public class RecclassDemo{
        double  length;
        double width;
void  setLW(double l, double w){
                length=l;
                width=w;
        }
        double area(){
                return length*width;
}
public static void main(String args[]){
                RecclassDemo r1;                              //聲明類的對象
                r1=new RecclassDemo ();                  //創建對象，為對象分配空間
                r1.setLW(10,20);                                 //調用成員方法進行賦值
                System.out.println("The Rectangle area is :  "+r1.area());    //調用成員方法計算面積
}
}