Java AWT调整窗口大小时Panel移位、点击坐标异常问题

使用Java AWT开发GUI程序时遇到如下异常：

• 窗口初始显示时，内部Panel组件存在移位，点击Panel区域获取的坐标显示错误
• 手动拖拽调整窗口大小后，Panel位置自动恢复正常，点击坐标可以正确显示
• 预期效果：点击Panel左上角时输出坐标(0,8)，该结果仅在手动调整窗口大小后才能得到

import java.awt.*;
import java.awt.event.*;
import java.awt.Color;

public class Proj07Runner {
  Proj07Runner(){//overloaded constructor
  //The purpose of this constructor is to display
  // the certification: modified very slightly from Proj03Runner
    System.out.println(
    "Terminal Text");
    new GUI(); //instantiate a GUI
  }//end main
}//end class Proj07Runner
//=======================================================//


class MyFrame extends Frame{
  int clickX;
  int clickY;

  MyFrame(){
    setTitle("zzz");
    setSize(300,100);
  }

  public void paint(Graphics g){
    g.setColor(Color.BLACK);
    g.drawString(
             "" + clickX + ", " + clickY, clickX, clickY);
  }//end paint()
}//end class MyFrame
//=======================================================//


//Subclass Panel in order to override the paint method.
class MyPanel extends Panel{
  int clickX;
  int clickY;
 
  MyPanel(){//constructor
    setSize(300,100); // set size of Panel
    setBackground(Color.yellow); // make panel yellow
  }//end constructor
  
  public void paint(Graphics g){//override paint() method
    //display coordinate information on the object
    g.drawString(
            "" + clickX + ", " + clickY, clickX, clickY);
  }//end paint()
}//end class MyFrame
//---------------------------------------------------------


class GUI {
  public GUI(){//constructor
    //Instantiate an object of type MyPanel
    MyPanel myPanel = new MyPanel();    
    //Instantiate an object of type MyFrame
    MyFrame displayWindow = new MyFrame();
    displayWindow.setVisible(true);
    displayWindow.add(myPanel);

    WProc1 winProcCmd1 = new WProc1();
    displayWindow.addWindowListener(winProcCmd1);  
    //Instantiate and register Listener object which will 
    // process mouse events on MyFrame or myPanel object.
    MouseProc mouseProcCmd = new MouseProc(displayWindow, myPanel);
    displayWindow.addMouseListener(mouseProcCmd);
    myPanel.addMouseListener(mouseProcCmd); 
  }//end constructor
}//end class GUI definition
//=======================================================//


class MouseProc extends MouseAdapter{
  //save references to the objects here
  MyFrame refToFrame;
  MyPanel refToPanel;
  
  MouseProc(MyFrame inFrame,MyPanel inPanel){//constructor
    refToFrame = inFrame;//save references to the frames
    refToPanel = inPanel;
  }//end constructor

  public void mousePressed(MouseEvent e){
    refToPanel.clickX = e.getX();
    refToPanel.clickY = e.getY();
    refToPanel.repaint();
  }//end mousePressed()
}//end class MouseProc
//---------------------------------------------------------

class WProc1 extends WindowAdapter{
  public void windowClosing(WindowEvent e){
    System.exit(0);
  }//end windowClosing()
}//end class WProc1
//=======================================================//

1. 代码执行顺序错误：GUI构造方法中先调用displayWindow.setVisible(true)让窗口显示，之后才执行Panel添加、监听器注册操作。AWT容器仅在首次显示、窗口大小变化时自动触发全局布局计算，先显示窗口后添加组件不会自动触发布局更新，此时Panel没有被分配正确的位置和尺寸，坐标原点默认对齐Frame的全局左上角（包含标题栏的窗口装饰区域），而非标题栏下方的客户区左上角，因此点击获取的坐标存在固定偏移。
2. 手动拖拽调整窗口大小时，AWT会自动触发容器的validate()方法重新执行布局逻辑，将Panel自动放置到标题栏下方的客户区位置，顶部偏移量匹配标题栏高度，此时点击Panel左上角就能得到预期的(0,8)坐标。
3. 存在冗余无效代码：MyPanel构造方法中手动调用setSize(300,100)没有实际作用，Frame默认使用BorderLayout布局管理器，会自动将添加到默认位置的组件拉伸占满整个客户区，手动设置的尺寸会被布局管理器覆盖。

核心修改是调整窗口显示和组件配置的顺序，确保窗口第一次显示前就完成所有组件添加、监听器注册操作，让初始布局计算直接得到正确结果。如果业务需要在窗口显示后动态添加组件，添加完成后必须调用容器的validate()方法强制触发布局重算，再调用repaint()刷新界面。
修正后的GUI类构造方法代码如下：

class GUI {
  public GUI(){//constructor
    //Instantiate an object of type MyPanel
    MyPanel myPanel = new MyPanel();    
    //Instantiate an object of type MyFrame
    MyFrame displayWindow = new MyFrame();

    // 先完成所有组件添加、监听器注册操作
    displayWindow.add(myPanel);
    WProc1 winProcCmd1 = new WProc1();
    displayWindow.addWindowListener(winProcCmd1);  
    MouseProc mouseProcCmd = new MouseProc(displayWindow, myPanel);
    displayWindow.addMouseListener(mouseProcCmd);
    myPanel.addMouseListener(mouseProcCmd); 

    // 所有配置完成后再显示窗口
    displayWindow.setVisible(true);
  }//end constructor
}//end class GUI definition

额外优化：可删除MyPanel构造方法中setSize(300,100);这行无效代码，修正MyPanel类末尾的注释笔误（原注释误写为//end class MyFrame，应为//end class MyPanel）。

内容的提问来源于stack exchange，提问作者Kevin Adams