Recap: Choosing a Behavioural Pattern

The behavioural patterns most often confused are told apart by a few questions about the code.

• Who decides the behaviour? The caller, by passing an object in, is Strategy; the object itself, switching as it runs, is State; a subclass, when it is written, is Template Method.
• Is it a request or an algorithm? A request to keep, queue or undo is Command; a way of doing a job now is Strategy.
• Does one object decide for the others? Yes is Mediator; each listener deciding for itself is Observer.
• Can an object in the line stop the call? Yes, and one handles it, is Chain of Responsibility; every object passing it on, adding a layer, is Decorator.