多态

灵活调用

替换扩充性强

继承、引用，重写

写一个shap类，有一个方法getArea（），获取图形面积

public class Shap {
    /**
     *写一个方法获取图形面积
     */
    public void getArea(){
        System.out.println("面积");
    }
}

写一个cycle，写一个rectangle，继承shap

写好私有属性，重写getArea（）方法求面积

public class Cycle extends Shap {
    private double r;

    public Cycle() {
    }

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

    @Override
    public void getArea() {
        System.out.println("圆的面积：" + 3.14 * r * r);
    }

    public double getR() {
        return r;
    }

    public void setR(double r) {
        this.r = r;
    }
}

rectangle

私有化长乘宽，无参，有参构造器，方法重写

public class Rectangle extends Shap {
    private double length;
    private double wide;

    public Rectangle() {
    }

    public Rectangle(double length, double wide) {
        this.length = length;
        this.wide = wide;
    }

    @Override
    public void getArea() {
        System.out.println("矩形的面积:" + length * wide);
    }

    public double getLength() {
        return length;
    }

    public void setLength(double length) {
        this.length = length;
    }

    public double getWide() {
        return wide;
    }

    public void setWide(double wide) {
        this.wide = wide;
    }
}

写一个测试类ShapDemo,类里面有个getShapArea（Shap shap）用于获取任意图形面积。

打印一个圆和一个矩形对象面积

搞两个对象，写方法，注意static。

public class ShapDemo {
    public static void main(String[] args) {
        Shap shap = new Cycle(10);
        getShapDemo(shap);
        shap = new Rectangle(10.0, 20.0);
        getShapDemo(shap);
    }

    public static void getShapDemo(Shap shap) {
        shap.getArea();
    }
}