迭代器经常会出现问题，因为开发人员往往不知道它是如何工作的。下面的代码来源是ArrayList的源码：

最常见的问题是抛出java.util.ConcurrentModificationException异常。这异常实际通常是由remove方法抛出。当remove()调用是应该在 next()调用之后调用。如果remove()在next()调用之前调用，arraylist 的容量改变了，modCount != expectedModCount条件不满足，ConcurrentModificationException 异常就产生了。

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
...
 
public Iterator<E> iterator() {
    return new Itr();
}
 
/**
 * An optimized version of AbstractList.Itr
 */
private class Itr implements Iterator<E> {
    int cursor;       // index of next element to return
    int lastRet = -1; // index of last element returned; -1 if no such
    int expectedModCount = modCount;
 
    public boolean hasNext() {
        return cursor != size;
    }
 
    @SuppressWarnings("unchecked")
    public E next() {
        checkForComodification();
        int i = cursor;
        if (i >= size)
            throw new NoSuchElementException();
        Object[] elementData = ArrayList.this.elementData;
        if (i >= elementData.length)
            throw new ConcurrentModificationException();
        cursor = i + 1;
        return (E) elementData[lastRet = i];
    }
 
    public void remove() {
        if (lastRet < 0)
            throw new IllegalStateException();
        checkForComodification();
 
        try {
            ArrayList.this.remove(lastRet);
            cursor = lastRet;
            lastRet = -1;
            expectedModCount = modCount;
        } catch (IndexOutOfBoundsException ex) {
            throw new ConcurrentModificationException();
        }
    }
 
    final void checkForComodification() {
        if (modCount != expectedModCount)
            throw new ConcurrentModificationException();
    }
}
 
...

参考文档