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/* 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

the Free Software Foundation; either version 2, or (at your option)

any later version.

GNU Classpath is distributed in the hope that it will be useful, but

WITHOUT ANY WARRANTY; without even the implied warranty of

MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU

General Public License for more details.

You should have received a copy of the GNU General Public License

along with GNU Classpath; see the file COPYING.  If not, write to the

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

conditions of the GNU General Public License cover the whole

combination.

As a special exception, the copyright holders of this library give you

permission to link this library with independent modules to produce an

executable, regardless of the license terms of these independent

modules, and to copy and distribute the resulting executable under

terms of your choice, provided that you also meet, for each linked

independent module, the terms and conditions of the license of that

module.  An independent module is a module which is not derived from

or based on this library.  If you modify this library, you may extend

this exception to your version of the library, but you are not

obligated to do so.  If you do not wish to do so, delete this

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();

    }

  }

}