java的iterator接口_java Iterator接口和LIstIterator接口分析_java_脚本之家
java? Iterator接口和LIstIterator接口分析
目錄
1.Iterator接口
2.ListIterator
3.Iterator和ListIterator的區(qū)別
正文
在繼續(xù)看ArrayList源碼之前，先了解Iterator接口和ListIterator接口，下篇文章詳細(xì)講解ArrayList是如何實(shí)現(xiàn)它們的。
我們知道，接口只是一種規(guī)范，當(dāng)繼承接口并實(shí)現(xiàn)其中的方法時(shí)，要遵循接口對(duì)方法的說明。
1.Iterator接口
Iterator接口取代了Java集合框架中的Enumeratrion。Iterators不同于enumerations的地方主要有兩點(diǎn)：
Iterators允許調(diào)用者在迭代過程中從集合里移除元素；
方法名得到了改善。
Iterator源碼如下：
/**
* 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 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接口定義了四個(gè)方法以及各個(gè)方法的功能，如果有類實(shí)現(xiàn)了這個(gè)接口，且實(shí)現(xiàn)了這些方法，這方法需要實(shí)現(xiàn)定義的功能，遵循這些規(guī)則：
1).hasNext() 判斷容器是否有下一個(gè)元素，有則返回true；
2).next() 返回容器中的下一個(gè)元素；
3).remove() 移除當(dāng)前迭代器返回的最后一個(gè)元素。這個(gè)方法在每次調(diào)用next()方法之后只能調(diào)用一次；
4).Java 8 增加forEachRemaining方法，它可以實(shí)現(xiàn)對(duì)余下的所有元素執(zhí)行指定的操作。
更詳細(xì)的說明請(qǐng)閱讀源碼中的注釋。
2.ListIterator
ListIterator在Iterator基礎(chǔ)上提供了add、set、previous等對(duì)列表的操作。但是ListIterator跟Iterator一樣，仍是在原列表上進(jìn)行操作。
ListIterator源碼如下：
/**
* 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 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的功能更加強(qiáng)大，定義的方法有：
1).hasNext() 向前遍歷時(shí)，如果有下一個(gè)元素返回真；
2).next() 返回下一個(gè)元素的值，并將指針加1；
3).hasPrevious() 向相反方向遍歷時(shí)，如果還有元素返回真；
4).previous() 返回上一個(gè)元素的值，并將指針前移1；
5).nextIndex() 返回此時(shí)調(diào)用next()方法時(shí)返回的元素的索引；
6).previousIndex() 返回此時(shí)調(diào)用previous()方法時(shí)返回的元素的索引；
7).remove() 移除最近一次調(diào)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(shí)調(diào)用next()或previous()方法返回的元素替換掉；
9).add(E e) 添加元素到此時(shí)調(diào)用next()返回的元素之前，或此時(shí)調(diào)用previous()返回的元素之后。
更詳細(xì)的說明請(qǐng)閱讀源碼中的注釋。
3.Iterator和ListIterator的區(qū)別
Iterator和ListIterator的方法對(duì)比如下表：
Iterator
ListIterator
hasNext()
hasNext()
覆蓋
next()
next()
覆蓋
remove()
remove()
覆蓋
forEachRemaining(Consumer super E>?action)
forEachRemaining(Consumer super E>?action)
繼承
hasPrevious()
previous()
nextIndex()
previousIndex()
set(E e)
add(E e)
二者的不同之處主要有：
1).Iterator只能單向移動(dòng)，ListIterator可以雙向移動(dòng)；
2).ListIterator可以刪除、替換或添加元素，而Iterator只能刪除元素；
3).ListIterator可以返回當(dāng)前(調(diào)用next()或previous()返回的)元素的索引，而Iterator不能。
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