Java AWT-EventQueue-0错误

Question

can someone help me to debug this,

Edit 1: the error is not in the code below * Error occurs within the code below: *

   if (e.getSource().equals(btnRefresh))
        {
            PeerList.removeAllElements();
            FileList.removeAllElements();


            try {
                AllPeers=(List <String>)Services.getPeers();
            } catch (RemoteException e1) {
                // TODO Auto-generated catch block

            }

            for (String CurrentPeer:AllPeers)
            {
                System.out.println(CurrentPeer);
                PeerList.addElement(CurrentPeer);
            }
        }

Edit 1: Error is here

lstPeerList.addListSelectionListener(new ListSelectionListener(){
            @Override
            public void valueChanged(ListSelectionEvent e) 
            {
                try 
                {
                    SelectedPeer=(String)lstPeerList.getSelectedValue();
                    List <String> PeerFiles=Services.getPeerFiles(SelectedPeer);
                    FileList.removeAllElements();
                    for (String CurrentFile:PeerFiles)
                    {
                        FileList.addElement(CurrentFile);
                    }
                } 
                catch (RemoteException e1) 
                {
                    // TODO Auto-generated catch block

                }
            }});

in the code below, if am retrieving the variable as the following code, the error occurs:

SelectedPeer=(String)lstPeerList.getSelectedValue();
List <String> PeerFiles=Services.getPeerFiles(SelectedPeer);

but if i'm doing this, ie not using the retrieved variable, the error doesn't occur:

SelectedPeer=(String)lstPeerList.getSelectedValue();
List <String> PeerFiles=Services.getPeerFiles("Noor");

No error occurs,

Exception:

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
        at Server.getPeerFiles(Server.java:54)
        at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
        at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
        at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
        at java.lang.reflect.Method.invoke(Method.java:597)
        at sun.rmi.server.UnicastServerRef.dispatch(UnicastServerRef.java:305)
        at sun.rmi.transport.Transport$1.run(Transport.java:159)
        at java.security.AccessController.doPrivileged(Native Method)
        at sun.rmi.transport.Transport.serviceCall(Transport.java:155)
        at sun.rmi.transport.tcp.TCPTransport.handleMessages(TCPTransport.java:535)
        at sun.rmi.transport.tcp.TCPTransport$ConnectionHandler.run0(TCPTransport.java:790)
        at sun.rmi.transport.tcp.TCPTransport$ConnectionHandler.run(TCPTransport.java:649)
        at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:886)
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:908)
        at java.lang.Thread.run(Thread.java:637)
        at sun.rmi.transport.StreamRemoteCall.exceptionReceivedFromServer(StreamRemoteCall.java:255)
        at sun.rmi.transport.StreamRemoteCall.executeCall(StreamRemoteCall.java:233)
        at sun.rmi.server.UnicastRef.invoke(UnicastRef.java:142)
        at java.rmi.server.RemoteObjectInvocationHandler.invokeRemoteMethod(RemoteObjectInvocationHandler.java:178)
        at java.rmi.server.RemoteObjectInvocationHandler.invoke(RemoteObjectInvocationHandler.java:132)
        at $Proxy0.getPeerFiles(Unknown Source)
        at MainForm$1.valueChanged(MainForm.java:55)
        at javax.swing.JList.fireSelectionValueChanged(JList.java:1765)
        at javax.swing.JList$ListSelectionHandler.valueChanged(JList.java:1779)
        at javax.swing.DefaultListSelectionModel.fireValueChanged(DefaultListSelectionModel.java:167)
        at javax.swing.DefaultListSelectionModel.fireValueChanged(DefaultListSelectionModel.java:147)
        at javax.swing.DefaultListSelectionModel.fireValueChanged(DefaultListSelectionModel.java:194)
        at javax.swing.DefaultListSelectionModel.removeIndexInterval(DefaultListSelectionModel.java:660)
        at javax.swing.plaf.basic.BasicListUI$Handler.intervalRemoved(BasicListUI.java:2589)
        at javax.swing.AbstractListModel.fireIntervalRemoved(AbstractListModel.java:161)
        at javax.swing.DefaultListModel.removeAllElements(DefaultListModel.java:385)
        at MainForm.actionPerformed(MainForm.java:223)
        at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:1995)
        at javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2318)
        at javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel.java:387)
        at javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:242)
        at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(BasicButtonListener.java:236)
        at java.awt.Component.processMouseEvent(Component.java:6267)
        at javax.swing.JComponent.processMouseEvent(JComponent.java:3267)
        at java.awt.Component.processEvent(Component.java:6032)
        at java.awt.Container.processEvent(Container.java:2041)
        at java.awt.Component.dispatchEventImpl(Component.java:4630)
        at java.awt.Container.dispatchEventImpl(Container.java:2099)
        at java.awt.Component.dispatchEvent(Component.java:4460)
        at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4577)
        at java.awt.LightweightDispatcher.processMouseEvent(Container.java:4238)
        at java.awt.LightweightDispatcher.dispatchEvent(Container.java:4168)
        at java.awt.Container.dispatchEventImpl(Container.java:2085)
        at java.awt.Window.dispatchEventImpl(Window.java:2478)
        at java.awt.Component.dispatchEvent(Component.java:4460)
        at java.awt.EventQueue.dispatchEvent(EventQueue.java:599)
        at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:269)
        at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:184)
        at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:174)
        at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:169)
        at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:161)
        at java.awt.EventDispatchThread.run(EventDispatchThread.java:122)

Answer 1

The ListSelectionListener is invoked on every change of the selection. This could mean that now there is no selection, lstPeerList.getSelectedValue() returns null . Then you invoke getPeerFiles(null) , and this method seems to be unable to handle null parameters.

You could either change your getPeerFiles method, or invoke it only if the selected value is not null .

Answer 2

Whatever is on line 54

 at Server.getPeerFiles(Server.java:54)

Is null, so post line 54

Edit:

Possible things that could be null:

e, btnRefresh, PeerList, FileList, Services

Whatever one of those that is on line 54 is your cause.

(Sorry didn't know to add it to my original post)