Design Patterns discussion

[Michael Schuerig <schuerig AT acm.org>]

On Monday 19 July 2004 06:40, Devaiah Doni wrote:

> i could not find the difference in this two patterns..
> only definition is changed..but in the implementation both looks
> same. 

Adapter wraps another interface around an already existing object. The 
adapter itself is concrete.

Bridge, on the other hand, allows for different realizations of an 
abstraction. The main point here is *not* that you have an interface 
and various implementing classes. Rather, what you have is an abstract 
base class (GOF: Window) and various derived classes; the left-hand 
side in the GOF class diagram for Bridge. The point, then, is that the 
methods implemented in the abstract base class are realized by 
delegation to another object, the Implementor (GOF: WindowImp). The 
Implementor, in turn, is an interface with possibly several concrete 
implementing classes.

In the GOF example, there are two derived window classes, IconWindow and 
TransientWindow. These concrete classes extend the abstract base class 
Window and use the provided concrete methods. How these concrete 
methods are implemented is not fixed, rather it is defined by the 
concrete WindowImp object, which may even be dynamically exchanged.

In effect, Bridge is a way to exchange the implementation of a base 
class.

Michael

-- 
Michael Schuerig                   The Fifth Rider of the Apocalypse
mailto:michael AT schuerig.de
         is a programmer.
http://www.schuerig.de/michael/