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。