Decorator

Wrap an object to add behavior without changing it

Decorator wraps an object in another object that shares its interface and adds behavior on top. Start with a base walk price; wrap it to add a GPS surcharge; wrap that to add insurance. Each layer adds a bit and forwards the rest — the base class never changes.

This is the structural, object-wrapping Decorator — not the same as Python's @decorator syntax (which is a function-wrapping cousin of the same idea).