Iterator can traverse a Composite . Visitor can apply an operation over a Composite . Polymorphic Iterator s rely on Factory Method s to instantiate the appropriate Iterator subclass. Mediator and Observer are competing patterns. On the other hand, Mediator can leverage Observer for dynamically registering colleagues and communicating with them.
Mediator is similar to Facade in that it abstracts functionality of existing classes.
An Iterator can use a Memento to capture the state of an iteration. The Iterator stores the Memento internally. State is like Strategy except in its intent. State objects are often Singleton s. Strategy lets you change the guts of an object.