/* gnu.java.util.DoubleEnumeration
   Copyright (C) 1998, 1999, 2001, 2004 Free Software Foundation, Inc.

This file is part of GNU Classpath.

GNU Classpath is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
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any later version.

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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
General Public License for more details.

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Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
02110-1301 USA.

Linking this library statically or dynamically with other modules is
making a combined work based on this library.  Thus, the terms and
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permission to link this library with independent modules to produce an
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exception statement from your version. */

package gnu.java.util;

import java.util.Enumeration;
import java.util.NoSuchElementException;


/**
 * This is a helper class that combines two Enumerations.
 * It returns the elements of the first Enumeration until it has
 * no more elements and then returns the elements of the second
 * Enumeration.<br>
 * 
 * In the default case:
 * <pre>
 * doubleEnum = new DoubleEnumeration(enum1, enum2);
 * while (doubleEnum.hasMoreElements()) {
 *    Object o = doubleEnum.nextElement();
 *    do_something(o);
 * }
 * </pre>
 * it calls hasMoreElements of the Enumerations as few times as
 * possible.
 * The references to the Enumerations are cleared as soon as they have no
 * more elements to help garbage collecting.
 *
 * @author Jochen Hoenicke
 * @author Mark Wielaard (mark@klomp.org)
 */
public class DoubleEnumeration<T> implements Enumeration<T>
{
  /**
   * This is true as long as one of the enumerations has more
   * elements.  
   * Only valid when hasChecked is true.
   * Set in <code>hasMoreElements()</code>
   */
  private boolean hasMore;
  /**
   * This is true, if it is sure that hasMore indicates wether there are
   * more elements.
   * Set to true in <code>hasMoreElements()</code>.
   * Set to false in <code>getNextElement()</code>.
   */
  private boolean hasChecked;
  /**
   * The first enumeration.
   */
  private Enumeration<T> e1;
  /**
   * The second enumeration.
   */
  private Enumeration<T> e2;

  /**
   * Creates a new Enumeration combining the given two enumerations.
   * The enumerations mustn't be accessed by other classes.
   */
  public DoubleEnumeration(Enumeration<T> e1, Enumeration<T> e2)
  {
    this.e1 = e1;
    this.e2 = e2;
    hasChecked = false;
  }

  /**
   * Returns true, if at least one of the two enumerations has more
   * elements.
   */
  public boolean hasMoreElements()
  {
    if (hasChecked)
      return hasMore;

    hasMore = (e1 != null && e1.hasMoreElements());

    if (!hasMore) {
      e1 = e2;
      e2 = null;
      hasMore = (e1 != null && e1.hasMoreElements());
    }

    hasChecked = true;
    return hasMore;
  }

  /**
   * Returns the next element.  This returns the next element of the
   * first enumeration, if it has more elements, otherwise the next
   * element of the second enumeration. If both enumeration don't have
   * any elements it throws a <code>NoSuchElementException</code>.
   */
  public T nextElement()
  {
    if (!hasMoreElements())
      throw new NoSuchElementException();
    else {
      hasChecked = false;
      return e1.nextElement();
    }
  }
}