Alei最近和迭代器较上了劲,之前自以为深究过迭代器,不成想原来是坐井观天,以蠡测海。上文中写的东西哪里算什么深入探究?!但亡羊补牢,犹未迟也,经我多次试验,终于弄懂其中某些精巧机制,闲话少说,我们进入正题。
注意,之后所有的知识点都以 ArrayList 这个容器类为例来进行详细说明
在讨论这个问题之前我们得首先在意两个成员变量:
1、ArrayList 类里继承于 AbstractList 类的成员变量 modCount:
protected transient int modCount = 0;
2、ArrayList 类的私有内部类 Itr 里的成员变量 expectedModCount:
int expectedModCount = modCount;
再看下Itr类源码:
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;
public boolean hasNext() {
return cursor != size;
}
@SuppressWarnings("unchecked")
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];
}
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();
}
}
@Override
@SuppressWarnings("unchecked")
public void forEachRemaining(Consumer<? super E> consumer) {
Objects.requireNonNull(consumer);
final int size = ArrayList.this.size;
int i = cursor;
if (i >= size) {
return;
}
final Object[] elementData = ArrayList.this.elementData;
if (i >= elementData.length) {
throw new ConcurrentModificationException();
}
while (i != size && modCount == expectedModCount) {
consumer.accept((E) elementData[i++]);
}
// update once at end of iteration to reduce heap write traffic
cursor = i;
lastRet = i - 1;
checkForComodification();
}
final void checkForComodification() {
if (modCount != expectedModCount)
throw new ConcurrentModificationException();
}
}
当我们使用 ArrayList 容器的 iterator() 方法后,在栈空间里创建了一个此类特定的迭代器对象,同时将成员变量 modCount 的值赋予成员变量 expectedModCount。知道这个有趣的事情后可以先打住,让我们再来看看 ArrayList 类 remove() 方法的源码:
参数为 int 类型的 remove():
public E remove(int index) {
rangeCheck(index);
modCount++;
E oldValue = elementData(index);
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
return oldValue;
}
参数为 Object 类型的 remove():
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;
}