Really interesting bizarre bug in TreePath, I'm confused beyond belief

Question

Inside a large program, in which I use custom TreeModel instead of DefaultMutableTreeNode's etc, I have a following method which needs to recurse up a TreePath.
This seems to be a MAJOR bug, because if you look in the following ten lines, I verify several times that the argument is non-null (and even return early if it is).
However, at the line "while( ... )" it throws a NullPointerException! What, is the keyword "while" null ???!?!?! Because we know that tp is NOT null.
We also know that we can call "tp.getPathCount()" because I do so and even print it out!#
If you have any thoughts, please CC to [email protected], because at the moment I'm sure Sun will refuse a bug report (without more info) and I doubt I can make a small program that demonstrates this - otherwise surely it would have been found before!
public static Hashtable getMapping( TreePath tp )
System.out.println( "tp = "+tp );
Hashtable rc = new Hashtable();
if( tp.getPathCount() > 0 )
else
return rc;
System.out.println( "tp = "+tp );
int tpcount = tp.getPathCount();
System.out.println( "tp.count = "+tp.getPathCount() );
while( tp.getPathCount() > 0 )
return rc;
}

Answer 1

Oh dear. After prolonged testing, I realised the problem: when the TreePath is the root, it returns null as parent , which happens just before I do the test as to whether to carry on - I had originally put the test just before that, but had moved it to just afterwards, thereby causing the problem!
Very foolish, really.