I wrote a (working) Connect 4 program in Processing and am now trying to translate it over into Java via NetBeans. Yes, I am a student who's a Java newbie. Mostly everything has gone through ok, but for some strange reason I'm getting a NullPointerException when I run the program. There's a problem with the images being called from another class but I honestly can't figure out what the problem is. If anyone could help me that would be FANTASTIC.

The specific problem can be found in drawgamePiece() under the gamePiece class. The images are: image(Main.redPiece, xPos, yPos) and  image(Main.bluePiece, xPos, yPos). When I comment these two lines out the program works, but when I uncomment them it gives me the null pointer. Here's the Java/Processing program I have so far:

http://dm.lcc.gatech.edu/~jpiette3/Project_6.zip

Hi,

merc, in your case it really does not make sense for your gamePiece class to extend PApplet. But, unlike antiplastik stated, it is certainly possible to have several papplets in an application. But again, it does not make sense in you app.

Instead of extending PApplet or having inner classes, you better pass a reference of your PApplet to the gamePiece like so:
pieces.add(new gamePiece(true, this));

and in your gamePiece class would look something like this:
(so whenever you need to draw or other PApplet functionality like mouseX, put a 'parent.' in front of it. makes sense?

(...)
public class gamePiece {
   
   PApplet parent;
   float xPos, yPos;
   int r=0;
   boolean drop, hold, dropped;
   boolean red;
   
   gamePiece(boolean red1, PApplet parent){
       xPos=parent.mouseX;
       yPos=0;
       this.red = red1;
       this.parent = parent;
       hold = true;
   }
   
   public void drawgamePiece(){
       if(hold){
           holdGamePiece();
       }
       if( red ==true){
           parent.image(parent.redPiece, xPos, yPos);
       } else{
           parent.image(parent.bluePiece, xPos, yPos);
       }
   }
(...)