Hi all,

I'm working on a project with bluetooth communication with a 'robot' (conducted by an arduino microcontroller).
I would like the application (with processing) to keep a log of the activities of the robot.
This log would show images of the activities.

Now, I've made another program to implement the 'tactic' that I'd use.
This is a program with 4 buttons, and when these buttons are clicked, the program should show a log (in a rectangle) with the buttons that have been pressed.
I've thought of some possibilities to do this. I think this is the right one. But I've been having a 'nullPointerException' Error.
I, first of all, don't know what this error means and why this program won't work.

Here you have the code:
(The images that I use, can be downloaded at: https://docs.google.com/open?id=0B1oVMH7b0UhfM3paXzFKbXBKN2s )
I've highlighted the part, where I think it goes wrong.

1. color currentcolor;
2. 
   RectButton rect1, rect2, rect3, rect4, rect5, rect6;
   color achtergrond = color(102);
   
   PImage rood;
   PImage blauw;
   PImage groen;
   PImage geel;
   PImage niets;
   
   int xlogboek = 255;
   int ylogboek = 25;
   
   boolean locked = false;
   
   PImage[] logboek = new PImage[23];
   int teller;
   
   
   
   void setup()
   {
     for(int x = 0 ; x < 23 ; x = x + 1) {
     logboek[x] = niets;
     }
     teller = 1;
     size(400, 200);
     smooth();
     rood = loadImage("rood.jpg");
     blauw = loadImage("blauw.jpg");
     groen = loadImage("groen.jpg");
     geel = loadImage("geel.jpg");
     niets = loadImage("niets.jpg");
   
     color baseColor = color(102);
     currentcolor = baseColor;
    
     color buttoncolor = color(255,0,0);
     color highlight = color(178,34,34);
     rect1 = new RectButton(80, 20, 40, buttoncolor, highlight);
   
     buttoncolor = color(0,255,0);
     highlight = color(34,178,34); 
     rect2 = new RectButton(80, 70, 40, buttoncolor, highlight);
    
     buttoncolor = color(0,0,255);
     highlight = color(34,34,178); 
     rect3 = new RectButton(30, 70, 40, buttoncolor, highlight);
    
     buttoncolor = color(255,255,0);
     highlight = color(178,178,34); 
     rect4 = new RectButton(130, 70, 40, buttoncolor, highlight);
    
     buttoncolor = color(255,255,255);
     highlight = color(221,221,221); 
     rect5 = new RectButton(55, 140, 40, buttoncolor, highlight);
    
     buttoncolor = color(0,0,0);
     highlight = color(34,34,34); 
     rect6 = new RectButton(105, 140, 40, buttoncolor, highlight);
   }
   
   void draw()
   {
     background(currentcolor);
     stroke(255);
     rect(250,20,100,160);
    
     update(mouseX, mouseY);
     rect1.display();
     rect2.display();
     rect3.display();
     rect4.display();
     rect5.display();
     rect6.display();
    
    
   }
   
   void update(int x, int y)
   {
     if(locked == false) {
       rect1.update();
       rect2.update();
       rect3.update();
       rect4.update();
       rect5.update();
       rect6.update();
     }
     else {
       locked = false;
     }
   
    
     if (rect1.pressed() || rect2.pressed() || rect3.pressed() || rect4.pressed()) {
       currentcolor = achtergrond;
       if(rect1.pressed() && mousePressed) {
         currentcolor = rect1.basecolor;
         rood();
         }
       else if(rect2.pressed() && mousePressed) {
         currentcolor = rect2.basecolor;
         groen();
         }
       else if(rect3.pressed() && mousePressed) {
         currentcolor = rect3.basecolor;
         blauw();
         }
       else if(rect4.pressed() && mousePressed) {
         currentcolor = rect4.basecolor;
         geel();
         }
     }
     else if(rect5.pressed() || rect6.pressed()) {
       if(rect5.pressed() && mousePressed){
         currentcolor = rect5.basecolor;
       }
       else if(rect6.pressed() && mousePressed){
         currentcolor = rect6.basecolor;
       }   
     }
     else if(( keyCode == UP || keyCode == DOWN || keyCode == RIGHT || keyCode == LEFT )) {
       currentcolor = achtergrond;
       if(keyCode == UP){
         if(keyPressed == true) {
           currentcolor = color(255,0,0);
           rect1.basecolor = color(178,34,34);
           rood();
         }
       }
       else if(keyCode == DOWN){
         if(keyPressed == true) {
           currentcolor = color(0,255,0);
           rect2.basecolor = color(34,178,34);
           groen();
         }
       }
       else if(keyCode == LEFT){
         if(keyPressed == true) {
           currentcolor = color(0,0,255);
           rect3.basecolor = color(34,34,178);
           blauw();
         }
       }
       else if(keyCode == RIGHT){
         if(keyPressed == true) {
           currentcolor = color(255,255,0);
           rect4.basecolor = color(178,178,34);
           geel();
         }
       }
     }
     if(keyPressed == false) {
       keyCode = 0;
       rect1.basecolor = color(255,0,0);
       rect2.basecolor = color(0,255,0);
       rect3.basecolor = color(0,0,255);
       rect4.basecolor = color(255,255,0);
     }
    
    
     for(int i = 0 ; i < 23 ;  i = i + 1) {
         image(logboek[i] , xlogboek , ylogboek);
         if (logboek[i] != niets) {
           if(xlogboek == 330) {
             ylogboek = ylogboek + 25;
             xlogboek = 230;
           }
           xlogboek = xlogboek + 25;
       }
     }
      
   }
   
   
   void rood() {
         logboek[teller] = rood;
         teller = teller + 1;
         if(teller == 23){
           teller = 0;
         }
   }
   
   void groen() {
         logboek[teller] = groen;
         teller = teller + 1;
         if(teller == 23){
           teller = 0;
         }
   }
   
   void blauw() {
         logboek[teller] = blauw;
         teller = teller + 1;
         if(teller == 23){
           teller = 0;
         }
   }
   
   void geel() {
         logboek[teller] = geel;
         teller = teller + 1;
         if(teller == 23){
           teller = 0;
         }
   }
   
   class Button
   {
     int x, y;
     int size;
     color basecolor, highlightcolor;
     color currentcolor;
     boolean over = false;
     boolean pressed = false;  
   
     void update()
     {
       if(over()) {
         currentcolor = highlightcolor;
       }
       else {
         currentcolor = basecolor;
       }
     }
   
     boolean pressed()
     {
       if(over) {
         locked = false;
         return true;
       }
       else {
         locked = false;
         return false;
       }   
     }
   
     boolean over()
     {
       return true;
     }
   
     boolean overRect(int x, int y, int width, int height)
     {
       if (mouseX >= x && mouseX <= x+width &&
         mouseY >= y && mouseY <= y+height) {
         return true;
       }
       else {
         return false;
       }
     }
   }
   
   
   class RectButton extends Button
   {
     RectButton(int ix, int iy, int isize, color icolor, color ihighlight)
     {
       x = ix;
       y = iy;
       size = isize;
       basecolor = icolor;
       highlightcolor = ihighlight;
       currentcolor = basecolor;
     }
   
     boolean over()
     {
       if( overRect(x, y, size, size) ) {
         over = true;
         return true;
       }
       else {
         over = false;
         return false;
       }
     }
   
     void display()
     {
       stroke(255);
       fill(currentcolor);
       rect(x, y, size, size);
     }
   }