Chapter 10: The Bridge Pattern
Code Examples

Example 10-1

Example 10-2

Example 10-3

Proper use of inheritance

Example 10-1

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abstract public class Rectangle {
    private double _x1,_y1,_x2,_y2;
    public Rectangle (double x1,double y1,double x2,double y2) {
        x1= x1;_y1= y1;_x2= x2;_y2= y2;
    }
    public void draw() {
        drawLine(_x1,_y1,_x2,_y1);
        drawLine(_x2,_y1,_x2,_y2);
        drawLine(_x2,_y2,_x1,_y2);
        drawLine(_x1,_y2,_x1,_y1);
    }
    abstract protected void drawLine(double x1,double y1,double x2,double y2);
}

Designing with inheritance

Example 10-2

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abstract class Shape {
    abstract public void draw();
}
// the only change to Rectangle is
abstract class Rectangle extends Shape {
    //
    // V1Rectangle and V2Rectangle don't change
abstract public class Circle extends Shape {
    protected double _x,_y,_r;
    public Circle (double x, double y, double r) {
        _x= x;_y= y;_r= r;
    }
    public void draw() {
        drawCircle();
    }
    abstract protected void drawCircle();
}
public class V1Circle extends Circle {
    public V1Circle (double x,double y,double r) {
        super(x,y,r);
    }
    protected void drawCircle () {
        DP1.draw_a_circle(_x,_y,_r);
    }
}
public class V2Circle extends Circle {
    public V2Circle(double x,double y,double r) {
        super(x,y,r);
    }
    protected void drawCircle () {
        DP2.drawCircle(_x,_y,_r);
    }
}

Relating this to the inheritance-based design

Example 10-3

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public class Client {
    static public void main () {
        Shape myShapes[];
        Factory myFactory= new Factory();
        // get rectangles from some other source
        myShapes= myFactory.getShapes();
        for (int i= 0; i < myShapes.length; i++) {
            myShapes[i].draw();
        }
    }
}
abstract public class Shape {
    protected Drawing myDrawing;
    abstract public void draw();
    Shape (Drawing drawing) {
        myDrawing= drawing;
    }
    protected void drawLine (double x1,double y1,double x2,double y2) {
        myDrawing.drawLine(x1,y1,x2,y2);
    }
    protected void drawCircle (double x,double y,double r) {
        myDrawing.drawCircle(x,y,r);
    }
}
public class Rectangle extends Shape {
    private double _x1,_y1,_x2,_y2;
    public Rectangle (Drawing dp,double x1,double y1,double x2,double y2) {
        super( dp);
        x1= x1;_y1= y1;_x2= x2;_y2= y2;
    }
    public void draw() {
        drawLine(_x1,_y1,_x2,_y1);
        drawLine(_x2,_y1,_x2,_y2);
        drawLine(_x2,_y2,_x1,_y2);
        drawLine(_x1,_y2,_x1,_y1);
    }
    protected void drawLine(double x1,double y1,double x2,double y2) {
        myDrawing.drawLine(x1,y1,x2,y2);
    }
}
public class Circle extends Shape {
    private double _x,_y,_r;
    public Circle (Drawing dp,double x,double y,double r) {
        super(dp);
        x= x;_y= y;_r= r;
    }
    public void draw() {
        myDrawing.drawCircle(_x,_y,_r);
    }
}
abstract public class Drawing {
    abstract public void drawLine(double x1,double y1,double x2,double y2);
    abstract public void drawCircle(double x,double y,double r);
}
public class V1Drawing extends Drawing {
    public void drawLine (double x1,double y1,double x2,double y2) {
        DP1.draw_a_line(x1,y1,x2,y2);
    }
    public void drawCircle (double x,double y,double r) {
        DP1.draw_a_circle(x,y,r);
    }
}
public class V2Drawing extends Drawing {
    public void drawLine (double x1,double y1,double x2,double y2) {
        // arguments are different in DP2
        // and must be rearranged
        DP2.drawLine(x1,x2,y1,y2);
    }
    public void drawCircle (double x, double y,double r) {
        DP2.drawCircle(x,y,r);
    }
}