foreach与iterator总结

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我是靠谱客的博主完美啤酒，这篇文章主要介绍foreach与iterator总结，现在分享给大家，希望可以做个参考。

1.foreach

执行下面这段代码的时候会抛出异常

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public void testForeach() {
List<Integer> list = new ArrayList<>();
for (int i = 0; i < 10; i++) {
list.add(i);
}
//foreach
for (Integer integer : list) {
if (integer == 6) {
list.remove(integer);
}
}
}

java.util.ConcurrentModificationException
at java.util.ArrayList$Itr.checkForComodification(ArrayList.java:901)
at java.util.ArrayList$Itr.next(ArrayList.java:851)

然后跟踪到错误源，发现ArrayList中的iterator中有expectedModCount和modCount两个值，初始化的时候默认相等。

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public Iterator<E> iterator() {
return new Itr();
}
private class Itr implements Iterator<E> {
int cursor;
// index of next element to return
int lastRet = -1; // index of last element returned; -1 if no such
int expectedModCount = modCount;
...
}

执行到list.remove(integer)的时候，remove方法进入到fastRemove方法，然后modCount的值改变了。

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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; // clear to let GC do its work
}

继续循环遍历的时候会用到iterator中的next方法，其中checkForComodification()会检查expectedModCount和modCount的值是不是相等，不相等则抛出异常。

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public E next() {
checkForComodification();
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];
}
final void checkForComodification() {
if (modCount != expectedModCount)
throw new ConcurrentModificationException();
}

2.iterator

执行下面这段代码，输出结果为9。

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public void testIterator() {
List<Integer> list = new ArrayList<>();
for (int i = 0; i < 10; i++) {
list.add(i);
}
//Iterator
Iterator iterator = list.iterator();
while (iterator.hasNext()) {
Integer integer = (Integer) iterator.next();
if (integer == 6) {
iterator.remove();
}
}
System.out.println(list.size());
}

查看源码，当使用iterator.remove()的时候，删除完后会执行expectedModCount = modCount，所以不会抛出异常。

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private class Itr implements Iterator<E> {
...
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();
}
...
}