Recap: Bridge Pattern

Bridge splits a class that varies two ways into two hierarchies, joined by a reference from one to the other.

• Intent. Decouple an abstraction from its implementation so that the two can vary independently.
• Participants. The abstraction and its refined variants, the implementor interface and its concrete implementors, and the client.
• The count. M variants times N variants as subclasses becomes M plus N classes, and a new variant on either side costs one class.
• In the wild. JDBC: code written against java.sql interfaces, with a vendor driver behind them chosen by the URL.
• The trap. A bridge with one implementor varies nothing; extract it when the second variant arrives.
• Against Adapter and Strategy. An adapter is added after the fact; a strategy is one algorithm, where a bridge's abstraction has variants of its own.