Question

我到了某个地方，但现在我遇到了一个更大的障碍：我似乎无法理解如何创建一个监听器，将我的JLabel上的信息更改为存储在我的JTextArea中的输入和输出信息。

这是我的代码，可以给你一个想法：

public class DisplayStudent extends JFrame {
private JMenuItem jmiConnect, jmiClose, jmiExit, jmiSearch, jmiClear;
private JPanel contentPane;
private JPanel contentPane2;
private JTextArea info;
private String information;

public JPanel contentPane() {
    JPanel something = new JPanel(new BorderLayout());
    //something.setLayout();

    String information;

    info = new JTextArea(4, 30);
    info.setEditable(false);
    info.setLineWrap(true);
    info.setWrapStyleWord(true);

    JPanel one = new JPanel();
    one.setBackground(Color.WHITE);
    one.setSize(50, 50);
    one.add(info, BorderLayout.CENTER);
    something.add(one, BorderLayout.NORTH);

    JPanel two = new JPanel(new BorderLayout());
    two.setBackground(null);
    two.setSize(10, 10);
    two.add(new JLabel(" No Connection "), BorderLayout.WEST);
    two.add(new JLabel(" Number of Students = "), BorderLayout.EAST);
    something.add(two);

    something.setOpaque(true);
    return something;
}

public static void GUI() {
    JFrame frame = new JFrame("Display Student");

    DisplayStudent panel = new DisplayStudent();
    frame.setContentPane(panel.contentPane());

    DisplayStudent bar = new DisplayStudent();
    frame.setJMenuBar(bar.DisplayStudent());

    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    frame.setSize(400, 150);
    frame.setVisible(true);
    //frame.setJMenuBar(jmb);
}

public JMenuBar DisplayStudent() {
//Drop down menu
JMenuBar jmb = new JMenuBar();

  JMenu fileMenu = new JMenu("File");
  fileMenu.setMnemonic('F');
  jmb.add(fileMenu);


  JMenu editMenu = new JMenu("Edit");
  editMenu.setMnemonic('E');
  jmb.add(editMenu);

  //Buttons in menu - "File" menu
  JMenuItem jmiConnect = new JMenuItem("Connect", 'C');
  JMenuItem jmiClose = new JMenuItem("Close", 'L');
  JMenuItem jmiExit = new JMenuItem("Exit", 'E');

  fileMenu.add(jmiConnect); 
  fileMenu.add(jmiClose);
  fileMenu.add(jmiExit);

  //Buttons in menu - "Edit" menu
  JMenuItem jmiSearch = new JMenuItem("Search", 'S');
  JMenuItem jmiClear = new JMenuItem("Clear", 'B');

  editMenu.add(jmiSearch);
  editMenu.add(jmiClear);

  jmiConnect.addActionListener(new ActionListener() {
     @Override
     public void actionPerformed(ActionEvent e) {
         String database_name;
         System.out.println("Please enter your database.");
         Scanner input = new Scanner(System.in);
         database_name = input.next();
     }
  });

  jmiClose.addActionListener(new ActionListener() {
     @Override
     public void actionPerformed(ActionEvent e) {
         int reply = JOptionPane.showConfirmDialog(null, "Are you sure you want to quit?", "Question!", JOptionPane.YES_NO_OPTION);
    if (reply == JOptionPane.YES_OPTION) {
      JOptionPane.showMessageDialog(null, "Toodleloo!");
    }
     }
  });

  jmiExit.addActionListener(new ActionListener() {
     @Override
     public void actionPerformed(ActionEvent e) {
         System.exit(0);
     }
  });

  jmiSearch.addActionListener(new ActionListener() {
     @Override
     public void actionPerformed(ActionEvent e) {
         System.exit(0);
     }
  });

  jmiClear.addActionListener(new ActionListener() {
     @Override
     public void actionPerformed(ActionEvent e) {
         System.exit(0);
     }
  });

  return jmb;
}

public static void main(String[] args) {
  GUI();

}
   }

我很抱歉继续咨询。阅读不足以理解这一点。我再次感谢大家的帮助。

好的，当我回去时，我遇到了一个非常大的错误。这是：

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at displaystudent.DisplayStudent$1.actionPerformed(DisplayStudent.java:119)
at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:2018)
at javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2341)
at javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel.java:402)
at javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:259)
at javax.swing.AbstractButton.doClick(AbstractButton.java:376)
at javax.swing.plaf.basic.BasicMenuItemUI.doClick(BasicMenuItemUI.java:833)
at com.apple.laf.AquaMenuItemUI.doClick(AquaMenuItemUI.java:157)
at javax.swing.plaf.basic.BasicMenuItemUI$Handler.mouseReleased(BasicMenuItemUI.java:877)
at java.awt.Component.processMouseEvent(Component.java:6505)
at javax.swing.JComponent.processMouseEvent(JComponent.java:3320)
at java.awt.Component.processEvent(Component.java:6270)
at java.awt.Container.processEvent(Container.java:2229)
at java.awt.Component.dispatchEventImpl(Component.java:4861)
at java.awt.Container.dispatchEventImpl(Container.java:2287)
at java.awt.Component.dispatchEvent(Component.java:4687)
at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4832)
at java.awt.LightweightDispatcher.processMouseEvent(Container.java:4492)
at java.awt.LightweightDispatcher.dispatchEvent(Container.java:4422)
at java.awt.Container.dispatchEventImpl(Container.java:2273)
at java.awt.Window.dispatchEventImpl(Window.java:2719)
at java.awt.Component.dispatchEvent(Component.java:4687)
at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:735)
at java.awt.EventQueue.access$200(EventQueue.java:103)
at java.awt.EventQueue$3.run(EventQueue.java:694)
at java.awt.EventQueue$3.run(EventQueue.java:692)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:76)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:87)
at java.awt.EventQueue$4.run(EventQueue.java:708)
at java.awt.EventQueue$4.run(EventQueue.java:706)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:76)
at java.awt.EventQueue.dispatchEvent(EventQueue.java:705)
at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:242)
at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:161)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:150)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:146)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:138)
at java.awt.EventDispatchThread.run(EventDispatchThread.java:91)

如果我问得太多，我很抱歉。我现在已经知道了。

Answer 1

“将名称转移到JLabel，将其从”No Connection“更改为”Connected to database_name“

问题：您的JLabel没有对象引用。你这样做

two.add(new JLabel(" No Connection "), BorderLayout.WEST);

使难以（并非不可能）访问JLabel。而是给它一个像

这样的参考

JLabel statusLabel = new JLabel(" No Connection ");
public JPanel contentPane() {
    two.add(statusLabel, BorderLayout.WEST);

然后你可以从JMenuItem的听众那里访问它。

jmiConnect.addActionListener(new ActionListener(){
    public void actionPerformed(ActionEvent e) {
        ...
        statusLabel.setText(...);
    }
});

这是另一个建议，不要试图将控制台与GUI混合，即Scanner(System.in)。您可能希望使用类似JOptionPane.showInputDialog(..)甚至JCheckBoxMenuItem的内容，这是菜单中的复选框，您可以检查数据库名称。然后，只需将结果附加到侦听器的文本区域即可。

<强>更新

为什么要创建一个新的DisplayStudent来放入框架？由于该方法返回JPanel，只需执行此操作
```
JPanel panel = contentPane();
frame.setContentPane(panel);
```
不要让您的课程延长JFrame。您已经使用JFrame的实例作为顶级容器。摆脱extends JFrame
请勿调用返回与您的班级JMenuBar同名的DisplayStudent的方法。称之为createMenuBar()，然后执行此操作
```
JMenuMenuBar menuBar = createMenuBar();
frame.setJMenuBar(menuBar);
```
摆脱DisplayStudent .. = new DisplayStudent();

GUI问题：不要知道从哪里开始

1 个答案: