Source for gnu.java.util.EmptyEnumeration

   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., 51 Franklin Street, Fifth Floor, Boston, MA
  19: 02110-1301 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<T> implements Enumeration<T>, Serializable
  55: {
  56:   /** The only instance of this class */
  57:   private static final EmptyEnumeration<Object> instance =
  58:     new EmptyEnumeration<Object>();
  59: 
  60:   /**
  61:    * Returns an instance of this class for Object.
  62:    * It can be shared by multiple objects and threads.
  63:    *
  64:    * @return the common empty enumeration
  65:    */
  66:   public static EmptyEnumeration<Object> getInstance()
  67:   {
  68:     return instance;
  69:   }
  70: 
  71:   /**
  72:    * Returns false, since there are no elements.
  73:    *
  74:    * @return false
  75:    */
  76:   public boolean hasMoreElements()
  77:   {
  78:     return false;
  79:   }
  80: 
  81:   /**
  82:    * Always throws <code>NoSuchElementException</code>, since it is empty.
  83:    *
  84:    * @throws NoSuchElementException this is empty
  85:    */
  86:   public T nextElement()
  87:   {
  88:     throw new NoSuchElementException();
  89:   }
  90: }