Source for gnu.java.util.DoubleEnumeration

   1: /* gnu.java.util.DoubleEnumeration
   2:    Copyright (C) 1998, 1999, 2001, 2004 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.util.Enumeration;
  41: import java.util.NoSuchElementException;
  42: 
  43: 
  44: /**
  45:  * This is a helper class that combines two Enumerations.
  46:  * It returns the elements of the first Enumeration until it has
  47:  * no more elements and then returns the elements of the second
  48:  * Enumeration.<br>
  49:  *
  50:  * In the default case:
  51:  * <pre>
  52:  * doubleEnum = new DoubleEnumeration(enum1, enum2);
  53:  * while (doubleEnum.hasMoreElements()) {
  54:  *    Object o = doubleEnum.nextElement();
  55:  *    do_something(o);
  56:  * }
  57:  * </pre>
  58:  * it calls hasMoreElements of the Enumerations as few times as
  59:  * possible.
  60:  * The references to the Enumerations are cleared as soon as they have no
  61:  * more elements to help garbage collecting.
  62:  *
  63:  * @author Jochen Hoenicke
  64:  * @author Mark Wielaard (mark@klomp.org)
  65:  */
  66: public class DoubleEnumeration<T> implements Enumeration<T>
  67: {
  68:   /**
  69:    * This is true as long as one of the enumerations has more
  70:    * elements.
  71:    * Only valid when hasChecked is true.
  72:    * Set in <code>hasMoreElements()</code>
  73:    */
  74:   private boolean hasMore;
  75:   /**
  76:    * This is true, if it is sure that hasMore indicates wether there are
  77:    * more elements.
  78:    * Set to true in <code>hasMoreElements()</code>.
  79:    * Set to false in <code>getNextElement()</code>.
  80:    */
  81:   private boolean hasChecked;
  82:   /**
  83:    * The first enumeration.
  84:    */
  85:   private Enumeration<T> e1;
  86:   /**
  87:    * The second enumeration.
  88:    */
  89:   private Enumeration<T> e2;
  90: 
  91:   /**
  92:    * Creates a new Enumeration combining the given two enumerations.
  93:    * The enumerations mustn't be accessed by other classes.
  94:    */
  95:   public DoubleEnumeration(Enumeration<T> e1, Enumeration<T> e2)
  96:   {
  97:     this.e1 = e1;
  98:     this.e2 = e2;
  99:     hasChecked = false;
 100:   }
 101: 
 102:   /**
 103:    * Returns true, if at least one of the two enumerations has more
 104:    * elements.
 105:    */
 106:   public boolean hasMoreElements()
 107:   {
 108:     if (hasChecked)
 109:       return hasMore;
 110: 
 111:     hasMore = (e1 != null && e1.hasMoreElements());
 112: 
 113:     if (!hasMore) {
 114:       e1 = e2;
 115:       e2 = null;
 116:       hasMore = (e1 != null && e1.hasMoreElements());
 117:     }
 118: 
 119:     hasChecked = true;
 120:     return hasMore;
 121:   }
 122: 
 123:   /**
 124:    * Returns the next element.  This returns the next element of the
 125:    * first enumeration, if it has more elements, otherwise the next
 126:    * element of the second enumeration. If both enumeration don't have
 127:    * any elements it throws a <code>NoSuchElementException</code>.
 128:    */
 129:   public T nextElement()
 130:   {
 131:     if (!hasMoreElements())
 132:       throw new NoSuchElementException();
 133:     else {
 134:       hasChecked = false;
 135:       return e1.nextElement();
 136:     }
 137:   }
 138: }