Bridge pattern
From Wikipedia, the free encyclopedia
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 varies. The class itself can be thought of as the implementation and what the class can do as the abstraction.
Variant: The implementation can be decoupled even more by deferring the presence of the implementation to the point where the abstraction is utilized.
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[edit] Examples
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[edit] Shape abstraction
When the abstraction and implementation are separated, they can vary independently. Consider the abstraction of shapes. There are many types of shapes, each with its own properties. And there are things that all shapes do. One thing all shapes can do is draw themselves. However, drawing graphics to a screen can sometimes be dependent on different graphics implementations or operating systems. Shapes have to be able to be drawn on many types of operating systems. Having the shape itself implement them all, or modifying the shape class to work with different architectures is not practical. The bridge helps by allowing the creation of new implementation classes that provide the drawing implementation. The abstraction class, shape, provides methods for getting the size or properties of a shape. The implementation class, drawing, provides an interface for drawing graphics. If a new shape needs to be created or there is a new graphics API to be drawn on, then it is very easy to add a new implementation class that implements the needed features.[2]
[edit] Structure
- Abstraction
- defines the abstract interface
- maintains the Implementor reference
- Refined Abstraction
- extends the interface defined by Abstraction
- Implementor
- defines the interface for implementation classes
- ConcreteImplementor
- implements the Implementor interface
[edit] Code examples
[edit] Java
The following Java (SE 6) program illustrates the 'shape' example given above 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[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(); } } }
[edit] C#
The following C# program illustrates the "shape" example given above and will output:
API1.circle at 1:2 radius 7.5 API2.circle at 5:7 radius 27.5
using System; /** "Implementor" */ interface IDrawingAPI { void DrawCircle(double x, double y, double radius); } /** "ConcreteImplementor" 1/2 */ class DrawingAPI1 : IDrawingAPI { public void DrawCircle(double x, double y, double radius) { System.Console.WriteLine("API1.circle at {0}:{1} radius {2}", x, y, radius); } } /** "ConcreteImplementor" 2/2 */ class DrawingAPI2 : IDrawingAPI { public void DrawCircle(double x, double y, double radius) { System.Console.WriteLine("API2.circle at {0}:{1} radius {2}", x, y, radius); } } /** "Abstraction" */ interface IShape { void Draw(); // low-level (i.e. Implementation-specific) void ResizeByPercentage(double pct); // high-level (i.e. Abstraction-specific) } /** "Refined Abstraction" */ class CircleShape : IShape { private double x, y, radius; private IDrawingAPI drawingAPI; public CircleShape(double x, double y, double radius, IDrawingAPI 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) { IShape[] shapes = new IShape[2]; shapes[0] = new CircleShape(1, 2, 3, new DrawingAPI1()); shapes[1] = new CircleShape(5, 7, 11, new DrawingAPI2()); foreach (IShape shape in shapes) { shape.ResizeByPercentage(2.5); shape.Draw(); } } }
[edit] C# using generics
The following C# program illustrates the "shape" example given above and will output:
API1.circle at 1:2 radius 7.5 API2.circle at 5:7 radius 27.5
using System; /** "Implementor" */ interface IDrawingAPI { void DrawCircle(double x, double y, double radius); } /** "ConcreteImplementor" 1/2 */ struct DrawingAPI1 : IDrawingAPI { public void DrawCircle(double x, double y, double radius) { System.Console.WriteLine("API1.circle at {0}:{1} radius {2}", x, y, radius); } } /** "ConcreteImplementor" 2/2 */ struct DrawingAPI2 : IDrawingAPI { public void DrawCircle(double x, double y, double radius) { System.Console.WriteLine("API2.circle at {0}:{1} radius {2}", x, y, radius); } } /** "Abstraction" */ interface IShape { void Draw(); // low-level (i.e. Implementation-specific) void ResizeByPercentage(double pct); // high-level (i.e. Abstraction-specific) } /** "Refined Abstraction" */ class CircleShape<T> : IShape where T : struct, IDrawingAPI { private double x, y, radius; private IDrawingAPI drawingAPI = new T(); public CircleShape(double x, double y, double radius) { this.x = x; this.y = y; this.radius = radius; } // 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) { IShape[] shapes = new IShape[2]; shapes[0] = new CircleShape<DrawingAPI1>(1, 2, 3); shapes[1] = new CircleShape<DrawingAPI2>(5, 7, 11); foreach (IShape shape in shapes) { shape.ResizeByPercentage(2.5); shape.Draw(); } } }
[edit] C++
The following C++ program illustrates the "shape" example given above and will output:
API1.circle at 1:2 7.5 API2.circle at 5:7 27.5
#include <iostream> using namespace std; /* Implementor*/ class DrawingAPI { public: virtual void drawCircle(double x, double y, double radius) = 0; virtual ~DrawingAPI() {} }; /* Concrete ImplementorA*/ class DrawingAPI1 : public DrawingAPI { public: void drawCircle(double x, double y, double radius) { cout << "API1.circle at " << x << ':' << y << ' ' << radius << endl; } }; /* Concrete ImplementorB*/ class DrawingAPI2 : public DrawingAPI { public: void drawCircle(double x, double y, double radius) { cout << "API2.circle at " << x << ':' << y << ' ' << radius << endl; } }; /* Abstraction*/ class Shape { public: virtual ~Shape() {} virtual void draw() = 0; virtual void resizeByPercentage(double pct) = 0; }; /* Refined Abstraction*/ class CircleShape : public Shape { public: CircleShape(double x, double y,double radius, DrawingAPI *drawingAPI) : m_x(x), m_y(y), m_radius(radius), m_drawingAPI(drawingAPI) {} void draw() { m_drawingAPI->drawCircle(m_x, m_y, m_radius); } void resizeByPercentage(double pct) { m_radius *= pct; } private: double m_x, m_y, m_radius; DrawingAPI *m_drawingAPI; }; int main(void) { CircleShape circle1(1,2,3,new DrawingAPI1()); CircleShape circle2(5,7,11,new DrawingAPI2()); circle1.resizeByPercentage(2.5); circle2.resizeByPercentage(2.5); circle1.draw(); circle2.draw(); return 0; }
[edit] Python
The following Python program illustrates the "shape" example given above and will output:
API1.circle at 1:2 7.5 API2.circle at 5:7 27.5
# Implementor class DrawingAPI: def drawCircle(x, y, radius): pass # ConcreteImplementor 1/2 class DrawingAPI1(DrawingAPI): def drawCircle(self, x, y, radius): print "API1.circle at %f:%f radius %f" % (x, y, radius) # ConcreteImplementor 2/2 class DrawingAPI2(DrawingAPI): def drawCircle(self, x, y, radius): print "API2.circle at %f:%f radius %f" % (x, y, radius) # Abstraction class Shape: # low-level def draw(self): pass # high-level def resizeByPercentage(self, pct): pass # Refined Abstraction class CircleShape(Shape): def __init__(self, x, y, radius, drawingAPI): self.__x = x self.__y = y self.__radius = radius self.__drawingAPI = drawingAPI # low-level i.e. Implementation specific def draw(self): self.__drawingAPI.drawCircle(self.__x, self.__y, self.__radius) # high-level i.e. Abstraction specific def resizeByPercentage(self, pct): self.__radius *= pct def main(): shapes = [ CircleShape(1, 2, 3, DrawingAPI1()), CircleShape(5, 7, 11, DrawingAPI2()) ] for shape in shapes: shape.resizeByPercentage(2.5) shape.draw() if __name__ == "__main__": main()
[edit] See also
[edit] References
[edit] 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.
- Example of using Bridge pattern in Jt, J2EE Pattern Oriented Framework
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