Bridge Design Pattern - Structural Pattern

1. Design Pattern – Bridge
(Structural)
• INTENT
» DECOUPLE AN ABSTRACTION FROM ITS
IMPLEMENTATION SO THAT THE TWO CAN VARY
INDEPENDENTLY
» SEPARATE THE ABSTRACTION AND ITS
IMPLEMENTATION AND HAVE SEPARATE
INHERITANCE STRUCTURE FOR BOTH.
• Also known as
» Handle / Body
by
Shahriar Hyder
Oct 05, 2011

2. Motivation
 Write a program that will draw rectangles with either
of two drawing programs:
 drawing program 1 (DP1)
 drawing program 2 (DP2)

3. Example
DP 1 DP 2
draw a line draw_a_line( x1, y1, x2, y2) drawline( x1, x2, y1, y2)
draw a circle draw_a_circle( x, y, r) drawcircle( x, y, r)
The collection (the client of rectangles) doesn’t want to worry
about what type of drawing program it should use.

5. Cont..
 new requirement:
asked to support a new shape - circle
adding another level, called Shape, which
derives the Rectangle and Circle class.

7. new problems arise …
 if I get another drawing program – DP3
• I will have 6 different kinds of Shapes (two Shape concepts
times three drawing programs).
 if I get another type of Shape
• I will have nine different types of Shapes (three Shape
concepts times three drawing programs).
class
DP:Drawing Program S:Shapes
Need ≈DP*S new classes explosion!
Want ≈DP+S new classes
The new classes are hard wired for each type of Drawing Programs

8. Problem
 “Hardening of the software arteries” has occurred by
using sub-classing of an abstract base class to
provide alternative implementations. This locks in
compile-time binding between interface and
implementation. The abstraction and
implementation cannot be independently extended
or composed.

9. Try another alternate hierarchy
redundancy still there !

10. Step 1. Identify what is varying

11. Step 2. Represent the variations

12. Step3. Tie the class together

13. Step4. Expanding the design

14. Bridge – Structure

15. Bridge – Participants
• Abstraction
» Defines the abstractions interface
» Maintains a reference to an object of
type implementor
• RefinedAbstraction
» Extends the interface defined by
Abstraction

16. Bridge – Participants – 2
• Implementer
» Defines the interface for implementation classes
> Can be different from the Abstraction interface
– Implementer provides primitive operations
– Abstraction provides higher-level operations
• ConcreteImplementer
» Implements the Implementer interface
» Defines its concrete implementation

17. Demo Time!

18. Bridge – Applicability
• The normal method of dealing with an abstraction having
several implementations is through inheritance.
• Avoid permanent binding between an abstraction and its
implementation
» Especially if selection or switching of implementation is at
run-time rather than design time.
• Both abstractions and implementations should be extensible
by sub-classing
• Changes in implementation should have no impact on clients
• Share an implementation among multi objects and this fact
should be hidden from the client
• You have a proliferation of classes resulting from a coupled
interface and numerous implementations
• You need to map orthogonal class hierarchies.

19. Bridge – Applicability Summary
The Bridge pattern is useful when you have an
abstraction that has different implementations. It
allows the abstraction and the implementation to
vary independently of each other.
• Find what varies and encapsulate it.
• Favor composition over inheritance.

20. Collaborations
 Abstraction forwards client requests to its
Implementor object.

21. Bridge – Consequences
• Decouples interface and implementation
» Can configure implementation to use at runtime
» Encourages better structure through layering
> The client only has to know about Abstraction
and Implementer
• Improved extensibility
» Extend in Abstraction and Implementer
independently
• Hide implementation details from clients

22. Implementation
 Ditto from GoF Please!

23. Bridge – Related Patterns
• Abstract Factory can create and configure a particular Bridge
• Adapter is geared toward making unrelated classes work together
» Usually applied after systems are designed
» Bridge is used during design
• Structural difference: Bridge can abstract a complex
entity from its implementation; Adapter only abstracts a
single interface
• A bridge is by design. An adaptor is not. An adaptor is
a patch. A bridge is put in place on purpose.

24. Thank You