1 /* 2 * Copyright (c) 1994, 2022, 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} method return successive elements of the 32 * series. 33 * <p> 34 * For example, to print all elements of a {@code Vector<E>} <i>v</i>: 35 * <pre> 36 * for (Enumeration<E> 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}. 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 * @param <E> the type of elements returned by this enumeration 53 * 54 * @see java.util.Iterator 55 * @see java.io.SequenceInputStream 56 * @see java.util.Enumeration#nextElement() 57 * @see java.util.Hashtable 58 * @see java.util.Hashtable#elements() 59 * @see java.util.Hashtable#keys() 60 * @see java.util.Vector 61 * @see java.util.Vector#elements() 62 * 63 * @author Lee Boynton 64 * @since 1.0 65 */ 66 public interface Enumeration<E> { 67 /** 68 * Tests if this enumeration contains more elements. 69 * 70 * @return {@code true} if and only if this enumeration object 71 * contains at least one more element to provide; 72 * {@code false} otherwise. 73 */ hasMoreElements()74 boolean hasMoreElements(); 75 76 /** 77 * Returns the next element of this enumeration if this enumeration 78 * object has at least one more element to provide. 79 * 80 * @return the next element of this enumeration. 81 * @throws NoSuchElementException if no more elements exist. 82 */ nextElement()83 E nextElement(); 84 85 /** 86 * Returns an {@link Iterator} that traverses the remaining elements 87 * covered by this enumeration. Traversal is undefined if any methods 88 * are called on this enumeration after the call to {@code asIterator}. 89 * 90 * @apiNote 91 * This method is intended to help adapt code that produces 92 * {@code Enumeration} instances to code that consumes {@code Iterator} 93 * instances. For example, the {@link java.util.jar.JarFile#entries() 94 * JarFile.entries()} method returns an {@code Enumeration<JarEntry>}. 95 * This can be turned into an {@code Iterator}, and then the 96 * {@code forEachRemaining()} method can be used: 97 * 98 * <pre>{@code 99 * JarFile jarFile = ... ; 100 * jarFile.entries().asIterator().forEachRemaining(entry -> { ... }); 101 * }</pre> 102 * 103 * (Note that there is also a {@link java.util.jar.JarFile#stream() 104 * JarFile.stream()} method that returns a {@code Stream} of entries, 105 * which may be more convenient in some cases.) 106 * 107 * @implSpec 108 * The default implementation returns an {@code Iterator} whose 109 * {@link Iterator#hasNext hasNext} method calls this Enumeration's 110 * {@code hasMoreElements} method, whose {@link Iterator#next next} 111 * method calls this Enumeration's {@code nextElement} method, and 112 * whose {@link Iterator#remove remove} method throws 113 * {@code UnsupportedOperationException}. 114 * 115 * @return an Iterator representing the remaining elements of this Enumeration 116 * 117 * @since 9 118 */ asIterator()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 } 130