java 并发修改异常产生的原因

示例代码

     ArrayList arrayList = new ArrayList();
        arrayList.add("test1");
        arrayList.add("test2");
        arrayList.add("test3");
        Iterator iterator = arrayList.iterator();
        while (iterator.hasNext()) {
            Object next = iterator.next();
            if (next.equals("test1")) {
               arrayList.remove(next);
            }
        }

 

public Iterator iterator() {
        return new 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
        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 = 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();
            }
        }

        @Override
        @SuppressWarnings("unchecked")
        public void forEachRemaining(Consumer consumer) {
            Objects.requireNonNull(consumer);
            final int size = ArrayList.this.size;
            int i = cursor;
            if (i >= size) {
                return;
            }
            final Object[] elementData = ArrayList.this.elementData;
            if (i >= elementData.length) {
                throw new ConcurrentModificationException();
            }
            while (i != size && modCount == expectedModCount) {
                consumer.accept((E) elementData[i++]);
            }
            // update once at end of iteration to reduce heap write traffic
            cursor = i;
            lastRet = i - 1;
            checkForComodification();
        }

        final void checkForComodification() {
            if (modCount != expectedModCount)
                throw new ConcurrentModificationException();
        }
    }

    final void checkForComodification() {
            if (modCount != expectedModCount)
                throw new ConcurrentModificationException();
    }

当要删除的元素在集合的倒数第二个位置的时候,不会产生并发修改异常

因为在调用  public boolean hasNext() { 的时候,光标的值和集合的长度一样,此时就会发回false,因此就不会再调用next( ) 方法
                      return cursor != size;
                     }

获取集合的元素,  既然不会调用next() 方法 ,那么底层也就不会抛出 ConcurrentModificationException 的异常