这篇文章主要介绍了java Iterator接口和LIstIterator接口分析的相关资料,需要的朋友可以参考下
java Iterator接口和LIstIterator接口分析
目录
1.Iterator接口
2.ListIterator
3.Iterator和ListIterator的区别
正文
在继续看ArrayList源码之前，先了解Iterator接口和ListIterator接口，下篇文章详细讲解ArrayList是如何实现它们的。
我们知道，接口只是一种规范，当继承接口并实现其中的方法时，要遵循接口对方法的说明。
1.Iterator接口
Iterator接口取代了Java集合框架中的Enumeratrion。Iterators不同于enumerations的地方主要有两点：
Iterators允许调用者在迭代过程中从集合里移除元素；
方法名得到了改善。
Iterator源码如下：
/**
* An iterator over a collection. {@code Iterator} takes the place of
* {@link Enumeration} in the Java Collections Framework. Iterators
* differ from enumerations in two ways:
* Iterators allow the caller to remove elements from the underlying collection during the iteration with well-defined semantics.
* Method names have been improved.
* This interface is a member of the Java Collections Framework.
* @param <E> the type of elements returned by this iterator*/
public interface Iterator<E> {
/**
* Returns {@code true} if the iteration has more elements.
* (In other words, returns {@code true} if {@link #next} would
* return an element rather than throwing an exception.)
* @return {@code true} if the iteration has more elements
*/
boolean hasNext();
/**
* Returns the next element in the iteration.
* @return the next element in the iteration
* @throws NoSuchElementException if the iteration has no more elements
*/
E next();
/**
* Removes from the underlying collection the last element returned
* by this iterator (optional operation). This method can be called
* only once per call to {@link #next}. The behavior of an iterator
* is unspecified if the underlying collection is modified while the
* iteration is in progress in any way other than by calling this
* method.
*
* @implSpec
* The default implementation throws an instance of
* {@link UnsupportedOperationException} and performs no other action.
*
* @throws UnsupportedOperationException if the {@code remove}
* operation is not supported by this iterator
*
* @throws IllegalStateException if the {@code next} method has not
* yet been called, or the {@code remove} method has already
* been called after the last call to the {@code next}
* method
*/
default void remove() {
throw new UnsupportedOperationException("remove");
}
/**
* Performs the given action for each remaining element until all elements
* have been processed or the action throws an exception. Actions are
* performed in the order of iteration, if that order is specified.
* Exceptions thrown by the action are relayed to the caller.
*
* @implSpec
* <p>The default implementation behaves as if:
* <pre class="brush:php;toolbar:false">{@code
* while (hasNext())
* action.accept(next());
* }
*
* @param action The action to be performed for each element
* @throws NullPointerException if the specified action is null
* @since 1.8
*/
default void forEachRemaining(Consumer action) {
Objects.requireNonNull(action);
while (hasNext())
action.accept(next());
}
}Iterator接口定义了四个方法以及各个方法的功能，如果有类实现了这个接口，且实现了这些方法，这方法需要实现定义的功能，遵循这些规则：
1).hasNext() 判断容器是否有下一个元素，有则返回true；
2).next() 返回容器中的下一个元素；
3).remove() 移除当前迭代器返回的最后一个元素。这个方法在每次调用next()方法之后只能调用一次；
4).Java 8 增加forEachRemaining方法，它可以实现对余下的所有元素执行指定的操作。
更详细的说明请阅读源码中的注释。
2.ListIterator
ListIterator在Iterator基础上提供了add、set、previous等对列表的操作。但是ListIterator跟Iterator一样，仍是在原列表上进行操作。
ListIterator源码如下：
/**
* 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: ^ ^ ^ ^ ^
*
* Note that the {@link #remove} and {@link #set(Object)} methods are
* not 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()}.
*
* This interface is a member of the Java Collections Framework.*/
public interface ListIteratorListIterator的功能更加强大，定义的方法有：
1).hasNext() 向前遍历时，如果有下一个元素返回真；
2).next() 返回下一个元素的值，并将指针加1；
3).hasPrevious() 向相反方向遍历时，如果还有元素返回真；
4).previous() 返回上一个元素的值，并将指针前移1；
5).nextIndex() 返回此时调用next()方法时返回的元素的索引；
6).previousIndex() 返回此时调用previous()方法时返回的元素的索引；
7).remove() 移除最近一次调用next()或previous()方法返回的元素（可选）；
8).set(E e) 用元素e将如果此时调用next()或previous()方法返回的元素替换掉；
9).add(E e) 添加元素到此时调用next()返回的元素之前，或此时调用previous()返回的元素之后。
更详细的说明请阅读源码中的注释。
3.Iterator和ListIterator的区别
Iterator和ListIterator的方法对比如下表：
Iterator |
ListIterator |
|
hasNext() |
hasNext() | 覆盖 |
next() |
next() | 覆盖 |
remove() |
remove() | 覆盖 |
forEachRemaining(Consumer action) |
forEachRemaining(Consumer action) | 继承 |
| hasPrevious() | ||
| previous() | ||
| nextIndex() | ||
| previousIndex() | ||
| set(E e) | ||
| add(E e) |
二者的不同之处主要有：
1).Iterator只能单向移动，ListIterator可以双向移动；
2).ListIterator可以删除、替换或添加元素，而Iterator只能删除元素；
3).ListIterator可以返回当前（调用next()或previous()返回的）元素的索引，而Iterator不能。
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