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jlechner |
/* gnu.java.util.DoubleEnumeration
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Copyright (C) 1998, 1999, 2001 Free Software Foundation, Inc.
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This file is part of GNU Classpath.
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GNU Classpath is free software; you can redistribute it and/or modify
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it under the terms of the GNU General Public License as published by
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the Free Software Foundation; either version 2, or (at your option)
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any later version.
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GNU Classpath is distributed in the hope that it will be useful, but
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WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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General Public License for more details.
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You should have received a copy of the GNU General Public License
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along with GNU Classpath; see the file COPYING. If not, write to the
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Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
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02110-1301 USA.
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Linking this library statically or dynamically with other modules is
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making a combined work based on this library. Thus, the terms and
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conditions of the GNU General Public License cover the whole
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combination.
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As a special exception, the copyright holders of this library give you
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permission to link this library with independent modules to produce an
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executable, regardless of the license terms of these independent
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modules, and to copy and distribute the resulting executable under
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terms of your choice, provided that you also meet, for each linked
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independent module, the terms and conditions of the license of that
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module. An independent module is a module which is not derived from
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or based on this library. If you modify this library, you may extend
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this exception to your version of the library, but you are not
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obligated to do so. If you do not wish to do so, delete this
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exception statement from your version. */
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package gnu.java.util;
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import java.util.Enumeration;
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import java.util.NoSuchElementException;
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/**
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* This is a helper class that combines two Enumerations.
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* It returns the elements of the first Enumeration until it has
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* no more elements and then returns the elements of the second
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* Enumeration.<br>
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*
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* In the default case:
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* <pre>
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* doubleEnum = new DoubleEnumeration(enum1, enum2);
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* while (doubleEnum.hasMoreElements()) {
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* Object o = doubleEnum.nextElement();
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* do_something(o);
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* }
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* </pre>
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* it calls hasMoreElements of the Enumerations as few times as
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* possible.
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* The references to the Enumerations are cleared as soon as they have no
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* more elements to help garbage collecting.
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*
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* @author Jochen Hoenicke
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* @author Mark Wielaard (mark@klomp.org)
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*/
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public class DoubleEnumeration implements Enumeration
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{
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/**
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* This is true as long as one of the enumerations has more
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* elements.
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* Only valid when hasChecked is true.
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* Set in <code>hasMoreElements()</code>
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*/
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private boolean hasMore;
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/**
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* This is true, if it is sure that hasMore indicates wether there are
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* more elements.
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* Set to true in <code>hasMoreElements()</code>.
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* Set to false in <code>getNextElement()</code>.
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*/
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private boolean hasChecked;
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/**
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* The first enumeration.
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*/
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private Enumeration e1;
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/**
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* The second enumeration.
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*/
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private Enumeration e2;
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/**
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* Creates a new Enumeration combining the given two enumerations.
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* The enumerations mustn't be accessed by other classes.
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*/
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public DoubleEnumeration(Enumeration e1, Enumeration e2)
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{
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this.e1 = e1;
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this.e2 = e2;
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hasChecked = false;
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}
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/**
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* Returns true, if at least one of the two enumerations has more
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* elements.
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*/
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public boolean hasMoreElements()
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{
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if (hasChecked)
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return hasMore;
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hasMore = (e1 != null && e1.hasMoreElements());
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if (!hasMore) {
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e1 = e2;
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e2 = null;
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hasMore = (e1 != null && e1.hasMoreElements());
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}
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hasChecked = true;
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return hasMore;
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}
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/**
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* Returns the next element. This returns the next element of the
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* first enumeration, if it has more elements, otherwise the next
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* element of the second enumeration. If both enumeration don't have
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* any elements it throws a <code>NoSuchElementException</code>.
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*/
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public Object nextElement()
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{
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if (!hasMoreElements())
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throw new NoSuchElementException();
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else {
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hasChecked = false;
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return e1.nextElement();
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}
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}
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}
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