现有圆的抽象类或者接口： 2.1）实现圆的周长和面积的计算； 2.2）环，继承自圆。实现其面积的计算； 2.3）扇形，继承自圆。实现其周长和面积的计算； 2.4）圆锥，继承自圆。实现其表面积和体积的计算； 2.5）圆台，继承自圆锥。实现其表面积和体积的计算； 2.6）如果上述类不是通过继承机制，而是采用类的复用机制呢？例如，圆锥类的一个 成员变量为圆类的对象。将上述类按这种要求实现一遍。 java

1. 圆的抽象类或接口：

public abstract class Circle {
    protected double radius; // 半径

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

    public abstract double getPerimeter(); // 计算周长

    public abstract double getArea(); // 计算面积
}

2.1 环类：

public class Ring extends Circle {
    protected double innerRadius; // 内半径

    public Ring(double innerRadius, double outerRadius) {
        super(outerRadius);
        this.innerRadius = innerRadius;
    }

    @Override
    public double getPerimeter() {
        return 2 * Math.PI * (innerRadius + radius);
    }

    @Override
    public double getArea() {
        return Math.PI * (radius * radius - innerRadius * innerRadius);
    }
}

2.2 扇形类：

public class Sector extends Circle {
    protected double angle; // 扇形角度

    public Sector(double radius, double angle) {
        super(radius);
        this.angle = angle;
    }

    @Override
    public double getPerimeter() {
        return 2 * Math.PI * radius * angle / 360;
    }

    @Override
    public double getArea() {
        return Math.PI * radius * radius * angle / 360;
    }
}

2.3 圆锥类：

public class Cone extends Circle {
    protected double height; // 高

    public Cone(double radius, double height) {
        super(radius);
        this.height = height;
    }

    public double getSurfaceArea() {
        return Math.PI * radius * (radius + Math.sqrt(radius * radius + height * height));
    }

    public double getVolume() {
        return Math.PI * radius * radius * height / 3;
    }
}

2.4 圆台类：

public class Frustum extends Cone {
    protected double topRadius; // 顶部半径

    public Frustum(double bottomRadius, double topRadius, double height) {
        super(bottomRadius, height);
        this.topRadius = topRadius;
    }

    @Override
    public double getSurfaceArea() {
        double l = Math.sqrt((radius - topRadius) * (radius - topRadius) + height * height);
        return Math.PI * (radius * radius + topRadius * topRadius + radius * l + topRadius * l);
    }

    @Override
    public double getVolume() {
        return Math.PI * height / 3 * (radius * radius + topRadius * topRadius + radius * topRadius);
    }
}

2.5 类的复用，圆锥类中包含圆类的对象：

public class Cone {
    protected Circle base; // 圆锥的底面圆

    protected double height; // 高

    public Cone(Circle base, double height) {
        this.base = base;
        this.height = height;
    }

    public double getSurfaceArea() {
        return Math.PI * base.radius * (base.radius + Math.sqrt(base.radius * base.radius + height * height));
    }

    public double getVolume() {
        return Math.PI * base.radius * base.radius * height / 3;
    }
}