# ArrayDeque iterator throws "Concurrent Modification Exception"

**URL:** https://discourse.processing.org/t/arraydeque-iterator-throws-concurrent-modification-exception/28055
**Category:** Libraries
**Created:** [February 24, 2021, 2:30pm UTC](https://discourse.processing.org/t/arraydeque-iterator-throws-concurrent-modification-exception/28055 "2021-02-24T14:30:30Z")
**Posts on this page:** 5
**Page:** 1

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### Author: ![mxa](https://avatars.discourse-cdn.com/v4/letter/m/5f8ce5/32.png) [@mxa](https://discourse.processing.org/u/mxa)
#### Post date: [February 24, 2021, 2:30pm UTC](https://discourse.processing.org/t/arraydeque-iterator-throws-concurrent-modification-exception/28055/1 "2021-02-24T14:30:30Z")

</div>

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?

```auto
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();
}

```

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<div class="post-metadata">

### Author: ![Chrisir](https://yyz2.discourse-cdn.com/flex036/user_avatar/discourse.processing.org/chrisir/32/45_2.png) [@Chrisir](https://discourse.processing.org/u/Chrisir)
#### Post date: [February 24, 2021, 2:58pm UTC](https://discourse.processing.org/t/arraydeque-iterator-throws-concurrent-modification-exception/28055/2 "2021-02-24T14:58:06Z")

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> [@mxa](#):
>
> ( `for (int i = 2; i < width-2; i++)` )

Do you

maybe try looping backwards

> [@mxa](#):
>
> ` waveform.remove();`

What does this line do?

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<div class="post-metadata">

### Author: ![mxa](https://avatars.discourse-cdn.com/v4/letter/m/5f8ce5/32.png) [@mxa](https://discourse.processing.org/u/mxa)
#### Post date: [February 24, 2021, 3:27pm UTC](https://discourse.processing.org/t/arraydeque-iterator-throws-concurrent-modification-exception/28055/3 "2021-02-24T15:27:15Z")

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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 )](https://docs.oracle.com/javase/7/docs/api/java/util/ArrayDeque.html)

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<div class="post-metadata">

### Author: ![mxa](https://avatars.discourse-cdn.com/v4/letter/m/5f8ce5/32.png) [@mxa](https://discourse.processing.org/u/mxa)
#### Post date: [February 24, 2021, 9:54pm UTC](https://discourse.processing.org/t/arraydeque-iterator-throws-concurrent-modification-exception/28055/4 "2021-02-24T21:54:37Z")

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I found a “solution” on this page [Iterate through Queue in Java – Techie Delight](https://www.techiedelight.com/iterate-through-queue-java/)  
And modified my code as such:

```auto
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.

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<div class="post-metadata">

### Author: ![mxa](https://avatars.discourse-cdn.com/v4/letter/m/5f8ce5/32.png) [@mxa](https://discourse.processing.org/u/mxa)
#### Post date: [March 16, 2021, 2:23pm UTC](https://discourse.processing.org/t/arraydeque-iterator-throws-concurrent-modification-exception/28055/5 "2021-03-16T14:23:43Z")

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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?