ArrayList有兩種迭代器實現,都是按照索引查找,但比正常for循環多了併發操作的安全校驗:
1. Itr()的實現,主要功能-後序遍歷next()方法
public Iterator<E> iterator() {
return new Itr();
}
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];
}
其中i >= elementData.length的校驗非常疑惑,理論上elementData的擴容的等操作size應該小於elementData.length,不知道在什麼併發場景下會觸發??
public void remove() {
if (lastRet < 0)
throw new IllegalStateException(); // 調用remove()方法前,必須調用next()方法,給lastRet賦值
checkForComodification();
try {
ArrayList.this.remove(lastRet);
cursor = lastRet;
lastRet = -1; // 重置索引位
expectedModCount = modCount; // 重置modify次數,因此在iterator方法中,不能使用remove(object)
} catch (IndexOutOfBoundsException ex) {
throw new ConcurrentModificationException();
}
}
2. ListIterator繼承自Itr類,主要功能擴展-前序遍歷previous(), set()以及add()方法
public ListIterator<E> listIterator() {
return new ListItr(0);
}
public ListIterator<E> listIterator(int index) {
if (index < 0 || index > size) // 前序遍歷第一個元素cursor - 1
throw new IndexOutOfBoundsException("Index: "+index);
return new ListItr(index);
}
ListItr(int index) {
super();
cursor = index; // 初始值範圍 [0, size]
}
public E previous() {
checkForComodification();
int i = cursor - 1; // 對比和Itr迭代的區別
if (i < 0)
throw new NoSuchElementException();
Object[] elementData = ArrayList.this.elementData;
if (i >= elementData.length)
throw new ConcurrentModificationException();
cursor = i;
return (E) elementData[lastRet = i];
}