

• A class needs N orthogonal extensions (a coffee gets milk, sugar, foam, syrup … any combination). Subclassing yields N! variants; Decorator yields N small wrappers.
• Java I/O Streams (BufferedReader(InputStreamReader(FileInputStream(...)))) is the textbook real-world example.
• Adding cross-cutting concerns (logging, caching, validation, rate-limiting) around an existing object without touching its code.

When NOT to use this skill

• The extensions interact non-orthogonally (sugar must be added before milk, etc.) — Decorator gives unordered composition; if order matters, you have hidden coupling.
• Only one extension exists and no second is in sight — subclassing or a flag is simpler.
• The wrapping changes the interface, not just behavior — that's gof-adapter.

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