java中foreach与迭代器

foreach语法主要用于数组，也可以应用于任何的Collection对象。这是Java SE5中引入的被称为Iterable接口的原因。该接口包含一个产生Iterator的iterator()方法，并且Iterable接口被foreach用来在序列中移动。因此如果你创建了任何实现了Iterable的类，都可以将其用于foreach语句中。

一下是Iterable接口的源码：

/* * Copyright (c) 2003, 2013, Oracle and/or its affiliates. All rights reserved. * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms. * */package java.lang;import java.util.Iterator;import java.util.Objects;import java.util.Spliterator;import java.util.Spliterators;import java.util.function.Consumer;/** * Implementing this interface allows an object to be the target of * the "for-each loop" statement. See * <strong> * <a href="{@docRoot}/../technotes/guides/language/foreach.html">For-each Loop</a> * </strong> * * @param <T> the type of elements returned by the iterator * * @since 1.5 * @jls 14.14.2 The enhanced for statement */public interface Iterable<T> {    /**     * Returns an iterator over elements of type {@code T}.     *     * @return an Iterator.     */    Iterator<T> iterator();    /**     * Performs the given action for each element of the {@code Iterable}     * until all elements have been processed or the action throws an     * exception.  Unless otherwise specified by the implementing class,     * actions are performed in the order of iteration (if an iteration order     * is specified).  Exceptions thrown by the action are relayed to the     * caller.     *     * @implSpec     * <p>The default implementation behaves as if:     * <pre>{@code     *     for (T t : this)     *         action.accept(t);     * }</pre>     *     * @param action The action to be performed for each element     * @throws NullPointerException if the specified action is null     * @since 1.8     */    default void forEach(Consumer<? super T> action) {        Objects.requireNonNull(action);        for (T t : this) {            action.accept(t);        }    }    /**     * Creates a {@link Spliterator} over the elements described by this     * {@code Iterable}.     *     * @implSpec     * The default implementation creates an     * <em><a href="Spliterator.html#binding">early-binding</a></em>     * spliterator from the iterable's {@code Iterator}.  The spliterator     * inherits the <em>fail-fast</em> properties of the iterable's iterator.     *     * @implNote     * The default implementation should usually be overridden.  The     * spliterator returned by the default implementation has poor splitting     * capabilities, is unsized, and does not report any spliterator     * characteristics. Implementing classes can nearly always provide a     * better implementation.     *     * @return a {@code Spliterator} over the elements described by this     * {@code Iterable}.     * @since 1.8     */    default Spliterator<T> spliterator() {        return Spliterators.spliteratorUnknownSize(iterator(), 0);    }}

下面是一个实现Iterable接口的类，将其放入foreach语句中：

public class IterableClass implements Iterable<String> {String[] words = ("And that is how we konw"+ "the Earth to be banana-shaped.").split(" ");public Iterator<String> iterator() {return new Iterator<String>(){private int index = 0;public boolean hasNext() {return index < words.length;}public String next() {return words[index++];}};}}

测试类：

public class IterableClassTest {public static void main(String[] args) {for(String s : new IterableClass()){System.out.print(s + " ");}}}

输出如下：

And that is how we konwthe Earth to be banana-shaped. 

这里iterator()方法返回的是实现了Iterator<String>的匿名内部类的实例。该匿名内部类可以遍历数组中所有的单词，在main方法中，我们可以看到IterableClass确实可以用于foreach语句中。

下面咱们写个产生反向遍历的迭代器玩玩。

public class ReversibleArrayList<T> extends ArrayList<T> {/** *  */private static final long serialVersionUID = 8711835951216698394L;public ReversibleArrayList(Collection<T> c) {super(c);}public Iterable<T> reverse() {return new Iterable<T>() {public Iterator<T> iterator() {return new Iterator<T>(){int current = size() - 1;public boolean hasNext() {return current > -1;}public T next() {return get(current--);}};}};}}

测试类：

<pre name="code" class="java">public class ReversibleArrayListTest {public static void main(String[] args) {ReversibleArrayList<String> ral = new ReversibleArrayList<String>(Arrays.asList("To be or not to be".split(" ")));for (String s : ral) {System.out.print(s + " ");}System.out.println();for(String s : ral.reverse()){System.out.print(s + " ");}}}

输出结果如下：

To be or not to be 
be to not or be To