The Bridge Pattern in Retrospect

Now that you've seen how the Bridge pattern works, it is worth looking at it from a more conceptual point of view. As shown in Figure 9-13, the pattern has an abstraction part (with its derivations) and an implementation part. When designing with the Bridge pattern, it is useful to keep these two parts in mind. The implementation's interface should be designed considering the different derivations of the abstract class that it will have to support. Note that a designer shouldn't necessarily put in an interface that will implement all possible derivations of the abstract class (yet another possible route to paralysis by analysis). Only those derivations that actually are being built need be supported. Time and time again, the authors have seen that the mere consideration of flexibility at this point often greatly improves a design.

Note: In C++, the Bridge pattern's implementation must be implemented with an abstract class defining the public interface. In Java, either an abstract class or an interface can be used. The choice depends upon whether implementations share common traits that abstract classes can take advantage of. See Peter Coad's Java Design, discussed on page 316 of the Bibliography, for more on this.