问题
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, i.e. 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)
回答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.
回答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)
来源：https://stackoverflow.com/questions/5373752/java-awt-eventqueue-0-error