为什么JLabel会消失？

Question

任务：有2个标签按行连接，用户必须能够用鼠标移动每个标签，并且标签之间的连接必须相应地移动。

这是我的代码：

import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.event.MouseAdapter;
import java.awt.event.MouseEvent;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.SwingUtilities;


class BackgroundJPanel extends JPanel {

    BackgroundJPanel() {        
        this.setBounds(0, 0, 200, 200); 
    }

    @Override
    public void paintComponent(Graphics g) {
        super.paintComponent(g);
        g.setColor(Color.orange);
        g.fillRect(0, 0, 198, 198);
        g.setColor(Color.black);
        g.drawRect(0, 0, 198, 198);
    }
}


class LinkJPanel extends JPanel {

    JLabel[] labels = null;

    LinkJPanel(JLabel[] labelsIn) { 
        labels = labelsIn;
        this.setBounds(0, 0, 200, 200);
        this.setOpaque(false);
    }

    @Override
    public void paintComponent(Graphics g) {
        super.paintComponent(g);
        g.drawLine(labels[0].getX(), labels[0].getY(), 
                   labels[1].getX(), labels[1].getY());
    }
}


class LabelJPanel extends JPanel {

    AllPanel allPanel = null;

    LabelJPanel(AllPanel allPanelIn) {
        allPanel = allPanelIn;
        this.setOpaque(false);

        JLabel[] labels = allPanel.labels;
        labels[0] = new JLabel("11");              
        this.add(labels[0]);                 
        labels[1] = new JLabel("22");            
        this.add(labels[1]);

        MyMouseAdapter myMouseAdapter = new MyMouseAdapter();
        this.addMouseMotionListener(myMouseAdapter);
        this.addMouseListener(myMouseAdapter);
    }

    class MyMouseAdapter extends MouseAdapter {

        JLabel labelToMove = null;
        int startDeltaX = -1;
        int startDeltaY = -1;

        JLabel whichLabel(MouseEvent e) {            
            for (JLabel label : allPanel.labels) {                
                int deltaX = e.getX() - label.getX();
                int deltaY = e.getY() - label.getY();                
                if ((deltaX >= 0) 
                        && (deltaX < label.getPreferredSize().width) 
                        && (deltaY >= 0) 
                        && (deltaY < label.getPreferredSize().height)) {
                    return label;
                }
            }
            return null;
        }

        @Override
        public void mousePressed(MouseEvent e) {            
            labelToMove = whichLabel(e);              
            if (labelToMove != null) {                
                startDeltaX = labelToMove.getX() - e.getX();
                startDeltaY = labelToMove.getY() - e.getY();
            }
        }

        @Override
        public void mouseDragged(MouseEvent e) {
            if (labelToMove != null) {
                updateLocation(e);
            }
        }

        @Override
        public void mouseReleased(MouseEvent e) {
            labelToMove = null;
        }

        public void updateLocation(MouseEvent e) {            
            int newXCoord = e.getX() + startDeltaX;
            int newYCoord = e.getY() + startDeltaY;            
            labelToMove.setBounds(newXCoord, newYCoord, 
                    labelToMove.getPreferredSize().width, 
                    labelToMove.getPreferredSize().height);                

            //allPanel.backgroundJPanel.repaint();
            allPanel.linkJPanel.repaint();  
        }
    }
}


class AllPanel extends JPanel {  
    BackgroundJPanel backgroundJPanel = null;
    LinkJPanel linkJPanel = null;
    LabelJPanel labelJPanel = null;
    JLabel[] labels = null;

    AllPanel() {        
        this.setLayout(new BorderLayout());        
        this.setPreferredSize(new Dimension(200, 200));

        labels = new JLabel[2];  

        backgroundJPanel = new BackgroundJPanel();
        this.add(backgroundJPanel);

        linkJPanel = new LinkJPanel(labels);
        this.add(linkJPanel);  

        labelJPanel = new LabelJPanel(this);
        this.add(labelJPanel); 

        this.setComponentZOrder(backgroundJPanel, 2);
        this.setComponentZOrder(linkJPanel, 1);
        this.setComponentZOrder(labelJPanel, 0);
    }
}


class MyFrame extends JFrame {
    MyFrame() {

        this.setTitle("Test");
        this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        this.add(new AllPanel());        

        this.pack();
        this.setVisible(true);        
        this.setResizable(false);
    }
}


public class Test {

    public static void main(String[] args) {
        SwingUtilities.invokeLater(new Runnable() {            
            @Override
            public void run() {                
                new MyFrame();                
            }
        });
    }
}

我使用3个JPanel - 一个用于背景，一个用于绘制连接线，一个用于绘制2个标签。用于绘制标签的JPanel具有MouseMotion和Mouse侦听器。当出现一些鼠标操作时，代码检查必须移动哪个标签，然后更改该标签坐标并用连接线重新绘制JPanel。

所有工作正常，直到我尝试用背景重新绘制JPanel。如果我取消注释该行

//allPanel.backgroundJPanel.repaint();

行动后JLabel和行开始消失。 有人可以解释为什么会这样吗？