Adapter Pattern Structure: The Class Diagram

Adapter has four participants: the client, the target interface the client uses, the adaptee with the interface that does not fit, and the adapter that joins them. This card reads each role off the Gang of Four's diagram for the object adapter, then compares it with the class adapter, the other form the book gives.

The diagram shows the object adapter: the adapter holds a reference to an adaptee, the association arrow, and passes calls to it. The class adapter instead inherits from the adaptee and implements the target, so it is the adaptee with an extra interface. In Java that is class Adapter extends Adaptee implements Target, and in Python multiple inheritance does the same.

The object adapter is the form used almost everywhere. It adapts any subclass of the adaptee, because it holds whatever object it is given, and it exposes only the target's methods, so the client cannot reach past the translation. The class adapter saves one reference and can override the adaptee's methods, but it ties the adapter to one concrete class and shows the adaptee's whole interface alongside the target's.