GUI中线程“AWT-EventQueue-0”java.lang.NullPointerException中的异常？ [重复]

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• What is a NullPointerException, and how do I fix it? 12 answers

什么是NullPointerException,我该如何解决? 12个答案

I am beginner, I use java GUI, I don't understand why ther is an error. I have a class Phong(it is Frameclass) and class LoaiPhong. In Phong class, I get the error when I try to get a property in LoaiPhong class by method GetItem5() and GetPop(). However, label( MainPanel.add(phongThuong[0].GetLabel()); ) runs well. So, why do I get the error???

我是初学者,我使用java GUI,我不明白为什么这是一个错误。我有一个类Phong(它是Frameclass)和类LoaiPhong。在Phong类中,当我尝试通过方法GetItem5()和GetPop()获取LoaiPhong类中的属性时,我收到错误。但是,标签(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)

UPDATE: I tried to change private JMenuItem item5; to private JMenuItem item5=new JMenuItem("Move"); and private JPopupMenu pop; to private JPopupMenu pop = new JPopupMenu("option"); and it run well, what happed???? I think in contructor I have initialized JMenuItem item5 = new JMenuItem("Move"); but why initialize it in property?

更新:我试图改变私人JMenuItem item5;私人JMenuItem item5 = new JMenuItem(“Move”);和私人JPopupMenu流行;私有JPopupMenu pop =新JPopupMenu(“选项”);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?

1 个解决方案

#1

I reply this to your UPDATE: It ran well, because u didn't initialized properly your variables at first. You just said private JMenuItem item5 and then in constructor you said JMenuItem item5=new JMenuItem("Move"); which is wrong, because you needed to remove JMenuItem keyword from your constructor, and let only item5=new JMenuItem("Move"); Try to initialize them in class constructor (good way), not outside constructor (bad way).

我回复你的更新:它运行良好,因为你没有首先正确初始化你的变量。你刚才说私有JMenuItem item5然后在构造函数中你说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");
    }
 }

And also try to implement ActionListener to your Phong class, and add unimplemented methods.

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