Early problems with Java on my Macbook.

Question

Hi, I am a former high school computer science student and I'm having problems with Java as I've recently moved from Windows to a Mac. The following is a letter I wrote to my prior computer science teacher but he does not seem to be responding. I think all you need to know is that DrawHouse and JumpingJacks are simple Java programs we made in class that utilize an object called DrawingTool (which is a graphics environment). DrawingTool is contained in gpdraw.jar and also was contained in our class' apcslib.jar. Also, FileInput is a class contained in apcslib.jar.
I basically transferred all my JAR files (apcslib.jar, etc.) from my Windows computer to my Mac. I didn't think anything was wrong or corrupted with them because they're the same JAR files that I used for my Java programs on my Windows comp in high school and when I run those Java programs now, they still work. But for some reason if I try to run, say, DrawHouse or JumpingJacks with those same programs on my Macbook, it gives me the following errors on runtime (it compiles fine).
Exception in thread "main" java.lang.NullPointerException
     at apcslib.DrawingTool.move( DrawingTool.java:456)
     at java.awt.Component.setLocation( Component.java:1705)
     at java.awt.Window.init( Window.java:289)
     at java.awt.Window.<init>( Window.java:318)
     at java.awt.Frame.<init>( Frame.java:419)
     at java.awt.Frame.<init>( Frame.java:384)
     at javax.swing.JFrame.<init>( JFrame.java:150)
     at apcslib.DrawingTool.<init>( DrawingTool.java:45)
     at apcslib.DrawingTool.<init>( DrawingTool.java:106)
     at JumpingJacks.main( JumpingJacks.java:35)
So I thought, maybe I'm not importing the JAR files correctly. But if I import gsdraw.jar or a different apcslib.jar that I downloaded from some random teacher's website on the internet (and that also happens to have DrawingTool in it) then it compiles and runs fine. So I thought, ok, maybe it's a problem with this one particular apcslib.jar that I transferred from my Windows computer. But like I said, DrawHouse and JumpingJacks run fine on my windows comp, and most ridiculous of all, I tried running Tasmania on my Macbook (which uses the FileInput class from your apcslib.jar) and THAT works fine! So to recap, I'm not importing the JAR files wrong and at times there doesn't seem to be anything wrong with the apcslib.jar you gave us and probably not anything wrong with my IDE, seeing that importing of other JAR files works fine (I'm using eclipse and/or xcode, and the same problem occurs with both). Now of course, I could just go ahead and use the apcslib that I downloaded from the internet, but it's very inconvenient since it doesn't match your own apcslib's contents (for example, it doesn't have FileInput). By the way, I've also tried adding the JARS directly to the Java Extensions instead of importing them as external JARS, but that doesn't solve the problem. I've also attached the apcslib.jar in case you think that it's somehow corrupted or something....
Do you have any idea about what could be wrong or where I should go from here?
Thanks for all your help!
Bryan Huh

Answer 1

Thanks for the reply, but if it is a problem with how
I'm using DrawingTool, then shouldn't it run with
errors on my Windows comp too?Maybe, maybe not. If you're doing something that's correct on one but not the other - using file names for Windows on Linux Mac), for instance.
The same EXACT source
code will compile and run fine on my Windows comp,
but run with errors on my Macbook. And if it's a bug
with DrawingTool, then shouldn't my Windows comp also
encounter the problem and run with errors?No. the DT programmer could have done the same thing, using code that's correct on Windows but not on Linux. Or there could be a difference in the Java versions on the couputers and code that's valid in the later version is failing in the earlier, or vice versa.