JLayeredPane上的JLabel随机落后于下层

Question

我正在尝试使用自定义图标（在地图上显示为城市）创建程序，其中图标在鼠标悬停时显示标签（在本例中为城市1，城市2等）。它适用于城市2,3和4;然而，城市1有一些意想不到的行为。当第一次悬停时，标签会显示在图标上，但如果鼠标退出并重新进入城市，则标签会显示在城市图标的后面。如果鼠标首先进入任何其他城市，则城市1的标签默认为图标后面。看到所有四个城市都是同一个班级的实例，我对于为什么会发生这种情况感到非常困惑。

希望一切都有道理。这是主窗口的代码：

import java.awt.Color;
import java.awt.Dimension;
import java.awt.Point;
import java.io.IOException;

import javax.swing.ImageIcon;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JLayeredPane;

@SuppressWarnings("serial")
public class MainWindow extends JFrame{

static JLabel hoverText = new JLabel("", JLabel.CENTER);
static JLayeredPane panel = new JLayeredPane();

public MainWindow(){
    ImageIcon cityImg = new ImageIcon("Resources/mock_city.png");

    CustomButton city1, city2, city3, city4;
    city1 = new CustomButton(cityImg, "City 1");        
    city2 = new CustomButton(cityImg, "City 2");
    city3 = new CustomButton(cityImg, "City 3");
    city4 = new CustomButton(cityImg, "City 4");

    Dimension cityDims = new Dimension(cityImg.getIconWidth(),cityImg.getIconHeight());
    city1.setSize(cityDims);
    city1.setLocation(50,50);
    city2.setSize(cityDims);
    city2.setLocation(50,150);
    city3.setSize(cityDims);
    city3.setLocation(150,50);
    city4.setSize(cityDims);
    city4.setLocation(150,150);

    panel.add(city1, new Integer(2));
    panel.add(city2, new Integer(2));
    panel.add(city3, new Integer(2));
    panel.add(city4, new Integer(2));

    add(panel);

    setSize(300,300);
    setVisible(true);
}
public static void setHoverText(String label, Point p) {
    hoverText.setText(label);
    hoverText.setForeground(Color.RED);
    hoverText.setSize(77,13);
    hoverText.setLocation(p);
    panel.add(hoverText, new Integer(3));
}

public static void delHoverText() {
    hoverText.setText("");
    panel.add(hoverText, new Integer(3));
}

public static void main (String[] args) throws IOException {
    new MainWindow();

    }
}

以下是图标的代码：

import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.Toolkit;
import java.awt.event.MouseEvent;
import java.awt.event.MouseListener;

import javax.swing.ImageIcon;
import javax.swing.JComponent;


@SuppressWarnings("serial")
public class CustomButton extends JComponent implements MouseListener{
ImageIcon buttonImg;
String label;
public CustomButton(ImageIcon buttonImg, String label){
    super();

    addMouseListener(this);
    this.buttonImg = buttonImg;
    this.label = label;
}
public void paintComponent(Graphics g){
    super.paintComponent(g);
    Graphics2D g2d = (Graphics2D)g;
    g2d.drawImage(buttonImg.getImage(), 0, 0, this);

    Toolkit.getDefaultToolkit().sync();
    g.dispose();
}
@Override
public void mouseClicked(MouseEvent arg0) {

}
@Override
public void mouseEntered(MouseEvent arg0) {
    MainWindow.setHoverText(label, getLocation());

}
@Override
public void mouseExited(MouseEvent arg0) {
    MainWindow.delHoverText();
}
@Override
public void mousePressed(MouseEvent arg0) {

}
@Override
public void mouseReleased(MouseEvent arg0) {

    }
}