ConcurrentModificationException only in Java 1.8.0_45

Question

I\'ve got two question about this code:

import java.util.*;

public class TestClass {

    private static List list;   
    public static void          


        

Answer 1

@nbokmans already nailed the general reason why you get that exception. However, it's true that this seems to be version dependant. I'll fill in why you get that in java 8.0_45 but not 1.6.0_45, 1.7.0_79, 1.8.0_5.

This is due to fact that Collections.sort() was changed in java 8.0_20. There's an in-depth article about it here. In the new version, sort, according to the article, is like this:

public void sort(Comparator c) {
  final int expectedModCount = modCount;
  Arrays.sort((E[]) elementData, 0, size, c);
  if (modCount != expectedModCount) {
    throw new ConcurrentModificationException();
  }
  modCount++;
}

Like the article explains:

Contrary to the old Collections.sort, this implementation modifies the modCount of the collection (line 7 above) once the list has been sorted, even if the structure itself didn’t really change (still the same number of elements).

So it will do an internal change even if the collection is already sorted, whereas before that change it didn't do that. That's why you're getting an exception now.

The actual fix is to not to sort a collection using multiple threads at the same time. You shouldn't do that.