4.欲开发一个绘图软件，要求使用不同的绘图程序绘制不同的图形。以绘制直线和圆形为例，对应的绘图程序如表6-1所示。
表6-1不同的绘图程序
该绘图软件的扩展性要求，将不断扩充新的图形和新的绘图程序。为了避免出现类爆炸的情况，现采用桥接（Bridge）模式来实现上述要求，得到如图6-1所示的类图。
图6-1类图
【Java代码】
（1）Drawing{
（2）;
（3）;
}
class DP1{
static public void draw_a_line(double x1,double y1,double x2,double y2){/*代码省略*/}
static public void draw_a_circle(double x,double y,double r){/*代码省略*/}
}
class DP2{
static public void drawline(double x1,double y1,double x2,double y2){/*代码省略*/}
static public void drawcircle(double x,double y,double r){/*代码省略*/}
}
class V1Drawing implements Drawing{
public void drawLine(double x1,double y1,double x2,double y2){/*代码省略*/}
public void drawCircle(double x,double y,double r){（4）;}
}
class V2Drawing implements Drawing{
public void drawLine(double x1,double y1,double x2,double y2){/*代码省略*/}
public void drawCircle(double x,double y,double r){（5）;}
}
abstract class Shape{
private Drawing_dp;
（6）;
Shape(Drawing dp){_dp=dp;}
public void drawLine(double x1,double y1,double x2,double y2){_dp.drawLine(x1,y1,x2,y2);}
public void drawCircle(double x,double y,double r){_dp.drawCircle(x,y,r);}
}
class Rectangle extends Shape{
private double_x1,_x2,_y1,_y2;
public Rectangle(Drawing dp,double x1,double y1,double x2,double y2){/*代码省略*/}
public void draw（ ）{/*代码省略*/}
}
class Circle extends Shape{
private double_x,_y,_r;
public Circle(Drawing dp,double x,double y,double r){/*代码省略*/}
public void draw（ ）{drawCircle(_x,_y,_r);}
}
