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：

iterator	listiterator
hasnext()	hasnext()	覆盖
next()	next()	覆盖
remove()	remove()	覆盖
foreachremaining(consumer<? super e> action)	foreachremaining(consumer<? super e> action)	继承
	hasprevious()
	previous()
	nextindex()
	previousindex()
	set(e e)
	add(e e)

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。

感谢阅读，希望能帮助到大家，谢谢大家对本站的支持！