如何让MouseHandler在DrawPanel中生效并实现鼠标位置显示标签？

你遇到的问题核心是：你在DrawPanel里嵌套的mousePanel是空的，默认FlowLayout布局下，空JPanel的尺寸是0×0，鼠标根本碰不到这个面板，所以事件监听器完全触发不了。

解决方案一：直接给DrawPanel加监听器（推荐） #

没必要额外嵌套一个JPanel，直接把鼠标监听器绑定到DrawPanel本身：

private class DrawPanel extends JPanel {

    public DrawPanel() {
        MouseHandler handler = new MouseHandler();
        this.addMouseListener(handler);
        this.addMouseMotionListener(handler);
        // 可以加个背景色方便看到面板范围
        this.setBackground(Color.WHITE);
    }

    @Override
    public void paintComponent(Graphics g) {
        super.paintComponent(g);
        Graphics2D g2d = (Graphics2D) g;
    }

    private class MouseHandler extends MouseAdapter implements MouseMotionListener {

        @Override
        public void mousePressed(MouseEvent event) {
            System.out.print("Pressed");
        }

        @Override
        public void mouseReleased(MouseEvent event) {
            System.out.print("Released"); 
        }

        @Override
        public void mouseDragged(MouseEvent event) {
            System.out.print("Dragged"); 
            // lblStatus.setText(String.format("(%d,%d)",event.getX(),event.getY()));
        }

        @Override
        public void mouseMoved(MouseEvent event) {
            System.out.print("Moved");
            // System.out.print("("+event.getX()+","+event.getY()+")");
            // lblStatus.setText(String.format("(%d,%d)",event.getX(),event.getY()));
        }
    }
}

解决方案二：保留mousePanel，让它占满空间 #

如果一定要用内部的mousePanel，需要修改DrawPanel的布局，让mousePanel填满整个DrawPanel：

private class DrawPanel extends JPanel {

    public DrawPanel() {
        // 把DrawPanel的布局改成BorderLayout，让内部组件占满空间
        this.setLayout(new BorderLayout());
        JPanel mousePanel = new JPanel();
        // 加背景色方便观察mousePanel的范围
        mousePanel.setBackground(Color.LIGHT_GRAY);
        this.add(mousePanel, BorderLayout.CENTER);
        
        MouseHandler handler = new MouseHandler();
        mousePanel.addMouseListener(handler);
        mousePanel.addMouseMotionListener(handler);
    }

    // paintComponent和MouseHandler代码不变
}

补充：关于状态标签lblStatus #

如果要启用lblStatus.setText(...)的代码，需要确保lblStatus能被MouseHandler访问到：

1. 在DrawPanel类中添加成员变量private JLabel lblStatus;
2. 在DrawPanel的构造方法中传入这个标签，比如：

public DrawPanel(JLabel statusLabel) {
    this.lblStatus = statusLabel;
    // 后续监听器代码就可以正常调用lblStatus的方法了
}

内容的提问来源于stack exchange，提问作者JTD28