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如何实现JavaIterator接口

这篇文章主要讲解了如何实现Java Iterator接口,内容清晰明了,对此有兴趣的小伙伴可以学习一下,相信大家阅读完之后会有帮助。

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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:
 *
 * 
    *
  • 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 the type of elements returned by this iterator * * @author Josh Bloch * @see Collection * @see ListIterator * @see Iterable * @since 1.2 */ public interface Iterator { /** * 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 *

The default implementation behaves as if: *

{@code
   *   while (hasNext())
   *     action.accept(next());
   * }
* * @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
 * nextElement method return successive elements of the
 * series.
 * 

* For example, to print all elements of a Vector<E> v: *

 *  for (Enumeration<E> e = v.elements(); e.hasMoreElements();)
 *    System.out.println(e.nextElement());
*

* 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 * SequenceInputStream. *

* 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 { /** * Tests if this enumeration contains more elements. * * @return true if and only if this enumeration object * contains at least one more element to provide; * false 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 stringCollection = new ArrayList<>();
    stringCollection.add("Hello");
    stringCollection.add("World");
    stringCollection.add("!");
    Iterator 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 {
  /**
   * 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 extends AbstractCollection implements List {
  /**
   * The number of times this list has been structurally modified.
   * 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。
   *
   * 

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 * fail-fast 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。 * *

Use of this field by subclasses is optional. 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。

看完上述内容,是不是对如何实现Java Iterator接口有进一步的了解,如果还想学习更多内容,欢迎关注创新互联行业资讯频道。


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