Iterator接口
Iterator接口来自java.util包，主要方法如下：
package java.util;
public interface Iterator<E> {
boolean hasNext();
E next();
default void remove() {
throw new UnsupportedOperationException("remove");
}
}
Iterable接口
Iterable接口来自java.lang包，实现了Iterable接口的类可以使用for each去遍历
Iterable接口通过iterator()方法返回一个Iterator实例
package java.lang;
/**
* Implementing this interface allows an object to be the target of
* the "for-each loop" statement.
*/
public interface Iterable<T> {
/**
* Returns an iterator over elements of type
*
* @return an Iterator.
*/
Iterator<T> iterator();
/**
* @param action The action to be performed for each element
* @throws NullPointerException if the specified action is null
* @since 1.8
*/
default void forEach(Consumer<? super T> action) {
Objects.requireNonNull(action);
for (T t : this) {
action.accept(t);
}
}
}
为什么要把Iterable和Iterator变成两个接口呢？
因为实现了Iterable接口的类可以实现多个Iterator内部类，比如LinkedList中的ListItr和DescendingIterator这两个内部类，分别实现了链表的双向遍历和逆序遍历。通过返回不同的Iterator实例来实现不同的遍历方式，这样更灵活，如果将Iterable和Iterator合并，就没法返回不同的Iterator实例。Iterable相当于对Iterator的封装
Iterator的保留可以让子类去实现自己的迭代器，而Iterable接口更加关注于for each
LinkedList类中的这两个内部类如下：
package java.util;
public class LinkedList<E>
extends AbstractSequentialList<E>
implements List<E>, Deque<E>, Cloneable, java.io.Serializable
{
//省略了其他实现，只显示了这两个内部类的代码
public ListIterator<E> listIterator(int index) {
checkPositionIndex(index);
return new ListItr(index);
}
private class ListItr implements ListIterator<E> {
private LinkedList.Node<E> lastReturned;
private LinkedList.Node<E> next;
private int nextIndex;
private int expectedModCount = modCount;
ListItr(int index) {
// assert isPositionIndex(index);
next = (index == size) ? null : node(index);
nextIndex = index;
}
public boolean hasNext() {
return nextIndex < size;
}
public E next() {
checkForComodification();
if (!hasNext())
throw new NoSuchElementException();
lastReturned = next;
next = next.next;
nextIndex++;
return lastReturned.item;
}
public boolean hasPrevious() {
return nextIndex > 0;
}
public E previous() {
checkForComodification();
if (!hasPrevious())
throw new NoSuchElementException();
lastReturned = next = (next == null) ? last : next.prev;
nextIndex--;
return lastReturned.item;
}
public int nextIndex() {
return nextIndex;
}
public int previousIndex() {
return nextIndex - 1;
}
public void remove() {
checkForComodification();
if (lastReturned == null)
throw new IllegalStateException();
LinkedList.Node<E> lastNext = lastReturned.next;
unlink(lastReturned);
if (next == lastReturned)
next = lastNext;
else
nextIndex--;
lastReturned = null;
expectedModCount++;
}
public void set(E e) {
if (lastReturned == null)
throw new IllegalStateException();
checkForComodification();
lastReturned.item = e;
}
public void add(E e) {
checkForComodification();
lastReturned = null;
if (next == null)
linkLast(e);
else
linkBefore(e, next);
nextIndex++;
expectedModCount++;
}
public void forEachRemaining(Consumer<? super E> action) {
Objects.requireNonNull(action);
while (modCount == expectedModCount && nextIndex < size) {
action.accept(next.item);
lastReturned = next;
next = next.next;
nextIndex++;
}
checkForComodification();
}
final void checkForComodification() {
if (modCount != expectedModCount)
throw new ConcurrentModificationException();
}
}
public Iterator<E> descendingIterator() {
return new DescendingIterator();
}
/**
* Adapter to provide descending iterators via ListItr.previous
*/
private class DescendingIterator implements Iterator<E> {
private final LinkedList.ListItr itr = new LinkedList.ListItr(size());
public boolean hasNext() {
return itr.hasPrevious();
}
public E next() {
return itr.previous();
}
public void remove() {
itr.remove();
}
}
}
总结
Java容器中，所有的Collection子类会实现Iteratable接口以实现foreach功能，Iteratable接口的实现又依赖于实现了Iterator的内部类(参照LinkedList中listIterator()和descendingIterator()的JDK源码)。有的容器类会有多个实现Iterator接口的内部类，通过返回不同的迭代器实现不同的迭代方式。
扩展：ListIterator接口
ListIterator的注意点：
- 实现了Iterator接口
- 以任意方向遍历List
- 在遍历的时候修改list
- 获取iterator在list中的当前位置
- ListIterator没有当前的元素，它光标的位置总是在previous()和next()之间
- 长度为n的list有n+1种可能的光标位置
代码如下，注意读注释：
package java.util;
/**
* An iterator for lists that allows the programmer
* to traverse the list in either direction, modify
* the list during iteration, and obtain the iterator's
* current position in the list. A {@code ListIterator}
* has no current element; its <I>cursor position</I> always
* lies between the element that would be returned by a call
* to {@code previous()} and the element that would be
* returned by a call to {@code next()}.
* An iterator for a list of length {@code n} has {@code n+1} possible
* cursor positions, as illustrated by the carets ({@code ^}) below:
* <PRE>
* Element(0) Element(1) Element(2) ... Element(n-1)
* cursor positions: ^ ^ ^ ^ ^
* </PRE>
* Note that the {@link #remove} and {@link #set(Object)} methods are
* <i>not</i> defined in terms of the cursor position; they are defined to
* operate on the last element returned by a call to {@link #next} or
* {@link #previous()}.
*
*/
public interface ListIterator<E> extends Iterator<E> {
//Query Operations
/**
* Returns {@code true} if this list iterator has more elements when
* traversing the list in the forward direction.
*/
boolean hasNext();
/**
* Returns the next element in the list and advances the cursor position.
* @return the next element in the list
* @throws NoSuchElementException if the iteration has no next element
*/
E next();
/**
* Returns {@code true} if this list iterator has more elements when
* traversing the list in the reverse direction.
*/
boolean hasPrevious();
/**
* Returns the previous element in the list and moves the cursor
* position backwards.
* @return the previous element in the list
* @throws NoSuchElementException if the iteration has no previous
* element
*/
E previous();
/**
* Returns the index of the element that would be returned by a
* subsequent call to {@link #next}. (Returns list size if the list
* iterator is at the end of the list.)
*
* @return the index of the element that would be returned by a
* subsequent call to {@code next}, or list size if the list
* iterator is at the end of the list
*/
int nextIndex();
/**
* Returns the index of the element that would be returned by a
* subsequent call to {@link #previous}. (Returns -1 if the list
* iterator is at the beginning of the list.)
*
* @return the index of the element that would be returned by a
* subsequent call to {@code previous}, or -1 if the list
* iterator is at the beginning of the list
*/
int previousIndex();
// Modification Operations
/**
* Removes from the list the last element that was returned by {@link
* #next} or {@link #previous} (optional operation). This call can
* only be made once per call to {@code next} or {@code previous}.
* It can be made only if {@link #add} has not been
* called after the last call to {@code next} or {@code previous}.
*/
void remove();
/**
* Replaces the last element returned by {@link #next} or
* {@link #previous} with the specified element (optional operation).
* This call can be made only if neither {@link #remove} nor {@link
* #add} have been called after the last call to {@code next} or
* {@code previous}.
*/
void set(E e);
/**
* Inserts the specified element into the list (optional operation).
* The element is inserted immediately before the element that
* would be returned by {@link #next}, if any, and after the element
* that would be returned by {@link #previous}, if any. (If the
* list contains no elements, the new element becomes the sole element
* on the list.) The new element is inserted before the implicit
* cursor: a subsequent call to {@code next} would be unaffected, and a
* subsequent call to {@code previous} would return the new element.
* (This call increases by one the value that would be returned by a
* call to {@code nextIndex} or {@code previousIndex}.)
*/
void add(E e);
}