java  Iterator接口和LIstIterator接口分析

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 <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 <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：

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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