Java 面积计算：抽象类 Shape 及其子类实现

以下是以 Java 语言实现该设定的代码：

abstract class Shape {
    public abstract double getArea();
}

class Circle extends Shape {
    private double radius;

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

    public double getArea() {
        return Math.PI * radius * radius;
    }
}

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

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

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

class Triangle extends Shape {
    private double base;
    private double height;

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

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

class TestShape {
    static double area = 0;

    public static void countArea(Shape s) {
        area += s.getArea();
    }

    public static void main(String[] args) {
        Circle s1 = new Circle(5);
        Rectangle s2 = new Rectangle(4, 6);
        Triangle s3 = new Triangle(3, 8);

        countArea(s1);
        countArea(s2);
        countArea(s3);

        System.out.println('Total area: ' + area);
    }
}

运行以上代码，将输出三个形状的面积之和。