迭代器模式 (Iterator)

迭代器模式(Iterator), Provide a way to access the elements of an aggregate object sequentially without exposing its underlying representation. (它提供一各方法訪問一個窗口對象中各個元素，而又不需暴露該對象的內部細節。)

 

通用類圖：

角色：

1. Iterator抽象迭代器：負責定義訪問和遍歷元素的接口。在Java中，只迭代器爲interface，其中定義了三個方法，hasNext(), next(), remove().

2. ConcreteIterator具體迭代器：實現迭代器接口，完成元素的遍歷。持有容器的引用以完成遍歷。

3. Aggregate抽象容器： 負責建立具體迭代器角色的接口，必須提供相似createIterator()這樣的方法。在Java中，通常是iterator()方法。

4. ConcreteAggregate具體容器。

 

JDK中Iterator接口

/*
 * Copyright (c) 1997, 2010, Oracle and/or its affiliates. All rights reserved.
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
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package java.util;

/**
 * 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">
 * 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.
     *
     * @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
     */
    void remove();
}