今天同事写了几行类似这样的代码：

public static void main(String args[]) {    List
     
       famous = new ArrayList
      
       ();    famous.add("liudehua");    famous.add("madehua");    famous.add("liushishi");    famous.add("tangwei");    for (String s : famous) {        if (s.equals("madehua")) {            famous.remove(s);        }    }}
      
     

运行出异常:

Exception in thread "main" java.util.ConcurrentModificationException

at java.util.AbstractList$Itr.checkForComodification(AbstractList.java:372)

at java.util.AbstractList$Itr.next(AbstractList.java:343)

at com.bes.Test.main(Test.java:15)

Java新手最容易犯的错误，对JAVA集合进行遍历删除时务必要用迭代器。切记。

其实对于如上for循环，运行过程中还是转换成了如下代码：

for(Iterator
     
       it = famous.iterator();it.hasNext();){         String s = it.next();         if(s.equals("madehua")){             famous.remove(s);         }     }
     

仍然采用的是迭代器，但删除操作却用了错误的方法。如将famous.remove(s)改成it.remove()

则运行正常，结果也无误。

当然如果改成:

for (int i = 0; i < famous.size(); i++) {            String s = famous.get(i);            if (s.equals("madehua")) {                famous.remove(s);            }        }

这种方法，也是可以完成功能，但一般也不这么写。

为什么用了迭代码器就不能采用famous.remove(s)操作? 这种因为ArrayList与Iterator混合使用时会导致各自的状态出现不一样，最终出现异常。

我们看一下ArrayList中的Iterator实现:

private class Itr implements Iterator
     
       {   /**    * Index of element to be returned by subsequent call to next.    */   int cursor = 0;   /**    * Index of element returned by most recent call to next or    * previous.  Reset to -1 if this element is deleted by a call    * to remove.    */   int lastRet = -1;   /**    * 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;   public boolean hasNext() {           return cursor != size();   }   public E next() {           checkForComodification();       try {       E next = get(cursor);       lastRet = cursor++;       return next;       } catch (IndexOutOfBoundsException e) {       checkForComodification();       throw new NoSuchElementException();       }   }   public void remove() {       if (lastRet == -1)       throw new IllegalStateException();           checkForComodification();       try {       AbstractList.this.remove(lastRet);       if (lastRet < cursor)           cursor--;       lastRet = -1;       expectedModCount = modCount;       } catch (IndexOutOfBoundsException e) {       throw new ConcurrentModificationException();       }   }   final void checkForComodification() {       if (modCount != expectedModCount)       throw new ConcurrentModificationException();   }   }
     

基本上ArrayList采用size属性来维护自已的状态，而Iterator采用cursor来来维护自已的状态。

当size出现变化时，cursor并不一定能够得到同步，除非这种变化是Iterator主动导致的。

从上面的代码可以看到当Iterator.remove方法导致ArrayList列表发生变化时，他会更新cursor来同步这一变化。但其他方式导致的ArrayList变化，Iterator是无法感知的。ArrayList自然也不会主动通知Iterator们，那将是一个繁重的工作。Iterator到底还是做了努力：为了防止状态不一致可能引发的无法设想的后果，Iterator会经常做checkForComodification检查，以防有变。如果有变，则以异常抛出，所以就出现了上面的异常。