java java.util.ConcurrentModificationException 原因以及解決方案

public static void main(String[] args)
 {
  List<String> set = new ArrayList<String>();
  set.add("a10001");
  set.add("a10002");
  set.add("a10003");
  set.add("a10004");
  set.add("a10005");
  set.add("a10006");
  set.add("a10007");
  
  List<String> del = new ArrayList<String>();
  del.add("a10003");
  del.add("a10004");
  del.add("a10005");
  
  for(String str : set)
  {
   if(del.contains(str))
   {
    set.remove(str);
   }
  }
 }

Exception in thread "main" java.util.ConcurrentModificationException
 at java.util.AbstractList$Itr.checkForComodification(Unknown Source)
 at java.util.AbstractList$Itr.next(Unknown Source)
 at com.debug.Debug.main(Debug.java:28)

大家都知道for(String str : set)　這句話實際上是用到了集合的iterator() 方法

在iterator的時候是產生了一個List的內部類Itr

JDK源碼

public Iterator<E> iterator() {
　　 return new Itr();
}

在new Itr()時有一個關鍵性的操作　

/**
  * The modCount value that the iterator believes that the backing
  * List should have.  If this expectation is violated, the iterator
  * has detected concurrent modification.
  */
 int expectedModCount = modCount;

 

再回頭看一下List　的　remove方法

 public boolean remove(Object o) {
 if (o == null) {
            for (int index = 0; index < size; index++)
  if (elementData[index] == null) {
      fastRemove(index);
      return true;
  }
 } else {
     for (int index = 0; index < size; index++)
  if (o.equals(elementData[index])) {
      fastRemove(index);
      return true;
  }
        }
 return false;
    }
 
private void fastRemove(int index) {
        modCount++;
        int numMoved = size - index - 1;
        if (numMoved > 0)
            System.arraycopy(elementData, index+1, elementData, index,
                             numMoved);
        elementData[--size] = null; // Let gc do its work
    }
 

再看一下　iterator.next()操作

public E next() {
            checkForComodification();
     try {
  E next = get(cursor);
  lastRet = cursor++;
  return next;
     } catch (IndexOutOfBoundsException e) {
  checkForComodification();
  throw new NoSuchElementException();
     }
 }
 
final void checkForComodification() {
     if (modCount != expectedModCount)
  throw new ConcurrentModificationException();
 }
 

相信看到這兒大家已經應該明白了為什麼會出現在這個異常了。

總結：

　　iterator 時　將expectedModCount = modCount　在remove()時　modCount++　在next()時

 if (modCount != expectedModCount)
  throw new ConcurrentModificationException();

 

一旦刪除或添加元素後　modCount ，expectedModCount　這兩個值就會不一致　當next時就會報ConcurrentModificationException

 

了解了原由，解決方案就很簡單了　在遍曆時用一個集合存放要刪除的對象　在遍曆完後　調用removeAll(Collection<?> c)　就OK了。