- Bridge pattern
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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.
Contents
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
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
References
- ^ 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.
Creational Structural Behavioral Categories:- Software design patterns
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