實現(xiàn)要明白iterator的好處都有撒？
舉個例子：
現(xiàn)在有一個 list 里邊兒有5個字符串 分別是 “a“ ”b” "c" "b" "a"
現(xiàn)在需要刪除list里的字符串 “b”
使用 for(int i = 0; i < list.size(); i++) 去做就比較麻煩
直接使用iterator就能輕松實現(xiàn)
以下為ArrayList中iterator的內(nèi)部類Itr及ListItr實現(xiàn)
/**
* Returns a list iterator over the elements in this list (in proper
* sequence), starting at the specified position in the list.
* The specified index indicates the first element that would be
* returned by an initial call to {@link ListIterator#next next}.
* An initial call to {@link ListIterator#previous previous} would
* return the element with the specified index minus one.
*
*
The returned list iterator is fail-fast.
*
* @throws IndexOutOfBoundsException {@inheritDoc}
*/
public ListIteratorlistIterator(int index) {
if (index <0 || index >size)
throw new IndexOutOfBoundsException("Index: "+index);
? ? return new ListItr(index);
}
/**
* Returns a list iterator over the elements in this list (in proper
* sequence).
*
*
The returned list iterator is fail-fast.
*
* @see #listIterator(int)
*/
public ListIteratorlistIterator() {
return new ListItr(0);
}
/**
* Returns an iterator over the elements in this list in proper sequence.
*
*
The returned iterator is fail-fast.
*
* @return an iterator over the elements in this list in proper sequence
*/
public Iteratoriterator() {
return new Itr();
}
/**
* An optimized version of AbstractList.Itr
*/
private class Itrimplements Iterator {
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 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();
? ? }
}
/**
* An optimized version of AbstractList.ListItr
*/
private class ListItrextends Itrimplements ListIterator {
ListItr(int index) {
super();
? ? ? ? cursor = index;
? ? }
public boolean hasPrevious() {
return cursor !=0;
? ? }
public int nextIndex() {
return cursor;
? ? }
public int previousIndex() {
return cursor -1;
? ? }
@SuppressWarnings("unchecked")
public E previous() {
checkForComodification();
? ? ? ? int i =cursor -1;
? ? ? ? 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];
? ? }
public void set(E e) {
if (lastRet <0)
throw new IllegalStateException();
? ? ? ? checkForComodification();
? ? ? ? try {
ArrayList.this.set(lastRet, e);
? ? ? ? }catch (IndexOutOfBoundsException ex) {
throw new ConcurrentModificationException();
? ? ? ? }
}
public void add(E e) {
checkForComodification();
? ? ? ? try {
int i =cursor;
? ? ? ? ? ? ArrayList.this.add(i, e);
? ? ? ? ? ? cursor = i +1;
? ? ? ? ? ? lastRet = -1;
? ? ? ? ? ? expectedModCount =modCount;
? ? ? ? }catch (IndexOutOfBoundsException ex) {
throw new ConcurrentModificationException();
? ? ? ? }
}
}
