Multithreaded graphics applet gets NullPointerException

Question

Hi!
This is a complex issue and the code's big, so i'll try
to explain the main part.
I have five classes.
AppletClass (this one extends Applet)
GraphicItem (just a little sprite which can draw itself)
Thread1 (draws a bunch of GraphicItems in background on a Graphics which is set by setg() in this class)
Thread2 (draw a random clolor text line in background
on a Graphics which is set by setg() in this class)
Drawer (a thread that draws an image set in
the constructor ona graphics which is set by setg() in this class)
What is it for: Well, this is a dubble buffering example,
where two threads draw into one image which works
as a buffer and then another thread draw that image
into the applet area.
The problem is that when Thread1 in run() calls
gitem.draw(g) where g is Graphics i sometimes
get NullPointerException, also, it happenes in some
other place but i cannot pinpoint it. Also,it happenes
always when i restart applet w/o reloading in
JBuilder (nor the browser).
The thing is that i do not use synchronized or
imageUpdate() in the Drawer (imageUpdates() does
nothing), so it is possible for two or more object to
have access to the same Graphics object at the same
time. Also, while Drawer draws the image, the other
thread are drawing onto the image. So, there might
be a conflict, i think. The other other is that when
i comment out all Thread.sleep() the image get
clobbered. That is, the GraphicItem is a cross of
random color, so i shoudl see thousands of crosses of
random color, but i thousands of crosses of pretty
much the same color as it seems like Random is not
working or drawing is broken.
Anyway, any idea how to properly implement such
thing? What precautions shoudl i take and which
conflicts and how should i resolve?
And, of course, in what cases NullPointerException is
generated? I mean, what exxctly does it mean and
when usually occurs?
PS: Don't you think it is weird to get such
exception in a language which officially
has no pointers? :)

Answer 1

Why it SOUNDS like is the Image you are drawing may not be completly loaded yet.. so the first few times it's blitted ( and possibly if it gets bumped out of memory ) attempting to draw it will throw a null pointer.
I solved that in my own code by using a MediaTracker on the image when it was created to make sure that it is loaded and ready to be draw immediatly.