实现要明白iterator的好处都有撒？
举个例子：
现在有一个 list 里边儿有5个字符串 分别是 “a“ ”b” "c" "b" "a"
现在需要删除list里的字符串 “b”
使用 for(int i = 0; i < list.size(); i++) 去做就比较麻烦
直接使用iterator就能轻松实现
以下为ArrayList中iterator的内部类Itr及ListItr实现
/**
* 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();
}
}
}
