libjava/gnu/java/util/EmptyEnumeration.java

   1 /* EmptyEnumeration.java -- a constant empty enumeration
   2    Copyright (C) 2001, 2002 Free Software Foundation, Inc.
   3
   4 This file is part of GNU Classpath.
   5
   6 GNU Classpath is free software; you can redistribute it and/or modify
   7 it under the terms of the GNU General Public License as published by
   8 the Free Software Foundation; either version 2, or (at your option)
   9 any later version.
  10
  11 GNU Classpath is distributed in the hope that it will be useful, but
  12 WITHOUT ANY WARRANTY; without even the implied warranty of
  13 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  14 General Public License for more details.
  15
  16 You should have received a copy of the GNU General Public License
  17 along with GNU Classpath; see the file COPYING.  If not, write to the
  18 Free Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA
  19 02111-1307 USA.
  20
  21 Linking this library statically or dynamically with other modules is
  22 making a combined work based on this library.  Thus, the terms and
  23 conditions of the GNU General Public License cover the whole
  24 combination.
  25
  26 As a special exception, the copyright holders of this library give you
  27 permission to link this library with independent modules to produce an
  28 executable, regardless of the license terms of these independent
  29 modules, and to copy and distribute the resulting executable under
  30 terms of your choice, provided that you also meet, for each linked
  31 independent module, the terms and conditions of the license of that
  32 module.  An independent module is a module which is not derived from
  33 or based on this library.  If you modify this library, you may extend
  34 this exception to your version of the library, but you are not
  35 obligated to do so.  If you do not wish to do so, delete this
  36 exception statement from your version. */
  37
  38 package gnu.java.util;
  39
  40 import java.io.Serializable;
  41 import java.util.Enumeration;
  42 import java.util.NoSuchElementException;
  43
  44 /**
  45  * This is a helper class that produces an empty Enumerations. There is only
  46  * one instance of this class that can be used whenever one needs a
  47  * non-null but empty enumeration. Using this class prevents multiple
  48  * small objects and inner classes. <code>getInstance()</code> returns
  49  * the only instance of this class. It can be shared by multiple objects and
  50  * threads.
  51  *
  52  * @author Mark Wielaard (mark@klomp.org)
  53  */
  54 public final class EmptyEnumeration implements Enumeration, Serializable
  55 {
  56   /** The only instance of this class */
  57   private static final EmptyEnumeration instance = new EmptyEnumeration();
  58
  59   /**
  60    * Private constructor that creates a new empty Enumeration.
  61    */
  62   private EmptyEnumeration()
  63   {
  64   }
  65
  66   /**
  67    * Returns the only instance of this class.
  68    * It can be shared by multiple objects and threads.
  69    *
  70    * @return the common empty enumeration
  71    */
  72   public static EmptyEnumeration getInstance()
  73   {
  74     return instance;
  75   }
  76
  77   /**
  78    * Returns false, since there are no elements.
  79    *
  80    * @return false
  81    */
  82   public boolean hasMoreElements()
  83   {
  84     return false;
  85   }
  86
  87   /**
  88    * Always throws <code>NoSuchElementException</code>, since it is empty.
  89    *
  90    * @throws NoSuchElementException this is empty
  91    */
  92   public Object nextElement()
  93   {
  94     throw new NoSuchElementException();
  95   }
  96 }