Bridge pattern

The bridge pattern is a design pattern used in software engineering which is meant to "decouple an abstraction from its implementation so that the two can vary independently".^[1] The bridge uses encapsulation, aggregation, and can use inheritance to separate responsibilities into different classes.

When a class varies often, the features of object-oriented programming become very useful because changes to a program's code can be made easily with minimal prior knowledge about the program. The bridge pattern is useful when both the class as well as what it does vary often. The class itself can be thought of as the implementation and what the class can do as the abstraction. The bridge pattern can also be thought of as two layers of abstraction.

The bridge pattern is often confused with the adapter pattern. In fact, the bridge pattern is often implemented using the class adapter pattern, e.g. in the Java code below.

Variant: The implementation can be decoupled even more by deferring the presence of the implementation to the point where the abstraction is utilized.

Structure

Abstraction

defines the abstract interface

maintains the Implementor reference

RefinedAbstraction

extends the interface defined by Abstraction

Implementor

defines the interface for implementation classes

ConcreteImplementor

implements the Implementor interface

Bridge in LePUS3 (legend)

Example

The following Java (SE 6) program illustrates the 'shape' example given below and will output:

API1.circle at 1.000000:2.000000 radius 7.500000
API2.circle at 5.000000:7.000000 radius 27.500000

/** "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[] {
           new CircleShape(1, 2, 3, new DrawingAPI1()),
           new CircleShape(5, 7, 11, new DrawingAPI2()),
       };
 
       for (Shape shape : shapes) {
           shape.resizeByPercentage(2.5);
           shape.draw();
       }
   }
}

See also

• Template method pattern
• Strategy pattern
• Adapter pattern

References

1. ^ Gamma, E, Helm, R, Johnson, R, Vlissides, J: Design Patterns, page 151. Addison-Wesley, 1995

External links

• Bridge in UML and in LePUS3 (a formal modelling language)
• "C# Design Patterns: The Bridge Pattern". Sample Chapter. http://www.informit.com/articles/article.aspx?p=30297.  From: James W. Cooper. C# Design Patterns: A Tutorial. Addison-Wesley. ISBN 0201844532. http://www.informit.com/store/product.aspx?isbn=0201844532.