The Bridge

1
The Bridge

• A Structural Design Pattern
• Presented by Quddus Chong
• CS590L Winter 2002

2
The Problem

• Many implementations for one abstraction.
• High coupling - lacks flexibility.
• Inheritance property could be misused to
  specialize subclasses for different
  implementations - This leads to code duplication,
  platform dependence, and less manageable
  hierarchies.

Window
Window
Swing_ Window
MFC_ Window
IconWindow
Swing_ Window
MFC_ Window
Swing_ IconWindow
MFC_ IconWindow
3
The Solution

• Separate the abstraction class hierarchy from the
  implementation class hierarchy and use delegation
  as a bridge between the two.

• Abstraction defines the abstractions interface
• RefinedAbstraction extends the interface
  defined by Abstraction
• Implementor defines the interface for
  implementation classes can be different from
  Abstraction interface. Typically, Implementor
  provides only primitive operations, and
  Abstraction defines higher-level operations based
  on these primitives.
• ConcreteImplementor implements the Implementor
  interface and defines its concrete implementation.

4
The Solution (contd.)

• Consequences include
• Decoupling interface and implementation
• Improved extensibility
• Hiding implementation details from clients

bridge
Window
WindowImp
DrawText() DrawRect()
DevDrawText() DevDrawLine()
imp-gtDevDrawLine() imp-gtDevDrawLine() imp-gtDevDra
wLine() imp-gtDevDrawLine()
IconWindow
TransientWindow
Swing_ WindowImp
MFC_WindowImp
DrawBorder()
DrawCloseBox()
DevDrawText() DevDrawLine()
DevDrawText() DevDrawLine()
DrawRect() DrawText()
DrawRect()
Swing_ DrawLine()
Swing_ DrawString()
5
Applicability and Implementation

• Use the bridge when
• You want to avoid a permanent binding between an
  abstraction and its implementation.
• Both the abstractions and their implementations
  should be extensible by subclassing.
• Changes in the implementation of an abstraction
  should have no impact on clients (i.e. code
  should not have to be recompiled).
• You want to share an implementation among
  multiple objects, and want to hide this from the
  client.
• Some implementation issues
• Only one Implementor an abstract Implementor is
  not necessary when there is only one
  implemetation.
• Creating the right Implementor object - how to
  decide which Implementor to instantiate?
• Can be instantiated in the constructor for
  Abstraction choose a default implementation and
  change it later according to usage.
• A better approach Delegate the decision making
  to a Abstract Factory object.

6
Example J2EE Data Access Object (DAO)

• see http//java.sun.com/blueprints/patterns/j2ee
  _patterns/data_access_object/index.html
• In the example, Java PetStore application
  OrderEJB component uses an associated OrderDAO
  class to access a specific database

• At deployment time, the application
  administrator configures the implementation of
  OrderDAO to be one of OrderDAOCS, OrderDAOOracle,
  or OrderDAOSybase, depending on whether an
  Cloudscape, Oracle, or Sybase database is used.