Interface - java.util.Iterator

Created by : Mr Dk.

2019 / 11 / 05 23:24

Nanjing, Jiangsu, China

Definition

public interface ListIterator<E> extends Iterator<E> {

}

针对 List 的抽象迭代器：

  • 允许从 任意一个方向 遍历集合
  • 在遍历期间修改集合
  • 获得迭代器的当前位置
/**
 * 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()}.
 *
 * <p>This interface is a member of the
 * <a href="{@docRoot}/../technotes/guides/collections/index.html">
 * Java Collections Framework</a>.
 *
 * @author  Josh Bloch
 * @see Collection
 * @see List
 * @see Iterator
 * @see Enumeration
 * @see List#listIterator()
 * @since   1.2
 */

Has Next

遍历时是否还有更多的元素可以遍历。

/**
 * 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();

Next

返回下一个元素，并使指针前进。

/**
 * 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();

Has Previous

这个是 ListIterator 特有的，因为要支持双向的遍历。是否存在前一个元素。

/**
 * 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();

与 next() 配套。

/**
 * 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();

Next/Previous Index

返回下一次调用 next() 或 previous() 的迭代器位置。

/**
 * 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();

Remove

移除迭代器返回的上一个元素。

/**
 * 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();

Set

用指定元素替换迭代器返回的上一个元素。

/**
 * 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);

Add

元素被插入到下一个被返回的元素之前

  • 下一次调用 next() 不受影响
  • 下一次调用 previous() 返回新元素
/**
 * 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);

Summary

针对 List 定义了增、删、改的操作，查操作本来就是迭代器的主要功能，比较特殊的是双向性：所以很多操作都定义了两套，对应前后两个方向。