[说明]
以下程序的功能是计算三角形、矩形和正方形的面积并输出，程序由5个类组成：AreaTest是主类，类Trianlge、Rectangle和Square分别表示三角形、矩形和正方形，抽象类Figure提供了一个计算面积的抽象方法。
[程序]
public class AreaTest
public static void main(String args [])
Figure [] figures= new Triangle (2,3,3) ,new Rectangle (5,8) ,new Square (5) ;
for (int i=0; 1< figures .length; i++)
System. out .println (figures [i] +"area=" +figures [i]. getArea ());



public abstract class Figure
public abstract double getArea() ;

public class Rectangle extends (1)
double height;
double width;
public Rectangle(double height, double width)
this .height=height;
this.width=width;

public String toString ()
return "Rectangle: height= "+height+" , width= "+width+":";

public double getArea ()
return (2) ;

class Square extends (3)
public Square(double width)
(4) ;

public string toString ()
return "Square: width= "+width+" : " ;


class Triangle extends (5)
double la;
double lb;
double lc;
public Triangle(double la,double lb, double lc)
this.la=la; this.lb=lb; this.lc=lc;

public String toString()
return "Triangle : sides= "+la+" , "+lb+" , " +lc+" : " ;

public double getArea ()
double s= (la+lb+lc)/2.0;
return Math. sqrt (s* (s-la) * (s-lb) * (s-lc)) ;