摘自高等教育出版社《Java程序设计（许满武 主编）》
为方便测试，通常在类中实现一个toString方法，把当前对象转化成一个字符串以便显示
public String toString(){
return ......
}
示例：
class Point{
public double x,y;
public static Point origin=new Point();
Point(double x,double y){
this.x=x;
this.y=y;
}
Point(){}
public void clear(){
x=0;
y=0;
}
public double distance(Point that){
double xdiff,ydiff;
xdiff=x-that.x;
ydiff=y-that.y;
return Math.sqrt(xdiff*xdiff+ydiff*ydiff);
}
public String toString(){
return "x="+x+",y="+y;
}
public static double distance(Point p1,Point p2){
double xdiff,ydiff;
xdiff=p1.x-p2.x;
ydiff=p1.y-p2.y;
return Math.sqrt(xdiff*xdiff+ydiff*ydiff);
}
public static void main(String[] args){
Point A=new Point(1,1);
Point B=new Point(2,2);
System.out.println(B);
}
}
显示：x=2.0,y=2.0