2.定義圖形類Shape，該類中有獲得面積的方法getArea()；定義長方形類Rect，該類是Shape的子類，類中有矩形長和寬的變量double a,double b，設置長和寬的方法setWidth()、setHeight(),使用getArea()求矩形面積；利用getArea方法實現題1中圓面積的求解。

// 圖形類Shape
package d922B;

public class Shape {
double getArea(ShapePara x){
return x.getArea();
}
double getArea(Rect y)
{
return y.getA()*y.getB();
}

}
//矩形類
package d922B;

public class Rect extends Shape {
private double a, b;
public Rect(double a, double b) {
super();
this.a = a;
this.b = b;
}

public double getA() {
	return a;
}

public double getB() {
	return b;
}

public void setA(double a) {
	this.a = a;
}

public void setB(double b) {
	this.b = b;
}

}

//主類
package d922B;

public class Test {

public static void main(String[] args) {
	Shape a=new Shape();
	System.out.println(a.getArea(new Circle(1)));
	System.out.println(a.getArea(new Rect(15,20)));

}

}

//運行結果

3.0
300.0