Bridge

The Bridge pattern is intended to decouple an abstraction from its implementation so both can vary independently. The Bridge pattern is also know as Handle/Body.

UML Structure

Here is an example of the Bridge pattern.

Source: Downloaded from http://www.dofactory.com/Patterns/PatternBridge.aspx

Here is another example of the Bridge pattern where it decouples the abstraction from its implementation so that the two can vary independently
Source: Downloaded from http://home.earthlink.net/~huston2/dp/all_uml.html

Applicability

The Bridge pattern is applicable when:

we 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

we want to hide the implementation of an abstraction completely from clients

we want to share an implementation among multiple objects, and this fact should be hiddent from the client

Participants

The classes and/or objects participating in this pattern are:

• Abstraction
• defines the abstraction's interface
• maintains a reference to an object of type Implementor
• Refined Abstraction
• extends the interface defined by Abstraction.
• Implementor
• defines the interface for implementation classes. This interface does not have to correspond exactly to Abstraction's interface; in fact the two interfaces can be quite different. The Implementation interface provides only primitive operations, and Abstraction defines high-level operations based on these primitives.
• Concrete Implementor
• implements the Implementor interface and defines its concrete implementation.

Consequences

The Bridge pattern has the following consequence:

Decoupling interface and implementation - inheritance tightly couples an abstraction with an implementation at compile time. Bridge pattern can be used to avoid the binding between abstraction and implementation and to select implenetation at run time

Improved extensibility - extend the Abstraction and Implementor hierarchies independently

Hiding implementation detail from clients - shield clients from implentation details, like the sharing of implemntor objects and accompanying reference count merchanism

Interface and implementation can be varied independently - maintaining two different class hierarchies for interface and implementation entitles to vary one independent of the other

Lossely coupled client code - Abstraction separates the client code from the implementation

Reduction in the number of sub classes

Cleaner code and Reduction in executable size

Implementation

When applying the Bridge pattern, consider the following implementation issues:

Only one Implementor

Creating the right Implementor object

Sharing implementors

Using multiple inheritance

Code Examples

Real-world Sample code in C#

This real-world code demonstrates the Bridge pattern in which a BusinessObject abstraction is decoupled from the implementation in DataObject. The DataObject implementations can evolve dynamically without changing any clients.

// Bridge pattern -- Real World example  

using System;
using System.Collections;

namespace DoFactory.GangOfFour.Bridge.RealWorld
{

  // MainApp test application 
  
  class MainApp
  {
    static void Main()
    {
      // Create RefinedAbstraction 
      Customers customers = 
        new Customers("Chicago");

      // Set ConcreteImplementor 
      customers.Data = new CustomersData();

      // Exercise the bridge 
      customers.Show();
      customers.Next();
      customers.Show();
      customers.Next();
      customers.Show();
      customers.New("Henry Velasquez");

      customers.ShowAll();

      // Wait for user 
      Console.Read();
    }
  }

  // "Abstraction" 

  class CustomersBase
  {
    private DataObject dataObject;
    protected string group;

    public CustomersBase(string group)
    {
      this.group = group;
    }

    // Property 
    public DataObject Data
    {
      set{ dataObject = value; }
      get{ return dataObject; }
    }

    public virtual void Next()
    {
      dataObject.NextRecord();
    }

    public virtual void Prior()
    {
      dataObject.PriorRecord();
    }

    public virtual void New(string name)
    {
      dataObject.NewRecord(name);
    }

    public virtual void Delete(string name)
    {
      dataObject.DeleteRecord(name);
    }

    public virtual void Show()
    {
      dataObject.ShowRecord();
    }

    public virtual void ShowAll()
    {
      Console.WriteLine("Customer Group: " + group);
      dataObject.ShowAllRecords();
    }
  }

  // "RefinedAbstraction" 

  class Customers : CustomersBase
  {
    // Constructor 
    public Customers(string group) : base(group)
    {  
    }

    public override void ShowAll()
    {
      // Add separator lines 
      Console.WriteLine();
      Console.WriteLine ("------------------------");
      base.ShowAll();
      Console.WriteLine ("------------------------");
    }
  }

  // "Implementor" 

  abstract class DataObject
  {
    public abstract void NextRecord();
    public abstract void PriorRecord();
    public abstract void NewRecord(string name);
    public abstract void DeleteRecord(string name);
    public abstract void ShowRecord();
    public abstract void ShowAllRecords();
  }

  // "ConcreteImplementor" 

  class CustomersData : DataObject
  {
    private ArrayList customers = new ArrayList();
    private int current = 0;

    public CustomersData() 
    {
      // Loaded from a database 
      customers.Add("Jim Jones");
      customers.Add("Samual Jackson");
      customers.Add("Allen Good");
      customers.Add("Ann Stills");
      customers.Add("Lisa Giolani");
    }

    public override void NextRecord()
    {
      if (current <= customers.Count - 1)
      {
        current++;
      }
    }

    public override void PriorRecord()
    {
      if (current > 0)
      {
        current--;
      }
    }

    public override void NewRecord(string name)
    {
      customers.Add(name);
    }

    public override void DeleteRecord(string name)
    {
      customers.Remove(name);
    }

    public override void ShowRecord()
    {
      Console.WriteLine(customers[current]);
    }

    public override void ShowAllRecords()
    {
      foreach (string name in customers)
      {
        Console.WriteLine(" " + name);
      }
    }
  }
} 

Sample Code in Java

The following Java program illustrates the 'shape' example given above and will output:

import java.util.*;
/** "Implementor" */
interface DrawingAPI {
    public void drawCircle(double x, double y, double radius);
}

/** "ConcreteImplementor" 1/2 */
class DrawingAPI1 implements DrawingAPI {
   public void drawCircle(double x, double y, double radius) {
     System.out.printf("API1.circle at %f:%f radius %f\n", x, y, radius);
   }
}

/** "ConcreteImplementor" 2/2 */
class DrawingAPI2 implements DrawingAPI {
   public void drawCircle(double x, double y, double radius) 
  { System.out.printf("API2.circle at %f:%f radius %f\n", x, y, radius); }
}

/** "Abstraction" */
interface Shape {
   public void draw();                             // low-level
   public void resizeByPercentage(double pct);     // high-level
}

/** "Refined Abstraction" */
class CircleShape implements Shape {
   private double x, y, radius;
   private DrawingAPI drawingAPI;
   public CircleShape(double x, double y, double radius, DrawingAPI drawingAPI) {
       this.x = x;  this.y = y;  this.radius = radius; 
       this.drawingAPI = drawingAPI;
   }

   // low-level i.e. Implementation specific
   public void draw() { drawingAPI.drawCircle(x, y, radius); }   
   // high-level i.e. Abstraction specific
   public void resizeByPercentage(double pct) { radius *= pct; }
}

/** "Client" */
class BridgePattern {
   public static void main(String[] args) {
       Shape[] shapes = new Shape[2];
       shapes[0] = new CircleShape(1, 2, 3, new DrawingAPI1());
       shapes[1] = new CircleShape(5, 7, 11, new DrawingAPI2());

       for (Shape shape : shapes) {
           shape.resizeByPercentage(2.5);
           shape.draw();
       }
   }
}

References

Wikipedia.com - Bridge pattern

A Survey of Common Design Patterns

DoFactory.com - Bridge Design Pattern

Design Class Diagrams - Bridge

Gamma, E., Helm, R., Johnson, R., Vlissides, J. (1995). Design Patterns: Elements of Reusable Object-Oriented Software. Addison-Wesley. ISBN 0-201-63361-2