libjava/gnu/java/util/DoubleEnumeration.java

   1 /* gnu.java.util.DoubleEnumeration
   2    Copyright (C) 1998, 1999, 2001 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.*;
  41 import java.util.Enumeration;
  42 import java.util.NoSuchElementException;
  43
  44
  45 /**
  46  * This is a helper class that combines two Enumerations.
  47  * It returns the elements of the first Enumeration until it has
  48  * no more elements and then returns the elements of the second
  49  * Enumeration.<br>
  50  *
  51  * In the default case:
  52  * <pre>
  53  * doubleEnum = new DoubleEnumeration(enum1, enum2);
  54  * while (doubleEnum.hasMoreElements()) {
  55  *    Object o = doubleEnum.nextElement();
  56  *    do_something(o);
  57  * }
  58  * </pre>
  59  * it calls hasMoreElements of the Enumerations as few times as
  60  * possible.
  61  * The references to the Enumerations are cleared as soon as they have no
  62  * more elements to help garbage collecting.
  63  *
  64  * @author Jochen Hoenicke
  65  * @author Mark Wielaard (mark@klomp.org)
  66  */
  67 public class DoubleEnumeration implements Enumeration
  68 {
  69   /**
  70    * This is true as long as one of the enumerations has more
  71    * elements.
  72    * Only valid when hasChecked is true.
  73    * Set in <code>hasMoreElements()</code>
  74    */
  75   private boolean hasMore;
  76   /**
  77    * This is true, if it is sure that hasMore indicates wether there are
  78    * more elements.
  79    * Set to true in <code>hasMoreElements()</code>.
  80    * Set to false in <code>getNextElement()</code>.
  81    */
  82   private boolean hasChecked;
  83   /**
  84    * The first enumeration.
  85    */
  86   private Enumeration e1;
  87   /**
  88    * The second enumeration.
  89    */
  90   private Enumeration e2;
  91
  92   /**
  93    * Creates a new Enumeration combining the given two enumerations.
  94    * The enumerations mustn't be accessed by other classes.
  95    */
  96   public DoubleEnumeration(Enumeration e1, Enumeration e2)
  97   {
  98     this.e1 = e1;
  99     this.e2 = e2;
 100     hasChecked = false;
 101   }
 102
 103   /**
 104    * Returns true, if at least one of the two enumerations has more
 105    * elements.
 106    */
 107   public boolean hasMoreElements()
 108   {
 109     if (hasChecked)
 110       return hasMore;
 111
 112     hasMore = (e1 != null && e1.hasMoreElements());
 113
 114     if (!hasMore) {
 115       e1 = e2;
 116       e2 = null;
 117       hasMore = (e1 != null && e1.hasMoreElements());
 118     }
 119
 120     hasChecked = true;
 121     return hasMore;
 122   }
 123
 124   /**
 125    * Returns the next element.  This returns the next element of the
 126    * first enumeration, if it has more elements, otherwise the next
 127    * element of the second enumeration. If both enumeration don't have
 128    * any elements it throws a <code>NoSuchElementException</code>.
 129    */
 130   public Object nextElement()
 131   {
 132     if (!hasMoreElements())
 133       throw new NoSuchElementException();
 134     else {
 135       hasChecked = false;
 136       return e1.nextElement();
 137     }
 138   }
 139 }