Question

我是初学者，我使用java GUI，我不明白为什么这是一个错误。我有一个class Phong（它是Frameclass）和class LoaiPhong。在Phong课程中，当我尝试按方法LoaiPhong和GetItem5()获取GetPop()课程中的媒体资源时收到错误。但是，label( MainPanel.add(phongThuong[0].GetLabel()); )运行良好。那么，为什么我会收到错误???

public class Phong extends javax.swing.JFrame {
    private void jButton1ActionPerformed(java.awt.event.ActionEvent evt) {
         phongThuong[0] = new LoaiPhong();
         MainPanel.add(phongThuong[0].GetLabel());
         phongThuong[0].GetItem5().addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent e) { 
                 JOptionPane.showMessageDialog(null, "Time start:",
                  "Title", JOptionPane.WARNING_MESSAGE);
           }

      });
      phongThuong[0].GetPop().add(phongThuong[0].GetItem5());
   }
}
class LoaiPhong{
  private JMenuItem item5;
  private  JPopupMenu pop;
   LoaiPhong(){
      JMenuItem item5 = new JMenuItem("Move");
   }
   JPopupMenu GetPop(){
        return this.pop;
   }
   JMenuItem GetItem5(){
        return this.item5;
   }
}

error:
Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
    at doanjava3.Phong.jButton1ActionPerformed(Phong.java:160)
    at doanjava3.Phong.access$000(Phong.java:32)
    at doanjava3.Phong$1.actionPerformed(Phong.java:81)
    at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:2022)
    at javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2348)
    at javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel.java:402)
    at javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:259)
    at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(BasicButtonListener.java:252)
    at java.awt.Component.processMouseEvent(Component.java:6533)
    at javax.swing.JComponent.processMouseEvent(JComponent.java:3324)
    at java.awt.Component.processEvent(Component.java:6298)
    at java.awt.Container.processEvent(Container.java:2236)
    at java.awt.Component.dispatchEventImpl(Component.java:4889)
    at java.awt.Container.dispatchEventImpl(Container.java:2294)
    at java.awt.Component.dispatchEvent(Component.java:4711)
    at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4888)
    at java.awt.LightweightDispatcher.processMouseEvent(Container.java:4525)
    at java.awt.LightweightDispatcher.dispatchEvent(Container.java:4466)
    at java.awt.Container.dispatchEventImpl(Container.java:2280)
    at java.awt.Window.dispatchEventImpl(Window.java:2746)
    at java.awt.Component.dispatchEvent(Component.java:4711)
    at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:758)
    at java.awt.EventQueue.access$500(EventQueue.java:97)
    at java.awt.EventQueue$3.run(EventQueue.java:709)
    at java.awt.EventQueue$3.run(EventQueue.java:703)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:76)
    at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:86)
    at java.awt.EventQueue$4.run(EventQueue.java:731)
    at java.awt.EventQueue$4.run(EventQueue.java:729)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:76)
    at java.awt.EventQueue.dispatchEvent(EventQueue.java:728)
    at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:201)
    at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:116)
    at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:105)
    at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:101)
    at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:93)
    at java.awt.EventDispatchThread.run(EventDispatchThread.java:82)

更新：我试图改变private JMenuItem item5;到private JMenuItem item5=new JMenuItem("Move");和private JPopupMenu pop;到private JPopupMenu pop = new JPopupMenu("option");它运行良好，发生了什么？我认为在构造函数中我初始化了JMenuItem item5 = new JMenuItem("Move");，但为什么要在属性中初始化它？

Answer 1

我回复你的更新：它运行良好，因为你最初没有正确初始化你的变量。您刚才说private JMenuItem item5，然后在构造函数中说JMenuItem item5=new JMenuItem("Move");这是错误的，因为您需要从构造函数中删除JMenuItem关键字，并且只允许item5=new JMenuItem("Move"); 尝试在类构造函数中初始化它们（好的方法），而不是外部构造函数（坏方法）。

 class Phong()
 {
    private JMenuItem item5
    public Phong()
    {
         item5=new JMenuItem("Move");
    }
 }

并尝试将ActionListener实施到您的Phong类，并添加未实现的方法。

线程中的异常＆＃34; AWT-EventQueue-0＆＃34; GUI中的java.lang.NullPointerException？

1 个答案: