迭代ArrayList时出现ConcurrentModificationException <Integer>

Question

I'm trying to iterate through a Integer ArrayList and getting the value at each element, but I'm getting an error at the int value = ....

Not sure whats happening. Please advise.

Iterator<Integer> listItr = executeList.iterator(); // iterator for the execute list 
    while (listItr.hasNext()) { // iterate through list and do work!
        int robIndex = listItr.next();
        int timer = fakeRob.exeCountDown(robIndex); // decrement first then return timer
        if (timer == 0) {// check if instr finished execution
            System.out.println("timer expired. fire");
            executeList.remove(executeList.indexOf(robIndex)); // 1. remove instr from exeList
            transitState(robIndex, EX, WB); // 2. transit from EX state to WB state
            int tag = fakeRob.getTag(robIndex); // get producer tag
            regFile.setRdy(tag); // 3a. set register file ready flag for this tag
            fakeRob.wakeUp(tag); // 3b. wake up instructions with this tag
        }
    }

Error:

java.util.ConcurrentModificationException
at java.util.AbstractList$Itr.checkForComodification(AbstractList.java:372)
at java.util.AbstractList$Itr.next(AbstractList.java:343)
at sim.execute(sim.java:180)
at sim.<init>(sim.java:71)
at sim.main(sim.java:270

Thanks,

Hank

Answer 1

If the local variable value is the thing that you "do some stuff with", and you are not modifying the list, then some outside thread is modifying the list while you are in there.

Otherwise, see the link provided by @edalorozo for some ideas

EDIT ADDED

I never use the iterator.remove() idiom, because I never got all that familiar with the iterator idiom. And always mixed it up with the short lived and poorly implemented Enumerator stuff. Before the enhanced for loop, I typically looped using the very old fashioned for (int i-0; i<foo.length; i++) style. And not all Iterators support remove(), so what's the point?

Therefore, I got "used to" the "collect everything and delete it afterwards" style, even when I now use the enhanced for loop. In your code, that would be:

ArrayList <Integer> toBeRemoved = new ArrayList <Integer>();
for (Integer robIndex : executeList) {
   // note, I distrust auto-unboxing, (see all the Java Puzzlers books!)
   // so I'd probably add an explicit unbox here
   int robIndexi = robIndex.intValue();

   int timer = fakeRob.exeCountDown(robIndexi); // decrement first then return timer
   if (timer == 0) {// check if instr finished execution
      toBeRemoved.add(robIndex);
      // all that other stuff here...
   }
}

// remove everything now
executeList.removeAll(toBeRemoved);

Answer 2

Perhaps if you put what it is you're doing inside the loop it would help. If you're trying to remove an element from the list, you need to call listItr.remove() to do it. In general, you should not be calling any functions in the loop which modify the list (ie. add(), set(), etc...).

The following code would trigger this

Iterator<Integer> it = executeList.iterator();
while (it.hasNext()) {
  Integer i = it.next();
  executeList.remove(i);
}

The proper way to do it would be:

Iterator<Integer> it = executeList.iterator();
while (it.hasNext()) {
  Integer i = it.next();
  it.remove();
}

Also other threads (as mentioned above) could be the issue. Remember, iterators are backed by the list itself in all the java supplied collections. So if another thread modifies the list while you'e iterating, you'll run into this.