null pointer exception while closing a node of jtree

[naved momin]

  Object obj = node.getUserObject();

at the above line my application is throwing an exception , i had make ftp app , so whenever i close any of the tree's node , remember that icon that we use to close the jtree's node the one which has leaf nodes inside it , but i m not getting why is it throwing an null pointer exception while at the time of closing a node ??
any answers ?

but this exception doesnt affect my work or app , my app still works fine , but thing is making me curious to know what is happening , even though there is an exception my app is working fine
but still any how i need to find out the root cause of this exception ,
one more thing this statement

  Object obj = node.getUserObject();

is in the treeselectionlistener() method and gives me the last value selected by the user
i have also check whether closing the node gives me null or what , but no it is not giving any null value , but still exception is thrown , why ???

run:
Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
	at FTPClient.JTreeWithFTP.valueChanged(JTreeWithFTP.java:808)
	at javax.swing.JTree.fireValueChanged(JTree.java:2820)
	at javax.swing.JTree$TreeSelectionRedirector.valueChanged(JTree.java:3191)
	at javax.swing.tree.DefaultTreeSelectionModel.fireValueChanged(DefaultTreeSelectionModel.java:629)
	at javax.swing.tree.DefaultTreeSelectionModel.notifyPathChange(DefaultTreeSelectionModel.java:1078)
	at javax.swing.tree.DefaultTreeSelectionModel.removeSelectionPaths(DefaultTreeSelectionModel.java:497)
	at javax.swing.JTree.removeDescendantSelectedPaths(JTree.java:3516)
	at javax.swing.JTree.setExpandedState(JTree.java:3404)
	at javax.swing.JTree.collapsePath(JTree.java:2188)
	at javax.swing.plaf.basic.BasicTreeUI.toggleExpandState(BasicTreeUI.java:2215)
	at javax.swing.plaf.basic.BasicTreeUI.handleExpandControlClick(BasicTreeUI.java:2191)
	at javax.swing.plaf.basic.BasicTreeUI.checkForClickInExpandControl(BasicTreeUI.java:2149)
	at javax.swing.plaf.basic.BasicTreeUI$Handler.handleSelection(BasicTreeUI.java:3511)
	at javax.swing.plaf.basic.BasicTreeUI$Handler.mousePressed(BasicTreeUI.java:3459)
	at java.awt.AWTEventMulticaster.mousePressed(AWTEventMulticaster.java:262)
	at java.awt.Component.processMouseEvent(Component.java:6286)
	at javax.swing.JComponent.processMouseEvent(JComponent.java:3267)
	at java.awt.Component.processEvent(Component.java:6054)
	at java.awt.Container.processEvent(Container.java:2041)
	at java.awt.Component.dispatchEventImpl(Component.java:4652)
	at java.awt.Container.dispatchEventImpl(Container.java:2099)
	at java.awt.Component.dispatchEvent(Component.java:4482)
	at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4577)
	at java.awt.LightweightDispatcher.processMouseEvent(Container.java:4235)
	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:4482)
	at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:644)
	at java.awt.EventQueue.access$000(EventQueue.java:85)
	at java.awt.EventQueue$1.run(EventQueue.java:603)
	at java.awt.EventQueue$1.run(EventQueue.java:601)
	at java.security.AccessController.doPrivileged(Native Method)
	at java.security.AccessControlContext$1.doIntersectionPrivilege(AccessControlContext.java:87)
	at java.security.AccessControlContext$1.doIntersectionPrivilege(AccessControlContext.java:98)
	at java.awt.EventQueue$2.run(EventQueue.java:617)
	at java.awt.EventQueue$2.run(EventQueue.java:615)
	at java.security.AccessController.doPrivileged(Native Method)
	at java.security.AccessControlContext$1.doIntersectionPrivilege(AccessControlContext.java:87)
	at java.awt.EventQueue.dispatchEvent(EventQueue.java:614)
	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)
BUILD SUCCESSFUL (total time: 1 minute 47 seconds)

[Rob Spoor]

Well, something on line 808 of file JTreeWithFTP.java is null.

[naved momin]

Rob Spoor wrote:Well, something on line 808 of file JTreeWithFTP.java is null.

ya , this is line no 808

808 Object obj = node.getUserObject();  

[Campbell Ritchie]

So, node is null at this point. Or, there is something in the getUserObject() method which is null and causing an Exception. Since 808 appears to be the top line in your stack trace, the first explanation is probably correct and the second probably incorrect.

[naved momin]

Campbell Ritchie wrote:So, node is null at this point. Or, there is something in the getUserObject() method which is null and causing an Exception. Since 808 appears to be the top line in your stack trace, the first explanation is probably correct and the second probably incorrect.

ya , but what can i do to stop this exception to be thrown ?
it is a jtree , so what could be null while closing the node , i m not getting it ?
i know null porinter exception is thrown , when i i dont create any object but use the reference all over my code
but here i m not getting it
can you help me campbell finding and fixing this problem ?

[Campbell Ritchie]

Start with line 807½

System.out.printf("node is not null: %b%n", node); // test

See what that prints.

[naved momin]

Campbell Ritchie wrote:Start with line 807½

System.out.printf("node is not null: %b%n", node); // test

See what that prints.

ya , i got why it is throwing null pointer exception because when you browse inside a node , under it , there is leaf nodes , so when you select any of the leaf node
this line

Object obj = node.getUserObject();  

fetch the last thing we select , so suppose we select abc.txt and then we try to close the node by clicking its icon which close the node
then this obj fetches " null " because tree selection event is fired and when we close the node it try to fetch what we have selected but couldnt find anything except null

so , simple way to handle this is try catch , catch will catch(NullPointerException npe) , but when i did that it doesnt work , the code inside the catch is not printed on the console and again exception is thrown like i have given you above
so can you suggest me any simple solution to handle this ?
let me give you lines which are above this

Object obj = node.getUserObject();  

  
        DefaultMutableTreeNode node = new DefaultMutableTreeNode() ;
        
        node = (DefaultMutableTreeNode) tree.getLastSelectedPathComponent();
        
        Object obj = new Object();
        
        
       
        
        obj = node.getUserObject();
        

[Campbell Ritchie]

What’s this about null leaf nodes? Have you got any null leaf nodes?

Look at this tree

root here:                            node
                                      /  \
                                 node    node
                                /   |     |  \
                            node   leaf  leaf node
                            /                      \
                        node                        node
                       /   |                         |   \

But that isn’t how a binary tree works at all. You don’t have different node and leaf classes. Your leaves are nodes which don’t have children, but they still have pointers to those children, so they will point to null.
That means: Every binary tree is guaranteed to contain null pointers. In fact, I think you can work out how many nulls there are: I think it is n + 1 where n is the number of nodes.
Now I know a displayed tree is rather different from a binary tree, but you need to check

Do your leaf nodes have null pointers to their supposed children?

Similarly to their user Objects?

Should you be returning null if you pick up one of those leaves,

or should you throw an Exception, to be caught higher up the stack,

or should you do something different?

Have you set a user Object, or put one there by any other means?

And what happened when you printed my is not null test from earlier (line 807½)?

Not sure I can go much rather. Sorry, I have probably suggested all I can. You will have to look harder for that null.
Anybody else, please?

[naved momin]

Campbell Ritchie wrote:What’s this about null leaf nodes? Have you got any null leaf nodes?

Look at this tree

root here:                            node
                                      /  \
                                 node    node
                                /   |     |  \
                            node   leaf  leaf node
                            /                      \
                        node                        node
                       /   |                         |   \

But that isn’t how a binary tree works at all. You don’t have different node and leaf classes. Your leaves are nodes which don’t have children, but they still have pointers to those children, so they will point to null.
That means: Every binary tree is guaranteed to contain null pointers. In fact, I think you can work out how many nulls there are: I think it is n + 1 where n is the number of nodes.
Now I know a displayed tree is rather different from a binary tree, but you need to check

Do your leaf nodes have null pointers to their supposed children?

Similarly to their user Objects?

Should you be returning null if you pick up one of those leaves,

or should you throw an Exception, to be caught higher up the stack,

or should you do something different?

Have you set a user Object, or put one there by any other means?

And what happened when you printed my is not null test from earlier (line 807½)?

Not sure I can go much rather. Sorry, I have probably suggested all I can. You will have to look harder for that null.
Anybody else, please?

thanks for trying , and by the way ....at line 807 1/2 it prints out to be null
i know the reason ...but i m not getting any way to fix this
that try & catch thing is not working in this case , because null pointer in not checked exception
k fine thanks once again for trying i will look around

[Paul Clapham]

Come on, this isn't rocket science. Really. The simple solution is

if (node != null) {
  obj = node.getUserObject();
}

[Campbell Ritchie]

At least we know it’s null, which means Paul C’s suggestion shouldn’t have taken more than a few seconds to work out. But the try-catch will work just as well for an unchecked Exception.

[naved momin]

Paul Clapham wrote:Come on, this isn't rocket science. Really. The simple solution is

if (node != null) {
  obj = node.getUserObject();
}

thanks paul , after looking at the solution , i definetely laughed at me ,
because i cant figure out this tiny bug
anyways thanks to champbell and you
thank you very much
and i agree this isnt a rocket science ....lolz

[Campbell Ritchie]

You’re welcome