java ArrayList中modCount的作用

import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;

public class Test2 {
    public static void main(String[] args) {
        List list = new ArrayList();
        list.add("a");
        list.add("b");
        list.add("c");
        Iterator iterator = list.iterator();
        while(iterator.hasNext()){
            String str = iterator.next();
            list.remove(str);
        }
    }
}

Exception in thread "main" java.util.ConcurrentModificationException
    at java.util.ArrayList$Itr.checkForComodification(ArrayList.java:909)
    at java.util.ArrayList$Itr.next(ArrayList.java:859)
    at com.design.mode.Test2.main(Test2.java:15)

/**
     * 这里可以看出 迭代器是 ArrayList 的一个内部实现类 典型的迭代器模式
     */
    public Iterator iterator() {
        return new Itr();
    }

    /**
     * An optimized version of AbstractList.Itr
     */
    private class Itr implements Iterator {
        int cursor;       // index of next element to return
        int lastRet = -1; // index of last element returned; -1 if no such
         // 这里将 modCount 赋值给了 expectedModCount
        int expectedModCount = modCount;

        Itr() {}

        public boolean hasNext() {
            return cursor != size;
        }

        @SuppressWarnings("unchecked")
        public E next() {
            checkForComodification();
            int i = cursor;
            if (i >= size)
                throw new NoSuchElementException();
            Object[] elementData = this.elementData;
            if (i >= elementData.length)
                throw new ConcurrentModificationException();
            cursor = i + 1;
            return (E) elementData[lastRet = i];
        }

        public void remove() {
            if (lastRet )
                throw new IllegalStateException();
            checkForComodification();

            try {
                this.remove(lastRet);
                cursor = lastRet;
                lastRet = -1;
                expectedModCount = modCount;
            } catch (IndexOutOfBoundsException ex) {
                throw new ConcurrentModificationException();
            }
        }

        final void checkForComodification() {
            // 若修改后 modCount会变化 会与 expectedModCount 数值不相等
            if (modCount != expectedModCount)
                throw new ConcurrentModificationException();
        }
    }