/**
* The number of times this list has been structurally modified.Structural modifications are those that change the size of the list,
* or otherwise perturb it in such a fashion that iterations in progress may yield incorrect results.
*
* This field is used by the iterator and list iterator implementation returned by the {@code iterator} and {@code listIterator} methods.
* If the value of this field changes unexpectedly, the iterator (or list iterator) will throw a {@code ConcurrentModificationException} in
* response to the {@code next}, {@code remove}, {@code previous}, {@code set} or {@code add} operations. This provides
* fail-fast behavior, rather than non-deterministic behavior in the face of concurrent modification during iteration.
*
*
Use of this field by subclasses is optional. If a subclass wishes to provide fail-fast iterators (and list iterators), then it
* merely has to increment this field in its {@code add(int, E)} and {@code remove(int)} methods (and any other methods that it overrides
* that result in structural modifications to the list). A single call to {@code add(int, E)} or {@code remove(int)} must add no more than
* one to this field, or the iterators (and list iterators) will throw bogus {@code ConcurrentModificationExceptions}. If an implementation
* does not wish to provide fail-fast iterators, this field may be ignored.
*/
protected transient int modCount = 0;
public void deleteTest(){
List list = new ArrayList();
list.add("aaaaaa");
list.add("bbbbbb");
list.add("cccccc");
list.add("dddddd");
list.add("eeeeee");
Iterator it = list.iterator();
int i = 0;
String s = null;
while(it.hasNext()){
if(i==2){
it.remove();// 如果用list.remove(it.next());会报异常
}
System.out.println("第"+i+"个元素"+it.next());
i++ ;
}
System.out.println("----------------");
Iterator it2 = list.iterator();
while(it2.hasNext()){
System.out.println(it2.next());
}
}
注意：第14行，如果用list.remove(it.next());会报ConcurrentModificationException异常，原因参上。
public Iterator iterator() {
return new Itr();
}
返回的是一个Itr对象，这个Itr是ArrayList的内部类
private class Itr implements Iterator {
int cursor; // index of next element to return //下一个元素的游标
int lastRet = -1; // index of last element returned; -1 if no such //上一个元素的
int expectedModCount = modCount; //修改计数器期望值
public boolean hasNext() {
return cursor != size;
}
@SuppressWarnings("unchecked")
public E next() {
checkForComodification();
int i = cursor; //此时的游标，指向的是本次要遍历的对象，因为上一次已经++了，初始值为0，没有++的情况下是第一个元素
if (i >= size) //越界了
throw new NoSuchElementException();
Object[] elementData = ArrayList.this.elementData;
if (i >= elementData.length)
throw new ConcurrentModificationException();
cursor = i + 1; //游标指向了下一个元素, 但 i 的值没有变
return (E) elementData[lastRet = i]; //将 i 赋值给lastRet，取的值是方法开始时int i=cursor；中的cursor指向的值，而且最终这个游标的数值赋值给了lastRet
}
public void remove() {
if (lastRet < 0) // 如果没有next()操作就直接remove的话，lastRet=-1，会抛异常
throw new IllegalStateException();
checkForComodification();
try {
ArrayList.this.remove(lastRet); // remove之前，cursor、lastRet的值没有修改，都是上次next之后的值，因此此处的lastRet指向上次next获取的元素
cursor = lastRet;
lastRet = -1;
expectedModCount = modCount; // 手动将ArrayList.remove()后modCount的值赋给expectedModCount，避免引起不一致
} catch (IndexOutOfBoundsException ex) {
throw new ConcurrentModificationException();
}
}
@Override
@SuppressWarnings("unchecked")
public void forEachRemaining(Consumer 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();
}
}