[说明]
以下程序的功能时三角形、矩形和正方形的面积输出。
程序由5个类组成：areatest是主类，类Triangle，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。
[Java程序]
public class areatest {
public static viod main(string args[]){
Figure[]Figures={
New triangle(2,3,3),new rectangle(5,8),new square(5)
};
for(int i=0; i＜Figures.length;i++){
system.out.println(Figures+"area="+Figures.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)
}
}
public class square exends (3)
{
public square(double width){
(4);
}
public string tostring(){
return"square:width="+width":";
}
}
public class triangle entends (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 get area(){
double s=(la+lb+lc)/2.0;
return math．sqrt(s*(s-la)*(s-lb)*(s-lc));
}
}