1 /*
   2  * Copyright (c) 1994, 2015, Oracle and/or its affiliates. All rights reserved.
   3  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
   4  *
   5  * This code is free software; you can redistribute it and/or modify it
   6  * under the terms of the GNU General Public License version 2 only, as
   7  * published by the Free Software Foundation.  Oracle designates this
   8  * particular file as subject to the "Classpath" exception as provided
   9  * by Oracle in the LICENSE file that accompanied this code.
  10  *
  11  * This code is distributed in the hope that it will be useful, but WITHOUT
  12  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
  13  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
  14  * version 2 for more details (a copy is included in the LICENSE file that
  15  * accompanied this code).
  16  *
  17  * You should have received a copy of the GNU General Public License version
  18  * 2 along with this work; if not, write to the Free Software Foundation,
  19  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
  20  *
  21  * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
  22  * or visit www.oracle.com if you need additional information or have any
  23  * questions.
  24  */
  25 
  26 package java.util;
  27 
  28 /**
  29  * An object that implements the Enumeration interface generates a
  30  * series of elements, one at a time. Successive calls to the
  31  * <code>nextElement</code> method return successive elements of the
  32  * series.
  33  * <p>
  34  * For example, to print all elements of a <tt>Vector&lt;E&gt;</tt> <i>v</i>:
  35  * <pre>
  36  *   for (Enumeration&lt;E&gt; e = v.elements(); e.hasMoreElements();)
  37  *       System.out.println(e.nextElement());</pre>
  38  * <p>
  39  * Methods are provided to enumerate through the elements of a
  40  * vector, the keys of a hashtable, and the values in a hashtable.
  41  * Enumerations are also used to specify the input streams to a
  42  * <code>SequenceInputStream</code>.
  43  * 
  44  * @apiNote
  45  * The functionality of this interface is duplicated by the {@link Iterator}
  46  * interface.  In addition, {@code Iterator} adds an optional remove operation,
  47  * and has shorter method names.  New implementations should consider using
  48  * {@code Iterator} in preference to {@code Enumeration}. It is possible to
  49  * adapt an {@code Enumeration} to an {@code Iterator} by using the
  50  * {@link #asIterator} method.
  51  *
  52  * @see     java.util.Iterator
  53  * @see     java.io.SequenceInputStream
  54  * @see     java.util.Enumeration#nextElement()
  55  * @see     java.util.Hashtable
  56  * @see     java.util.Hashtable#elements()
  57  * @see     java.util.Hashtable#keys()
  58  * @see     java.util.Vector
  59  * @see     java.util.Vector#elements()
  60  *
  61  * @author  Lee Boynton
  62  * @since   1.0
  63  */
  64 public interface Enumeration<E> {
  65     /**
  66      * Tests if this enumeration contains more elements.
  67      *
  68      * @return  <code>true</code> if and only if this enumeration object
  69      *           contains at least one more element to provide;
  70      *          <code>false</code> otherwise.
  71      */
  72     boolean hasMoreElements();
  73 
  74     /**
  75      * Returns the next element of this enumeration if this enumeration
  76      * object has at least one more element to provide.
  77      *
  78      * @return     the next element of this enumeration.
  79      * @exception  NoSuchElementException  if no more elements exist.
  80      */
  81     E nextElement();
  82 
  83     /**
  84      * Returns an {@link Iterator} that traverses the remaining elements
  85      * covered by this enumeration. Traversal is undefined if any methods
  86      * are called on this enumeration after the call to {@code asIterator}.
  87      *
  88      * @apiNote
  89      * This method is intended to help adapt code that produces
  90      * {@code Enumeration} instances to code that consumes {@code Iterator}
  91      * or {@code Iterable} instances. For example, the
  92      * {@link java.util.jar.JarFile#entries JarFile.entries()}
  93      * method returns an {@code Enumeration<JarEntry>}. This can be adapted
  94      * for use in an enhanced-for loop as follows:
  95      *
  96      * <pre>{@code
  97      *     JarFile jf = ... ;
  98      *     Iterable<JarEntry> entries = () -> jf.entries().asIterator();
  99      *     for (JarEntry je : entries) {
 100      *         doSomethingWithEntry(je);
 101      *     }
 102      * }</pre>
 103      * 
 104      * Note that the lambda expression being used as an {@code Iterable}
 105      * fetches a new {@code Enumeration} each time, so that the resulting
 106      * {@code Iterator} instances do not interfere with each other.
 107      *
 108      * @implSpec
 109      * The returned Iterator's {@link Iterator#hasNext hasNext} method calls and returns
 110      * the value from this Enumeration's {@code hasMoreElements} method; its
 111      * {@link Iterator#next next} method calls and returns the value from this Enumeration's
 112      * {@code nextElement} method; and its {@link Iterator#remove remove} method throws
 113      * {@code UnsupportedOperationException}.
 114      *
 115      * @return an Iterator representing the remaining elements of this Enumeration
 116      *
 117      * @since 1.9
 118      */
 119     default Iterator<E> asIterator() {
 120         return new Iterator<>() {
 121             @Override public boolean hasNext() {
 122                 return hasMoreElements();
 123             }
 124             @Override public E next() {
 125                 return nextElement();
 126             }
 127         };
 128     }
 129 }