面试题

阅读更多
对JAVA集合进行遍历删除时务必要用迭代器

public static void main(String args[]) {
    List famous = new ArrayList();
    famous.add("haha");
    famous.add("hehe");
    famous.add("xixi");
    famous.add("heihei");
    for (String s : famous) {
        if (s.equals("haha")) {
            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("haha")){
             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("haha")) {
                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。

你可能感兴趣的:(面试,java)