java语言的科学与艺术-编程练习10.4(改进）

/*
* 程序可以用鼠标拖拽话出一个矩形；
*/
import acm.graphics.*;
import acm.program.*;
import java.awt.event.*;

public class DrawRectangle extends GraphicsProgram{
//鼠标侦听
public void run(){
addMouseListeners();
}
//鼠标按下事件
public void mousePressed(MouseEvent e){
press = new GPoint(e.getPoint());
rect = new GRect(0,0);
}
//鼠标拖拽事件
public void mouseDragged(MouseEvent e){
double x = press.getX();
double y = press.getY();
double width = e.getX() - press.getX();
double height = e.getY() - press.getY();
//确保向左下方拖拽也能画出矩形；
if(width < 0 ){
x = e.getX();
width = -width;
}
//确保向上方拖拽也能画出矩形；
if(height < 0){
y = e.getY();
height = -height;
}

rect.setSize(width,height);
rect.setFilled(true);
add(rect, x, y);
}
private GPoint press; //记录鼠标按下的点；
private GRect rect;
}