這篇文章主要介紹了java Iterator介面和LIstIterator介面分析的相關資料,需要的朋友可以參考下
#java Iterator介面和LIstIterator介面分析
#目錄
1.Iterator介面
2.ListIterator
3.Iterator和ListIterator的區別
正文
在繼續看ArrayList源碼之前，先了解Iterator接口和ListIterator接口，下篇文章詳細講解ArrayList是如何實現它們的。
我們知道，介面只是一種規範，當繼承介面並實作其中的方法時，要遵循介面對方法的說明。
1.Iterator介面
Iterator介面取代了Java集合框架中的Enumeratrion。 Iterators不同於enumerations的地方主要有兩點：
Iterators允許呼叫者在迭代過程中從集合中移除元素；
方法名稱得到了改善。
Iterator原始碼如下：
/**
* 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介面定義了四個方法以及各個方法的功能，如果有類別實作了這個接口，且實作了這些方法，這個方法需要實作定義的功能，遵循這些規則：
1).hasNext() 判斷容器是否有下一個元素，有則傳回true;
2).next() 返回容器中的下一個元素;
3).remove() 移除目前迭代器傳回的最後一個元素。這個方法在每次呼叫next()方法之後只能呼叫一次；
4).Java 8 增加forEachRemaining方法，它可以實作對餘下的所有元素執行指定的操作。
更詳細的說明請閱讀原始碼中的註解。
2.ListIterator
ListIterator在Iterator基礎上提供了add、set、previous等對列表的操作。但是ListIterator跟Iterator一樣，仍是在原始列表上進行操作。
ListIterator原始碼如下：
/**
* 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的功能更強大，定義的方法有：
1).hasNext() 向前遍歷時，如果有下一個元素傳回真；
2).next() 傳回下一個元素的值，並將指標加1;
3).hasPrevious() 往相反方向遍歷時，如果還有元素傳回真；
4).previous() 傳回上一個元素的值，並將指標前移1;
5).nextIndex() 傳回此時呼叫next()方法時傳回的元素的索引;
6).previousIndex() 傳回此時呼叫previous()方法時傳回的元素的索引；
7).remove() 移除最近一次調用next()或previous()方法返回的元素（可選）；
8).set(E e) 用元素e將如果此時調用next()或previous()方法傳回的元素被替換掉；
9).add(E e) 新增元素到此時呼叫next()傳回的元素之前，或此時呼叫previous()返回的元素之後。
更詳細的說明請閱讀原始碼中的註解。
3.Iterator與ListIterator的差異
# Iterator與ListIterator的方法比較如下表：
#Iterator |
ListIterator |
# |
hasNext() |
hasNext() | 覆寫 |
#next() |
next() | 覆寫 |
remove() |
#remove() | 已覆寫 |
forEachRemaining(Consumer action) |
forEachRemaining(Consumer action) | 繼承 |
| hasPrevious() | ||
| # | previous() | ##|
| nextIndex() | ||
| previousIndex() | ||
| set(E e) | ||
| add(E e) |
二者的差異主要有：
1).Iterator只能單向移動，ListIterator可以雙向移動； 2).ListIterator可以# 3).ListIterator可以返回目前（呼叫next()或previous()傳回的）元素的索引，而Iterator不能。以上是java中關於Iterator介面和LIstIterator介面的具體介紹的詳細內容。更多資訊請關注PHP中文網其他相關文章！
