ArrayDeque iterator throws "Concurrent Modification Exception"

Processing Libraries
1

I’m sending values from another application (Pure Data) to Processing via OSC. That data is then visualized in Processing by scrolling by bars like a waveform using an ArrayDeque. It seems to run fine, except after a few minutes running I’m getting ConcurrentModificationException suggesting that I access data somwhere whil it is being deleted or written to. That’s surprising to me as it sounds like I’m running a threaded program, something I’m not aware of.
I’ve tried to remedy that by not drawing (and though accessing the first and last two items of the deque (for (int i = 2; i < width-2; i++)) but that did not help. As another try, I set up a blocking boolean while the OSC data is incoming, so that while the deque is written the read access is paused (boolean block_drawing). That also did not change the fact that I’m getting the ConcurrentModificationException error after a while.
Anyone has an idea what else to try?

import java.util.Queue;
import java.util.ArrayDeque;
import java.util.Iterator;
import oscP5.*;
import netP5.*;
OscP5 oscP5;

float WAVELEVEL = 0;
Queue<Integer> waveform = new ArrayDeque<Integer>();
boolean block_drawing = false;

void setup() {
  size(640, 360);
  oscP5 = new OscP5(this,12000);
  for (int i=0; i<width; i++) {
    waveform.add(int(0));
  }
}

void draw() {
    background(255);
    if (block_drawing == false){
      draw_waveform();
    }
    circle(mouseX,mouseY,20);
}

void oscEvent(OscMessage theOscMessage) {
  if(theOscMessage.checkAddrPattern("/wavelevel")==true) {
    if(theOscMessage.checkTypetag("f")) {
       float wavelevel = theOscMessage.get(0).floatValue();
       WAVELEVEL = wavelevel;
       block_drawing = true;
       waveform.remove();
       waveform.add(int(WAVELEVEL*(height/2)));
       block_drawing = false;
       return;
    }
  }
}

void draw_waveform(){
  pushMatrix();
  translate(0,height/2);
  Iterator<Integer> it = waveform.iterator();
  for (int i = 2; i < width-2; i++) {
    strokeWeight(2);
    stroke(50);
    int wave = it.next();
    line(i, wave, i, -wave);
  }
  popMatrix();
}
2

Do you

maybe try looping backwards

What does this line do?

3

Looping backwards will scroll my waveform from left to right… not ideal visually.

remove() Retrieves and removes the head of the queue represented by this deque.
See: ArrayDeque (Java Platform SE 7 )

4

I found a “solution” on this page Iterate through Queue in Java – Techie Delight
And modified my code as such:

import java.util.ConcurrentModificationException;
import java.util.Queue;
import java.util.ArrayDeque;
import java.util.Iterator;
import oscP5.*;
import netP5.*;
OscP5 oscP5;

float WAVELEVEL = 0;
Queue<Integer> waveform = new ArrayDeque<Integer>();
boolean block_drawing = false;

void setup() {
  //frameRate(50);
  size(640, 360);
  oscP5 = new OscP5(this,12000);
  for (int i=0; i<width; i++) {
    waveform.add(int(0));
  }
}

void draw() {
    background(255);
    if (block_drawing == false){
      draw_waveform();
    }
    circle(mouseX,mouseY,20);
}

void oscEvent(OscMessage theOscMessage) {
  if(theOscMessage.checkAddrPattern("/wavelevel")==true) {
    if(theOscMessage.checkTypetag("f")) {
       float wavelevel = theOscMessage.get(0).floatValue();
       WAVELEVEL = wavelevel;
       block_drawing = true;
       waveform.remove();
       waveform.add(int(WAVELEVEL*(height/2)));
       block_drawing = false;
       return;
    }
  }
}

void draw_waveform(){
  pushMatrix();
  translate(0,height/2);
  try{
    Iterator<Integer> it = waveform.iterator();
    int i = 0;
    while (it.hasNext()){
      strokeWeight(2);
      stroke(50);
      int wave = it.next();
      line(i, wave, i, -wave);
      i++;
    }
  }
   catch (ConcurrentModificationException ex) {
            System.out.println(ex);
   }
  popMatrix();
}

It’s not particularly beautiful, in essence I just catch the error and ignore it, so the program can go on. Solves my issue for the moment but I gladly accept a real solution.

1 Like
5

The rendering is not 100% smooth, there is a slight stutter. It’s not very strong, but noticeable. I am wondering if there is a way to make sure it is always smooth. I’m thinking of rendering it as a texture and then moving the object it is rendered on smoothly… or something like that. I also wonder how to have more parameters, like for example changing the color of the waveform for each bar. How to get more parameters/dimensions into the ArrayDeque?