菜单项鼠标单击时面板背景颜色不会改变

Question

下面是我的代码。它相当简单，只需在选择三个菜单项之一时更改面板颜色，我认为它的设置正确（？）但它似乎没有做任何事情，也没有错误。有什么提示吗？

import java.awt.Color;
import java.awt.Graphics;
import java.awt.Graphics2D;
/*
* To change this license header, choose License Headers in Project     Properties.
* To change this template file, choose Tools | Templates
* and open the template in the editor.
*/

/**
*
* @author Joe
*/
public class menuframe extends javax.swing.JFrame {

/**
 * Creates new form menuframe
 */
public menuframe() {
    initComponents();
}

/**
 * This method is called from within the constructor to initialize the form.
 * WARNING: Do NOT modify this code. The content of this method is always
 * regenerated by the Form Editor.
 */
@SuppressWarnings("unchecked")
// <editor-fold defaultstate="collapsed" desc="Generated Code">                          
private void initComponents() {

    panel = new javax.swing.JPanel();
    jMenuBar1 = new javax.swing.JMenuBar();
    jMenu1 = new javax.swing.JMenu();
    jMenuItem1 = new javax.swing.JMenuItem();
    jMenuItem2 = new javax.swing.JMenuItem();
    jMenuItem3 = new javax.swing.JMenuItem();

    setDefaultCloseOperation(javax.swing.WindowConstants.EXIT_ON_CLOSE);

    javax.swing.GroupLayout panelLayout = new javax.swing.GroupLayout(panel);
    panel.setLayout(panelLayout);
    panelLayout.setHorizontalGroup(
        panelLayout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
        .addGap(0, 400, Short.MAX_VALUE)
    );
    panelLayout.setVerticalGroup(
        panelLayout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
        .addGap(0, 279, Short.MAX_VALUE)
    );

    jMenu1.setText("Color");

    jMenuItem1.setText("Red");
    jMenuItem1.addMouseListener(new java.awt.event.MouseAdapter() {
        public void mouseClicked(java.awt.event.MouseEvent evt) {
            jMenuItem1MouseClicked(evt);
        }
    });
    jMenu1.add(jMenuItem1);

    jMenuItem2.setText("Green");
    jMenuItem2.addMouseListener(new java.awt.event.MouseAdapter() {
        public void mouseClicked(java.awt.event.MouseEvent evt) {
            jMenuItem2MouseClicked(evt);
        }
    });
    jMenu1.add(jMenuItem2);

    jMenuItem3.setText("Blue");
    jMenuItem3.addMouseListener(new java.awt.event.MouseAdapter() {
        public void mouseClicked(java.awt.event.MouseEvent evt) {
            jMenuItem3MouseClicked(evt);
        }
    });
    jMenu1.add(jMenuItem3);

    jMenuBar1.add(jMenu1);

    setJMenuBar(jMenuBar1);

    javax.swing.GroupLayout layout = new javax.swing.GroupLayout(getContentPane());
    getContentPane().setLayout(layout);
    layout.setHorizontalGroup(
        layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
        .addComponent(panel, javax.swing.GroupLayout.Alignment.TRAILING, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, Short.MAX_VALUE)
    );
    layout.setVerticalGroup(
        layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
        .addComponent(panel, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, Short.MAX_VALUE)
    );

    pack();
}// </editor-fold>                        



private void jMenuItem1MouseClicked(java.awt.event.MouseEvent evt) {                                        
   panel.setBackground(Color.red);
}                                       

private void jMenuItem2MouseClicked(java.awt.event.MouseEvent evt) {                                        
   panel.setBackground(Color.green);
}                                       

private void jMenuItem3MouseClicked(java.awt.event.MouseEvent evt) {                                        
   panel.setBackground(Color.blue);
}                                       

/**
 * @param args the command line arguments
 */
public static void main(String args[]) {
    /* Set the Nimbus look and feel */
    //<editor-fold defaultstate="collapsed" desc=" Look and feel setting code (optional) ">
    /* If Nimbus (introduced in Java SE 6) is not available, stay with the default look and feel.
     * For details see http://download.oracle.com/javase/tutorial/uiswing/lookandfeel/plaf.html 
     */
    try {
        for (javax.swing.UIManager.LookAndFeelInfo info : javax.swing.UIManager.getInstalledLookAndFeels()) {
            if ("Nimbus".equals(info.getName())) {
                javax.swing.UIManager.setLookAndFeel(info.getClassName());
                break;
            }
        }
    } catch (ClassNotFoundException ex) {
        java.util.logging.Logger.getLogger(menuframe.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
    } catch (InstantiationException ex) {
        java.util.logging.Logger.getLogger(menuframe.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
    } catch (IllegalAccessException ex) {
        java.util.logging.Logger.getLogger(menuframe.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
    } catch (javax.swing.UnsupportedLookAndFeelException ex) {
        java.util.logging.Logger.getLogger(menuframe.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
    }
    //</editor-fold>

    /* Create and display the form */
    java.awt.EventQueue.invokeLater(new Runnable() {
        public void run() {
            new menuframe().setVisible(true);
        }
    });
}

// Variables declaration - do not modify                     
private javax.swing.JMenu jMenu1;
private javax.swing.JMenuBar jMenuBar1;
private javax.swing.JMenuItem jMenuItem1;
private javax.swing.JMenuItem jMenuItem2;
private javax.swing.JMenuItem jMenuItem3;
private javax.swing.JPanel panel;
// End of variables declaration                   

}