构造方法，类创建对象的调用，创建对象，为对象分配成员变量。。（Example4_2）

class Lader{
 double above,bottom,height;
 Lader(){}
 Lader(double a,double b,double h){
  above=a;
  bottom=b;
  height=h;
 }
 public void setAbove(double a){
  above=a;
 }
 public void setBottom(double b){
  bottom=b;
 }
 public void setHeight(double h){
  height=h;
 }
 double computeArea(){
  return (above+bottom)*height/2.0;
 }
 }
 public class Example4_3{
 public static void main(String args[]){
  double area1=0,area2=0;
  Lader laderOne,laderTwo;
  laderOne=new Lader();
  laderTwo=new Lader(10,88,20);
  laderOne.setAbove(16);
  laderOne.setBottom(26);
  laderOne.setHeight(100);
  laderTwo.setAbove(300);
  laderTwo.setBottom(500);
  area1=laderOne.computeArea();
  area2=laderTwo.computeArea();
  System.out.println("laderOne的above,bottom和heigh:"+laderOne.above+","+laderOne.bottom+","+laderOne.height);
  System.out.println("laderOne的面积："+area1);
  System.out.println("laderTwo的above,bottom和height:"+laderTwo.above+","+laderTwo.bottom+","+laderTwo.height);
  System.out.println("laderTwo的面积:"+area2);
  
 }
}