java/w4/Shape.java

    abstract class Shape {//定义一个抽象类
        public abstract double getArea();//强制所有图形都必须能计算面积
    }

    // 圆形
    class Circle extends Shape {//继承shape，必须实现getarea
        private double radius;//半径私有属性

        public Circle(double radius) {
            this.radius = radius;
        }//构造方法无返回值，给对象赋初始值

        @Override//重写父类方法
        public double getArea() {
            return Math.PI * radius * radius;
        }
    }

    // 矩形
    class Rectangle extends Shape {
        private double length;
        private double width;

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

        @Override
        public double getArea() {
            return length * width;
        }
    }

    // 三角形
    class Triangle extends Shape {
        private double base;
        private double height;

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

        @Override
        public double getArea() {
            return 0.5 * base * height;
        }
    }

    // 工具类
    class ShapeUtil {//专门用来打印图形面积
        public static void printArea(Shape shape) {//任意接受一个图形
            System.out.println("图形面积：" + shape.getArea());
        }
    }