定义一个抽象类Shape代表一个形状类，一个计算形状的面积的抽象方法，一个计算形状的周长的抽象方法。

定义一个抽象类Shape代表一个形状类，一个计算形状的面积的抽象方法，一个计算形状的周长的抽象方法。

abstract class Shape {
public abstract double area();// 面积

public abstract double perimeter();// 周长
}

class Circle extends Shape {
private double r;

public Circle(double r) {
this.r = r;
}

public double area() {
return Math.PI * r * r;
}

@Override
public double perimeter() {
return 2 * Math.PI * r;
}
}

class Retangle extends Shape {
private double height, width;

public Retangle(double height, double width) {
this.height = height;
this.width = width;
}

@Override
public double area() {
return this.height * width;
}

@Override
public double perimeter() {
return 2 * (this.height + this.width);
}
}

public class Test {
public static void main(String[] args) {
Shape shape1 = new Circle(3.0);
System.out.println(shape1.area());
System.out.println(shape1.perimeter());
Shape shape2 = new Retangle(4, 7.3);
System.out.println(shape2.area());
System.out.println(shape2.perimeter());
}
}

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