定义 Shape 类表示一般二维图形。Shape 具有抽象方法 area 和 perimeter，分别计算形状的面积和周长。试定义一些二维形状类（如矩形、三角形、圆形等），这些均为 Shape 类的子类并计算出这些形状的面积和周长，打印输出相关信息。

形状类:

public abstract class Shape {
    public abstract double area();
    public abstract double perimeter();
} 

矩形类:

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

    public double getLength() {
        return length;
    }

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

    public double getWidth() {
        return width;
    }

    public void setWidth(double width) {
        this.width = width;
    }

    @Override
    public double area() {
        return getLength() * getWidth();
    }

    @Override
    public double perimeter() {
        return 2 * (getWidth() + getWidth());
    }
}

三角形类:

public class Triangle extends Shape {
    private double a, b, c;

    public double getA() {
        return a;
    }

    public void setA(double a) {
        this.a = a;
    }

    public double getB() {
        return b;
    }

    public void setB(double b) {
        this.b = b;
    }

    public double getC() {
        return c;
    }

    public void setC(double c) {
        this.c = c;
    }

    @Override
    public double area() {
        double p = (getA() + getB() + getC()) / 2;
        return Math.sqrt(p * (p - getA()) * (p - getB()) * (p - getC()));
    }

    @Override
    public double perimeter() {
        return getA() + getB() + getC();
    }
}

圆形类:

public class Circle extends Shape {
    private double diameter;

    public double getDiameter() {
        return diameter;
    }

    public void setDiameter(double diameter) {
        this.diameter = diameter;
    }

    @Override
    public double area() {
        return Math.PI * Math.pow(getDiameter() / 2, 2);
    }

    @Override
    public double perimeter() {
        return Math.PI * getDiameter();
    }
}

测试代码:

public class Test {
    public static void main(String [] args){
        Rectangle rec = new Rectangle();
        rec.setLength(10);
        rec.setWidth(5);
        double rec_area = rec.area();
        double rec_perimeter = rec.perimeter();
        System.out.println("矩形的面积："+rec_area+"，周长"+rec_perimeter);

        Triangle tri = new Triangle();
        tri.setA(3);
        tri.setB(4);
        tri.setC(5);
        double tri_area = tri.area();
        double tri_perimeter = tri.perimeter();
        System.out.println("三角形的面积："+tri_area+"，周长"+tri_perimeter);

        Circle cir = new Circle();
        cir.setDiameter(10);
        double cir_area = cir.area();
        double cir_perimeter = cir.perimeter();
        System.out.println("圆形的面积："+cir_area+"，周长"+cir_perimeter);
    }
}