行为型之迭代模式

迭代模式

Iterator

行为型之一

把对容器中包含的内部对象的访问委让给外部类，使用Iterator（遍历）按顺序进行遍历访问

想一想

如果没有Iterator，那么会有什么问题？

1. 各自的容器内部需要实现各自容器内元素的遍历方法

   • 容器类承担了太多功能

     • 一方面需要提供删除添加等本身应有的功能

     • 一方面还需要提供遍历访问功能

   • 往往容器在实现遍历的过程中，需要保存遍历状态，当根元素的添加删除功能混合夹杂在一起时，很容易引起混乱和程序运行错误

2. 容器的访问者需要自己去实现容器的访问方法

   这样直接就把数据细节暴露给了外部

而且，最重要的是什么？

对容器的遍历基本上不都是相似的吗？

所以直接抽离出来，统一实现一个泛型不就解决了问题了吗？

而且通过Iterator还有效的屏蔽了聚集对象的容器中数据细节

Java中对这个实现很常见，过于简单，直接看源码吧

/**
* List
*/
 /**
     * Returns an iterator over the elements in this list in proper sequence.
     *
     * @return an iterator over the elements in this list in proper sequence
     */
    Iterator<E> iterator();

/**
* iterator接口中
本质上还是老套路，通过Consumer接口中默认方法实现遍历而已
*/
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">
 * 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());
    }
}

/**
* Comsumer
*/
/**
 * Represents an operation that accepts a single input argument and returns no
 * result. Unlike most other functional interfaces, {@code Consumer} is expected
 * to operate via side-effects.
 *
 * <p>This is a <a href="package-summary.html">functional interface</a>
 * whose functional method is {@link #accept(Object)}.
 *
 * @param <T> the type of the input to the operation
 *
 * @since 1.8
 */
@FunctionalInterface
public interface Consumer<T> {

    /**
     * Performs this operation on the given argument.
     *
     * @param t the input argument
     */
    void accept(T t);

    /**
     * Returns a composed {@code Consumer} that performs, in sequence, this
     * operation followed by the {@code after} operation. If performing either
     * operation throws an exception, it is relayed to the caller of the
     * composed operation.  If performing this operation throws an exception,
     * the {@code after} operation will not be performed.
     *
     * @param after the operation to perform after this operation
     * @return a composed {@code Consumer} that performs in sequence this
     * operation followed by the {@code after} operation
     * @throws NullPointerException if {@code after} is null
     */
    default Consumer<T> andThen(Consumer<? super T> after) {
        Objects.requireNonNull(after);
        return (T t) -> { accept(t); after.accept(t); };
    }
}
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评价

• 实现了功能分离，简化容器接口。让容器只实现本身的基本功能，把迭代的功能委让给外部类实现，符合类的设计原则
• 隐藏了容器的实现细节
• 为容器或者子接口提供了一个统一接口，一方面方便调用；另一方面时的调用者不必关注迭代器的实现细节
• 可以为容器或其子容器实现不同的迭代方法或多个迭代方法