abstract class Shapes {		//定义抽象类 Shapes
	abstract public double area();  	//周长
	abstract public double square();	//面积
}

class Triangle extends Shapes {  		//继承 Shapes，定义三角型
	private double a, b, c;

	public Triangle(double a, double b, double c) {
		this.a = a; 
		this.b = b;
		this.c = c;	
	}
	public double square()  {
		double p = (a + b +c) / 2;
		return Math.sqrt(p * (p - a) * (p- b) * (p- c));
	}
	public double area()  {
		double cir = (a + b +c);
		return cir;
	}
	public String toString() {
		return "三角形：边长"+a+","+b+","+c;
	}
}

class Rectangle extends Shapes {  //定义矩形
	private double width, height;//长，宽
	public Rectangle(double j, double k) {
		width = j; height = k;
	}
	public double area() {
		double Rectangle =(width + height)*2;
		return Rectangle;
	}
	public String toString() {
		return "矩形：宽 "+width+" 高 "+height;
	}
	public double square() {
		double Rectangle =width* height;
		return Rectangle;
	}
}

class Circle extends Shapes {   //定义圆
	private double r;
	public Circle(double r) {
		this.r = r;
	}
	public double area() {
		return 3.14 * 2 * r;
	}
	public double square() {
		return 3.14 * r * r;
	}
	public String toString() {
		return "圆：半径= "+ r;
	}   
}

以上构造了 三角形、矩形、圆的函数，定义了 计算周长（area）面积（square）的方法。
下面是测试类代码如下：

//测试类
public class TestShapes {
	 public static void main(String args[])  {
			Shapes s[] = new Shapes[3];   
			s[0] = new Triangle(3, 4, 5);
			s[1] = new Rectangle(5.2,4.8);
			s[2] = new Circle(5.2);
			for (int k= 0; k < s.length; k ++)
			    System.out.println(s[k]+", 面积："+s[k].square()+", 周长："+s[k].area());
		  } 
}