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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 <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>{@code * while (hasNext()) * action.accept(next()); * }</pre> * * @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 <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: ^ ^ ^ ^ ^ * </PRE> * Note that the {@link #remove} and {@link #set(Object)} methods are * <i>not</i> 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<E> extends Iterator<E> { // 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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本文標(biāo)題:Iterator與LIstIterator接口在java中的區(qū)別有哪些
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