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 the type of elements returned by this iterator*/

public interface Iterator {

/**

* 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

*

The default implementation behaves as if:

*

{@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 super E> 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 cursor position 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:

*

* 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 ListIterator extends Iterator {

// Query Operations

/**

* Returns {@code true} if this list iterator has more elements when

* traversing the list in the forward direction. (In other words,

* returns {@code true} if {@link #next} 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 forward direction

*/

boolean hasNext();

/**

* Returns the next element in the list and advances the cursor position.

* This method may be called repeatedly to iterate through the list,

* or intermixed with calls to {@link #previous} to go back and forth.

* (Note that alternating calls to {@code next} and {@code previous}

* will return the same element repeatedly.)

*

* @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. (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();

// 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}.

*

* @throws UnsupportedOperationException if the {@code remove}

* operation is not supported by this list iterator

* @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 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}.

*

* @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);

}

ListIterator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 super E> action)

forEachRemaining(Consumer super E> 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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