001    /*
002     *  This file is part of the Jikes RVM project (http://jikesrvm.org).
003     *
004     *  This file is licensed to You under the Eclipse Public License (EPL);
005     *  You may not use this file except in compliance with the License. You
006     *  may obtain a copy of the License at
007     *
008     *      http://www.opensource.org/licenses/eclipse-1.0.php
009     *
010     *  See the COPYRIGHT.txt file distributed with this work for information
011     *  regarding copyright ownership.
012     */
013    package org.jikesrvm.util;
014    
015    import java.util.Enumeration;
016    import java.util.NoSuchElementException;
017    
018    /**
019     * Implements an empty enumeration.<p>
020     *
021     * The Collections class from Java 7 provides an {@code emptyEnumeration()} method
022     * which will make this class unnecessary once we switch to Java 7.<p>
023     *
024     * TODO Remove this class when we require Java 7 for building and from
025     * our class libraries.
026     *
027     * @param <E> type parameter
028     */
029    public final class EmptyEnumeration<E> implements Enumeration<E> {
030    
031      @SuppressWarnings("unchecked")
032      private static final EmptyEnumeration<?> INSTANCE = new EmptyEnumeration();
033    
034      /**
035       * Non-instantiable. Use {@link #emptyEnumeration()} to
036       * create instances.
037       */
038      private EmptyEnumeration() {
039        // prevent instantiation
040      }
041    
042      @Override
043      public boolean hasMoreElements() {
044        return false;
045      }
046    
047      @Override
048      public E nextElement() {
049        throw new NoSuchElementException("The empty enumeration has no elements!");
050      }
051    
052      @SuppressWarnings("unchecked")
053      public static <T> Enumeration<T> emptyEnumeration() {
054        return (EmptyEnumeration<T>) INSTANCE;
055      }
056    
057    }