please help in solving: Exception in thread "main" java.lang.nullpointerexception

vkiran_v · #1 08-May-2006, 06:37

hi all,

i'm using the following code and i'm getting
exception in thread "main" java.lang.nullpointerexception at the line which is in bold.

JAVA Code:

public class ProjectDemo
{	
	JTree tree1;
	public ProjectDemo(String title)
	{
		//MENU CREATION CODE HERE
		menuConnect.addActionListener(new ActionListener()
		{
			public void actionPerformed(ActionEvent e)
			{ connectDlg(); }
		});
	       [b]tree1.addTreeSelectionListener(new TreeSelectionListener()[/b]
		{
			public void valueChanged(TreeSelectionEvent e) 
			{//SOME CODE HERE}
		});
	}
	public void connectDlg()
	{
		//SOME DIALOG BOX WITH CONNECT BUTTON
		connButton.addActionListener( new ActionListener()
		{
			public void actionPerformed(ActionEvent e)
			{ populateTree(userTxt.getText()); }
		});
	}
	public void populateTree(String user)
	{//TREE CREATION CODE HERE}
	public static void main(String[] args)
	{ProjectDemo projectDemo = new ProjectDemo("SOLAP Browser");}
}

even though i create 'tree1' in populateTree(), it seems that 'tree1' is again null in the bold line.

can anyone help me please...

LuciWiz · #2 08-May-2006, 07:23

The constructor of the ProjectDemo class will be called before the function populateTree (which I don't even know where in the code you will call). So the tree1 member will be not instantiated, hence the "null pointer" exception.
I suggest you move that piece of code from the function body in the constructor, before using tree1.

Best regards,
Lucian

vkiran_v · #3 08-May-2006, 14:02

as u can see in the code, i'm calling populateTree() in the connectDlg() which triggers when user connects to database.

also i have just declared 'tree1', and all the initialization and adding of root and childs is done in populateTree(). after doing this only the 'tree1' becomes visible. now as i think the treeselectionListener will be triggered only after populateTree() executes, but why do i get the nullPointerException ?

LuciWiz · #4 08-May-2006, 14:40

Quote:

Originally Posted by vkiran_v

as u can see in the code, i'm calling populateTree() in the connectDlg() which triggers when user connects to database.

also i have just declared 'tree1', and all the initialization and adding of root and childs is done in populateTree(). after doing this only the 'tree1' becomes visible. now as i think the treeselectionListener will be triggered only after populateTree() executes, but why do i get the nullPointerException ?

I studied Java some time ago, and I never was a "master" in this programming language, therefore I may be terrible wrong.
However, it seems to me that you will not be able to call a non-static function of the ProjectDemo class (like connectDlg), and will need on instance of it for that. You actually instantiate the class in main:

Quote:

Originally Posted by vkiran_v

JAVA Code:

public static void main(String[] args)
	{ProjectDemo projectDemo = new ProjectDemo("SOLAP Browser");}

This is where the constructor will be called - before the call to the connectDlg function, IMHO.
I respectfully suggest you try my previous suggestion, unless someone else offers more insightful help.

Best regards,
Lucian

TreyAU21 · #5 08-May-2006, 15:28

Quote:

Originally Posted by vkiran_v

JAVA Code:

	JTree tree1;

If your code sample is the whole example, then you first of all should change the visibility of this JTree to private (as is good practice in any class... unless you need it otherwise).

Secondly, though you have declared the variable... that doesn't mean it is instantiated. You need something like this at least...

JAVA Code:

	JTree tree1 = new JTree();

Let me know if that works.

LuciWiz · #6 08-May-2006, 16:02

Quote:

Originally Posted by TreyAU21

Secondly, though you have declared the variable... that doesn't mean it is instantiated. You need something like this at least...

JAVA Code:

	JTree tree1 = new JTree();

Actually, he is doing that, just not in the right place, I argue.

TreyAU21 · #7 09-May-2006, 00:27

Ah yes... good call. Then you are correct in that any use of the tree1 object before it is instantiated will cause a nullpointerexception... and since the tree (according to the poster) is being initialized in the populateTree() function... any use of the object before that will throw this exception.

So, the bolded line in the constructor will always be called before the populateTree() function causing this error. I overlooked that ... sorry

It looks like a redesign is needed. You need to initialize the tree in the constructor OR create a function that creates the TreeSelectionListener and make sure it is called after the tree is populated.

vkiran_v · #8 09-May-2006, 04:59

Thank u for the suggestion u made. i have solved my problem by using DefaultTreeModel.

Anyway i would like to ask one more thing, i.e., if i initialize the tree as u said, i.e.,

JAVA Code:

JTree tree1 = new JTree();

then a sample tree is coming up. at a later moment how can i assign a new tree i have created to the 'tree1'?

TreyAU21 · #9 09-May-2006, 08:06

An object assignment is just like a variable assignment. If you create two integers, and you want to assign one integer the value of the other integer, it would go like this:

JAVA Code:

int x;
int y=4;

x=y; //<---

Similarly, you do the same for objects:

JAVA Code:

Object x;
Object y;

// Do some initialization and manupulation of y and then...

x=y;

It's that simple.

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please help in solving: Exception in thread "main" java.lang.nullpointerexception

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