Enumeration.java

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 * accompanied this code).
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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} method return successive elements of the
 * series.
 * <p>
 * For example, to print all elements of a {@code Vector<E>} <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}.
 *
 * @author Lee Boynton
 * @apiNote The functionality of this interface is duplicated by the {@link Iterator}
 * interface.  In addition, {@code Iterator} adds an optional remove operation,
 * and has shorter method names.  New implementations should consider using
 * {@code Iterator} in preference to {@code Enumeration}. It is possible to
 * adapt an {@code Enumeration} to an {@code Iterator} by using the
 * {@link #asIterator} method.
 * @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()
 * @since 1.0
 */
/*
 * 枚举器，用来遍历元素
 *
 * 枚举器可以看做是一个阉割版的外部迭代器，它只能用于遍历元素，不能移除元素
 * 枚举器Enumeration可以通过asIterator()方法转换为外部迭代器Iterator来使用
 *
 * 注：区别于外部迭代器Iterator
 * 注：区别于内部迭代器Iterable
 */
public interface Enumeration<E> {
    /**
     * Tests if this enumeration contains more elements.
     *
     * @return {@code true} if and only if this enumeration object
     * contains at least one more element to provide;
     * {@code false} otherwise.
     */
    // 是否存在未遍历元素
    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.
     *
     * @throws NoSuchElementException if no more elements exist.
     */
    // 返回下一个元素
    E nextElement();
    
    /**
     * Returns an {@link Iterator} that traverses the remaining elements
     * covered by this enumeration. Traversal is undefined if any methods
     * are called on this enumeration after the call to {@code asIterator}.
     *
     * @return an Iterator representing the remaining elements of this Enumeration
     *
     * @apiNote This method is intended to help adapt code that produces
     * {@code Enumeration} instances to code that consumes {@code Iterator}
     * instances. For example, the {@link java.util.jar.JarFile#entries
     * JarFile.entries()} method returns an {@code Enumeration<JarEntry>}.
     * This can be turned into an {@code Iterator}, and then the
     * {@code forEachRemaining()} method can be used:
     *
     * <pre>{@code
     *     JarFile jarFile = ... ;
     *     jarFile.entries().asIterator().forEachRemaining(entry -> { ... });
     * }</pre>
     *
     * (Note that there is also a {@link java.util.jar.JarFile#stream
     * JarFile.stream()} method that returns a {@code Stream} of entries,
     * which may be more convenient in some cases.)
     * @implSpec The default implementation returns an {@code Iterator} whose
     * {@link Iterator#hasNext hasNext} method calls this Enumeration's
     * {@code hasMoreElements} method, whose {@link Iterator#next next}
     * method calls this Enumeration's {@code nextElement} method, and
     * whose {@link Iterator#remove remove} method throws
     * {@code UnsupportedOperationException}.
     * @since 9
     */
    // Enumeration转换为Iterator
    default Iterator<E> asIterator() {
        return new Iterator<>() {
            @Override
            public boolean hasNext() {
                return hasMoreElements();
            }
            
            @Override
            public E next() {
                return nextElement();
            }
        };
    }
}