集合的 modCount

AbstractList迭代器认为集合应该返回的修改值,如果和期望的值不一致，那么迭代器检测到了并发修改protected transient int modCount = 0;List迭代时定义了期望的修改值：int expectedModCount = modCount;判定迭代是否结束条件：迭代器的 next()方法在调用时会检测期望值与集合修改值是否一致

AbstractList<E>

定义了一个modCount属性。

       protected transient int modCount = 0; 

解释：

         这个字段被用于  iterator 和  list  iterator ， 如果这个值被意外的修改， 这个迭代器将会抛出一个 

ConcurrentModificationException异常，这提供了 失败快速 行为，在迭代期间面对并发修改。

适应范围： 

           {@code next},  {@code remove}, {@code previous},

           {@code set} or {@code add}

??????ArrayList<E>

add（E e）方法, 对 modCount字段维护，每次新增元素 + 1

 public boolean add(E e) {ensureCapacityInternal(size + 1);  // Increments modCount!!elementData[size++] = e;return true;}private static int calculateCapacity(Object[] elementData, int minCapacity) {if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {return Math.max(DEFAULT_CAPACITY, minCapacity);}return minCapacity;}private void ensureCapacityInternal(int minCapacity) {ensureExplicitCapacity(calculateCapacity(elementData, minCapacity));}private void ensureExplicitCapacity(int minCapacity) {modCount++;// overflow-conscious codeif (minCapacity - elementData.length > 0)grow(minCapacity);}

remove( ) 移除：

public E remove(int index) {rangeCheck(index);modCount++;E oldValue = elementData(index);int numMoved = size - index - 1;if (numMoved > 0)System.arraycopy(elementData, index+1, elementData, index,numMoved);elementData[--size] = null; // clear to let GC do its workreturn oldValue;}

迭代器迭代过程中对 modCount进行检查 

?private class Itr implements Iterator<E> {int cursor;       // index of next element to returnint lastRet = -1; // index of last element returned; -1 if no suchint expectedModCount = modCount;Itr() {}// 判断是否退出迭代的条件public boolean hasNext() {return cursor != size;}@SuppressWarnings("unchecked")public E next() {// 检查modCountcheckForComodification();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() {// 集合的modCount与期望值是否一致if (modCount != expectedModCount)throw new ConcurrentModificationException();}?