QChí Nguyễn QChí Nguyễn - 16 days ago 6
Java Question

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException in GUI?

I am beginner, I use java GUI, I don't understand why it error. I have a class Phong(it is Frameclass) and class LoaiPhong. In Phong class, I get a property in LoaiPhong class by method GetItem5() and GetPop() but it error. I tried use this way with label( MainPanel.add(phongThuong[0].GetLabel()); ) it run well. So, why it error???

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

Answer

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).

 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.

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