Java Iterator接口实现代码解析

iterator接口

源代码

package java.util;

import java.util.function.consumer;

/**
 * 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:
 *
 * <ul>
 *   <li> iterators allow the caller to remove elements from the
 *      underlying collection during the iteration with well-defined
 *      semantics.
 *   <li> method names have been improved.
 * </ul>
 *
 * <p>this interface is a member of the
 * <a href="{@docroot}/../technotes/guides/collections/index.html" rel="external nofollow" >
 * java collections framework</a>.
 *
 * @param <e> the type of elements returned by this iterator
 *
 * @author josh bloch
 * @see collection
 * @see listiterator
 * @see iterable
 * @since 1.2
 */
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());
  }
}

阅读笔记

1. iterator接口与enumeration接口的关系/iterator接口在java集合库中的作用

iterator接口是java集合框架的一部分，被用于替代原有的enumeration接口。（“iterator”比“enumeration”更简短、表意更清晰、功能更多，具体的信息下面enumeration接口的注解中说的挺清楚，且enumeration注解中也建议编程人员改用iterator接口）

java类库中，集合类的基本接口是collection接口，而collection接口实现了iterable接口，iterable接口中有一个iterator()方法用于获取iterator对象。

package java.util;

/**
 * an object that implements the enumeration interface generates a
 * series of elements, one at a time. successive calls to the
 * <code>nextelement</code> method return successive elements of the
 * series.
 * <p>
 * for example, to print all elements of a <tt>vector&lt;e&gt;</tt> <i>v</i>:
 * <pre>
 *  for (enumeration&lt;e&gt; e = v.elements(); e.hasmoreelements();)
 *    system.out.println(e.nextelement());</pre>
 * <p>
 * methods are provided to enumerate through the elements of a
 * vector, the keys of a hashtable, and the values in a hashtable.
 * enumerations are also used to specify the input streams to a
 * <code>sequenceinputstream</code>.
 * <p>
 * note: the functionality of this interface is duplicated by the iterator
 * interface. in addition, iterator adds an optional remove operation, and
 * has shorter method names. new implementations should consider using
 * iterator in preference to enumeration.
 *
 * @see   java.util.iterator
 * @see   java.io.sequenceinputstream
 * @see   java.util.enumeration#nextelement()
 * @see   java.util.hashtable
 * @see   java.util.hashtable#elements()
 * @see   java.util.hashtable#keys()
 * @see   java.util.vector
 * @see   java.util.vector#elements()
 *
 * @author lee boynton
 * @since  jdk1.0
 */
public interface enumeration<e> {
  /**
   * tests if this enumeration contains more elements.
   *
   * @return <code>true</code> if and only if this enumeration object
   *      contains at least one more element to provide;
   *     <code>false</code> otherwise.
   */
  boolean hasmoreelements();

  /**
   * returns the next element of this enumeration if this enumeration
   * object has at least one more element to provide.
   *
   * @return   the next element of this enumeration.
   * @exception nosuchelementexception if no more elements exist.
   */
  e nextelement();
}

2.hasnext()、next()、remove()方法的关系

hasnext()方法：判断是否还有元素可以进行迭代；

next()方法：迭代元素；

remove()方法：

/**
* remove from the underlying collection the last element returned by this iterator
*（optional operation）. 
* 移除当前迭代器上一次从基础集合中迭代的元素（可选操作）
*
* this method can be called only once per call to next().
* 调用remove()方法前必须先调用next()方法，调用完一次remove()方法后想要再次调用remove()方法，
* 必须先调用next()方法。
*
* the behavior of an iterator is unspecified if the underlying collection is modifyed while
* the iteration is in progress is any way other than by call this method.
* 如果在迭代进行过程中修改了基础集合，则迭代器的行为是不确定的。
*/
public static void main(string[] args) {
    collection<string> stringcollection = new arraylist<>();
    stringcollection.add("hello");
    stringcollection.add("world");
    stringcollection.add("!");
    iterator<string> stringiterator = stringcollection.iterator();

    stringiterator.next();
    stringiterator.remove();//ok
  }
public static void main(string[] args) {
    ......
    stringiterator.next();
    stringcollection.add("abc");//基本集合被改变
    stringiterator.remove();//error - java.util.concurrentmodificationexception
  }
public static void main(string[] args) {
    ......
    stringiterator.next();
    stringcollection.add("abc");//基本集合被改变
    stringiterator.next();//error - java.util.concurrentmodificationexception
  }
public static void main(string[] args) {
    ......
    stringiterator.next();
    stringcollection.add("abc");//基本集合改变
    stringiterator = stringcollection.iterator();//重新获取迭代器
    stringiterator.next();//ok
    stringiterator.remove();//ok
  }

三者关系：调用remove()方法前必须先调用next()方法，调用next()方法前最好先调用hasnext()方法。

3.具体实现类

abstractlist类中定义了一个实现了iterator接口的内部类：

private class itr implements iterator<e> {
  /**
   * index of element to be returned by subsequent call to next.
   */
  int cursor = 0;

  /**
   * index of element returned by most recent call to next or
   * previous. reset to -1 if this element is deleted by a call
   * to remove.
   */
  int lastret = -1;

  /**
   * the modcount value that the iterator believes that the backing
   * list should have. if this expectation is violated, the iterator
   * has detected concurrent modification.
   */
  int expectedmodcount = modcount;

  public boolean hasnext() {
    return cursor != size();
  }

  public e next() {
    checkforcomodification();
    try {
      int i = cursor;
      e next = get(i);
      lastret = i;//最近一次调用next()方法返回的元素的下标。
      cursor = i + 1;//下一次调用next()方法返回的元素的下标。
      return next;
    } catch (indexoutofboundsexception e) {
      checkforcomodification();
      throw new nosuchelementexception();
    }
  }

  public void remove() {
    if (lastret < 0)
      throw new illegalstateexception();//所以，调用remove()前必须先调用next()
    checkforcomodification();

    try {
      abstractlist.this.remove(lastret);
      if (lastret < cursor)
        cursor--;//因为移除了一个元素
      lastret = -1;//所以，不能连续调用两次remove()方法
      expectedmodcount = modcount;
    } catch (indexoutofboundsexception e) {
      throw new concurrentmodificationexception();
    }
  }

  final void checkforcomodification() {
    if (modcount != expectedmodcount)
      throw new concurrentmodificationexception();
  }
}

看完上面的代码，我对modcount、expectedmodcount变量以及checkforcomodification()方法的作用比较好奇，所以尝试着去搞清楚。

先来看modecount变量，这个变量被声明在内部类的外部：

public abstract class abstractlist<e> extends abstractcollection<e> implements list<e> {
  /**
   * the number of times this list has been <i>structurally modified</i>.
   * structural modifications are those that change the size of the
   * list, or otherwise perturb it in such a fashion that iterations in
   * progress may yield incorrect results.
   * 用于表示该列表发生结构性修改的次数。结构性修改是指*更改列表的大小*或*以其他
   * 方式干扰列表*，即正在进行的迭代可能会产生错误的结果。
   *
   * <p>this field is used by the iterator and list iterator implementation
   * returned by the {@code iterator} and {@code listiterator} methods.
   * if the value of this field changes unexpectedly, the iterator (or list
   * iterator) will throw a {@code concurrentmodificationexception} in
   * response to the {@code next}, {@code remove}, {@code previous},
   * {@code set} or {@code add} operations. this provides
   * <i>fail-fast</i> behavior, rather than non-deterministic behavior in
   * the face of concurrent modification during iteration.
   * 设计者认为，与其因为基本集合被并发修改从而使迭代产生不确定行为，不如尽早给出错误。
   *
   * <p><b>use of this field by subclasses is optional.</b> if a subclass
   * wishes to provide fail-fast iterators (and list iterators), then it
   * merely has to increment this field in its {@code add(int, e)} and
   * {@code remove(int)} methods (and any other methods that it overrides
   * that result in structural modifications to the list). a single call to
   * {@code add(int, e)} or {@code remove(int)} must add no more than
   * one to this field, or the iterators (and list iterators) will throw
   * bogus {@code concurrentmodificationexceptions}. if an implementation
   * does not wish to provide fail-fast iterators, this field may be
   * ignored.
   * 是否使用应需求决定。
   */
  protected transient int modcount = 0;
}

看完上面的源码注解，已经大概能够知道modcount、expectedmodcount以及checkforcomodification()的作用了。

假如把基础集合当作一个银行账号，基础集合中的元素表示存款。那么modcount就相当于银行为每个账号做的消费记录，expectedmodcount就相当于是账号持有人自己做的一份消费记录，一般银行和账号持有人自己做的消费记录都不会出错。

final void checkforcomodification() {
  if (modcount != expectedmodcount)
    throw new concurrentmodificationexception();
}

一旦银行那边的消费记录和自己手里的那份消费记录对不上，肯定是账号被盗用了。

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