java.util.ConcurrentModificationException while mutating an object

Question

I am iterating over a List of CustomObject and while doing this iteration, I am mutating this object by adding a tag to tags list of this custom object. I am not adding or removing any CustomObject to or from customObjects (List). I am still getting java.util.ConcurrentModifcationException.

public class CustomObject {
    private List<Tag> tags;
    // Getter for Tag
    // Setter for tag
}

public class DummyClass {


  List<CustomObject> addMissingTag(List<CustomObject> customObjects) {
    for (CustomObject object:customObjects) { // line 5
      // following method adds a tag to tags list of CustomObject
      mutateObjectByAddingField(object); // line 7
      // Some Code      
    }
    return customObjects;
  }

  void mutateObjectByAddingField(CustomObject customObject) {//line 13
    Boolean found = false;
    for (Tag tag:customObject.getTags()) { // line 15
      if ("DummyKey".equalsIgnoreCase(tag.getKey())) {
        found = true;
        break;
      }
    }
    if (!found) {
      Tag tag = new Tag("DummyKey", "false");
      List<Tag> tags = customObject.getTags();
      tags.add(tag);
      customObject.setTags(tags);
    }
  }

}

Here is the stack trace:

java.util.ConcurrentModificationException: null
    at java.util.ArrayList$Itr.checkForComodification(ArrayList.java:901) ~[?:1.8.0_131]
    at java.util.ArrayList$Itr.next(ArrayList.java:851) ~[?:1.8.0_131]
    at DummyClass.mutateObjectByAddingField(DummyClass.java:15)
    at DummyClass.addMissingTag(DummyClass.java:7)

Does this mean we can get ConcurrentModifcationException even if we just try to modify the object and not remove or add from/to the list/collection?

@Ismail: I updated the question with method's implementation — Mr Matrix
– Mr Matrix, Commented Sep 11, 2020 at 22:25
None of the code you’ve shown appears to be capable of causing a ConcurrentModificationException. Please create a minimal reproducible example that demonstrates the problem you’re having. — VGR
– VGR, Commented Sep 11, 2020 at 22:29
Your code shows no clue about the cause of the problem. This code works well. Could you show us the complete stacktrace? — Ismail
– Ismail, Commented Sep 11, 2020 at 22:32

Ismail · Accepted Answer · 2020-09-11 23:45:17Z

1

First, you are using a List type in the for loop to iterate the elements, so the enhanced for statement is equivalent to a for using an iterator, as mentioned here, because List implements Iterator. Also, it's obvious from the stack trace.

When using Iterator, you can't make modifications on the list you are iterating, as mentioned GeeksForGeeks, you will get ConcurrentModificationException.

So, to solve this issue, you can for example implement explicitly the for loop using integers like below:

 for (int i=0;i < customObjects.length(); i++) {

      CustomObject object = customObjects.get(i);

      // following method adds a tag to tags list of CustomObject
      mutateObjectByAddingField(object);
      // Some Code      
 }

edited Sep 11, 2020 at 23:45

answered Sep 11, 2020 at 23:36

Ismail

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3 Comments

Mr Matrix Over a year ago

This link does a good job of explaining differences between a traditional for loop and an iterator based for loop: stackoverflow.com/questions/33582050/… One of the main advantage of using traditional for loop is that it allows you to modify the list and this is exactly what I needed.

Ismail Over a year ago

Good, if the above lines answers your question, mark it as the right one to let other people know about it. If the answer need more detail, just edit it.

Mr Matrix Over a year ago

Thanks @Ismail, I will do that after validating it during my testing. But this seems to be the correct solution.

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