设计一个抽象类Shape，要求如下： 1、包含抽象方法：求面积的area()方法和求周长的perimeter()方法；以及普 通方法：设置颜色的setColor()方法和获取颜色的getColor方法()； 2、设计其子类Circle（圆形）类，包含属性半径r，并实现Shape类中的抽 象方法； 3、编写测试类ShapeTest，分别实现相应的求面积、求周长以及设置并获取 颜色的操作。

下面是代码实现：

// Shape.java
public abstract class Shape {
    private String color;
    
    public abstract double area();
    
    public abstract double perimeter();
    
    public void setColor(String color) {
        this.color = color;
    }
    
    public String getColor() {
        return color;
    }
}

// Circle.java
public class Circle extends Shape {
    private double r;
    
    public Circle(double r) {
        this.r = r;
    }
    
    public double area() {
        return Math.PI * r * r;
    }
    
    public double perimeter() {
        return 2 * Math.PI * r;
    }
}

// ShapeTest.java
public class ShapeTest {
    public static void main(String[] args) {
        Circle c = new Circle(3.0);
        c.setColor("red");
        System.out.println("The area of the circle is " + c.area());
        System.out.println("The perimeter of the circle is " + c.perimeter());
        System.out.println("The color of the circle is " + c.getColor());
    }
}

运行结果如下：

The area of the circle is 28.274333882308138
The perimeter of the circle is 18.84955592153876
The color of the circle is red