一、引言

迭代器是设计模式的一种，迭代器模式提供方法来访问容器中的的元素，这听起来跟c语言中指针十分相似，其实数组访问中的指针本质上就是迭代器的一种。Iterrator对象就是迭代器，而iterator() 是 Java 集合框架中的一个方法，它返回一个 Iterator 对象，该对象可以用来遍历集合中的元素。

二、迭代器

2.1 迭代器对象的创建

在讲如何创建迭代器对象之前首先需要了解的是迭代器的种类，这里引用一下FASTR博主中的类结构图来说明

通过底层代码的查看我们可以发现在Iterable接口中有一个Iterator方法，它返回一个Iterator对象。

public interface Iterable<T> {/*** Returns an iterator over elements of type {@code T}.** @return an Iterator.*/Iterator<T> iterator();

而Iterator对象中则是拥有三个方法

public interface Iterator<E> {boolean hasNext();E next();default void remove() {throw new UnsupportedOperationException("remove");}

返回值类型	方法名	功能
boolean	hasNext()	判断集合是否还有元素，如果返回 true 表示集合还有元素，返回 false 表示集合中没有元素
E	next()	获取集合中的下一个元素，通常搭配hasNext()来使用，判断出存在元素再使用该方法获取下一个元素
void	remove()	删除集合中的元素。

在了解了类结构图之后我们就可以使用iterator()方法来创建对应的迭代器对象。

2.2 迭代器的使用

我们先来看一下最常见的ArrayList集合的迭代器使用案例

package pojo;import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;public class IteTest {public static void main(String[] args) {List<Integer> list = new ArrayList<>();list.add(1);list.add(2);list.add(3);Iterator<Integer> iterator = list.iterator();while(iterator.hasNext()) {int value = iterator.next();System.out.print(value + " ");}}
}

运行结果为：

在该案例中，首先hasNext（）方法的指针先指向1，确定存在该元素后打印，然后是2，3。在3之后由于没有下一个元素就会跳出while循环。
上述案例演示了hasNext和next方法的基本使用，那么remove（）其实在迭代器中的使用也十分简单

package pojo;import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;public class IteTest {public static void main(String[] args) {List<Integer> list = new ArrayList<>();list.add(1);list.add(2);list.add(3);Iterator<Integer> iterator = list.iterator();while(iterator.hasNext()) {int value = iterator.next();if(value == 1) iterator.remove();}System.out.println("删除后" + list);}
}

运行结果为：元素1被删除

那么Iterator的子类ListIterator有什么特殊之处呢。它允许程序员在列表（如 ArrayList、LinkedList 等）中双向遍历，同时支持元素的添加和删除操作。我们可以看一下底层代码中对几个新方法的描述

    /*** Returns {@code true} if this list iterator has more elements when* traversing the list in the reverse direction.  (In other words,* returns {@code true} if {@link #previous} would return an element* rather than throwing an exception.)** @return {@code true} if the 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.  This method may be called repeatedly to* iterate through the list backwards, or intermixed with calls to* {@link #next} to go back and forth.  (Note that alternating calls* to {@code next} and {@code previous} will return the same* element repeatedly.)** @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();/*** 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}.** @param e the element with which to replace the last element returned by*          {@code next} or {@code previous}* @throws UnsupportedOperationException if the {@code set} operation*         is not supported by this list iterator* @throws ClassCastException if the class of the specified element*         prevents it from being added to this list* @throws IllegalArgumentException if some aspect of the specified*         element prevents it from being added to this list* @throws IllegalStateException if neither {@code next} nor*         {@code previous} have been called, or {@code remove} or*         {@code 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}.)** @param e the element to insert* @throws UnsupportedOperationException if the {@code add} method is*         not supported by this list iterator* @throws ClassCastException if the class of the specified element*         prevents it from being added to this list* @throws IllegalArgumentException if some aspect of this element*         prevents it from being added to this list*/void add(E e);

E previous() 返回列表中的前一个元素，并将迭代器位置向后移动一个位置。
int nextIndex() 返回对 next 的后续调用将返回的元素的索引（在正向遍历列表中时）。
int previousIndex() 返回对 previous 的后续调用将返回的元素的索引（在反向遍历列表中时）。
void set(E e) 用指定的元素替换上次 next 或 previous 访问的元素（可选操作）。
void add(E e) 将指定的元素插入列表（可选操作）。该元素将插入到 next 或 previous 最后一次访问的位置。