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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