異常在GUI線程 「AWT-EventQueue的-0」 顯示java.lang.NullPointerException？

Question

我是初學者，我使用java GUI，我不明白爲什麼它是一個錯誤。我有一個class Phong（它是Frameclass）和class LoaiPhong。在Phong類，我得到的錯誤，當我試圖通過方法GetItem5()和GetPop()獲得財產LoaiPhong類。但是，label(MainPanel.add(phongThuong[0].GetLabel());)運行良好。那麼，爲什麼我會得到錯誤？

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： 我試圖改變private JMenuItem item5;到private JMenuItem item5=new JMenuItem("Move");和private JPopupMenu pop;到private JPopupMenu pop = new JPopupMenu("option");，它運行良好，什麼發梗????我認爲在構造函數中，我初始化了JMenuItem item5 = new JMenuItem("Move");，但爲什麼在屬性中初始化它？

Answer 1

我回復給你的UPDATE： 它運行良好，因爲ü沒有在第一次正確初始化的變量。你只是說private JMenuItem item5，然後在構造函數中說你說JMenuItem item5=new JMenuItem("Move");這是錯誤的，因爲你需要從你的構造函數中刪除JMenuItem關鍵字，並且只允許item5=new JMenuItem("Move"); 嘗試在類構造函數中初始化它們（好方法），而不是在構造函數外（壞方法） 。

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

而且也試圖實現ActionListener您Phong類，並添加未實現的方法。