I am trying to make a sketch that visualizes midi data sent from Ableton Live, using the proMidi library. Every midi note is represented by a little ball that moves across the screen. However, I can't seem to write it in a way as to have it deal well with the real-time and potentially infinite nature of the incoming midi data. I've been trying to do this by using ArrayList and adding objects on the fly, but I keep getting an "IndexOutOfBoundsException" error. Any ideas?

My code:

1. import promidi.*;
   
   MidiIO midiIO;
   
   int notecount = 0;
   
   List<Ball> balls;
   
   void noteOn(
     Note note,
     int deviceNumber,
     int midiChannel
   ){
     pitch = note.getPitch();
    
     notecount = notecount + 1;
   }
   
   void setup() {
     size(640, 200);
     noStroke();
     smooth();
    
     midiIO = MidiIO.getInstance(this);
     println("printPorts of midiIO");
     midiIO.printDevices();
     midiIO.openInput(0,0);
    
     balls = new ArrayList();
    
     for (int i = 0; i < notecount; ++i) {
       Ball freshBall = new Ball(width, pitch, 20);
       balls.add(freshBall);
     }
   }
   
   void draw()
   {
     background(0);
   
     for (int i = 0; i < notenum; i++) {
       Ball ball = (Ball) balls.get(i);
       ball.display();
     }
   }
   
   class Ball {
     float x, y;
    
     Ball(float xin, float yin) {
       x = xin;
       y = yin;
       diameter = din;
   
     }
    
     void display() {
       if (x > -50) {
         x = x - 1;
       }
       fill(255, 204);
       ellipse(x, y, diameter, diameter);
     }
   }