Java – do not modify the list, but still get concurrentmodificationexception

This is not a typical concurrent modificationexception problem until you look at another concurrent modificationexception problem

I understand concurrent modificationexceptions, but I don't understand why I get one in the following code In the following 'for', it seems that the iterator continues to exist outside the for loop (using the debugger in eclipse, I can see that the iterator retains the modcount value of its JList when restarting the loop.)

public class ForEachTest {
  public static void main(String[] args) {
    List<Integer> zList = new LinkedList<Integer>();
    List<Integer> jList = new LinkedList<Integer>();
    jList.add(1);

    while (true) {
      for (Integer j : jList) {
        zList.add(17);
        System.out.println(j);
      }
      jList = zList;
    }
  }
}

At first I thought it might be a syntax sugar problem, but I rewritten it with an explicit iterator and ran the same behavior

Finally, I can rewrite for by dividing it into a period of time and performing a separate initialization step, so I don't just want it to be rewritten to make it work But I want to know what the problem here is If possible, I prefer to use the for - each syntax

Solution

Look at your cycle:

while (true) {
  for (Integer j : jList) {
    zList.add(17);
    System.out.println(j);
  }
  jList = zList;
}

In the second iteration of the external loop, JList and zlist refer to the same list (because JList = zlist; statement)... So when you add zlist to the internal loop, you are modifying the list. You are an iteration Bang