//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 测试 {    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);    }}