001/*  A placeholder token that represents the absence of a token.
002
003 Copyright (c) 1998-2013 The Regents of the University of California.
004 All rights reserved.
005 Permission is hereby granted, without written agreement and without
006 license or royalty fees, to use, copy, modify, and distribute this
007 software and its documentation for any purpose, provided that the above
008 copyright notice and the following two paragraphs appear in all copies
009 of this software.
010
011 IN NO EVENT SHALL THE UNIVERSITY OF CALIFORNIA BE LIABLE TO ANY PARTY
012 FOR DIRECT, INDIRECT, SPECIAL, INCIDENTAL, OR CONSEQUENTIAL DAMAGES
013 ARISING OUT OF THE USE OF THIS SOFTWARE AND ITS DOCUMENTATION, EVEN IF
014 THE UNIVERSITY OF CALIFORNIA HAS BEEN ADVISED OF THE POSSIBILITY OF
015 SUCH DAMAGE.
016
017 THE UNIVERSITY OF CALIFORNIA SPECIFICALLY DISCLAIMS ANY WARRANTIES,
018 INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
019 MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. THE SOFTWARE
020 PROVIDED HEREUNDER IS ON AN "AS IS" BASIS, AND THE UNIVERSITY OF
021 CALIFORNIA HAS NO OBLIGATION TO PROVIDE MAINTENANCE, SUPPORT, UPDATES,
022 ENHANCEMENTS, OR MODIFICATIONS.
023
024 PT_COPYRIGHT_VERSION_2
025 COPYRIGHTENDKEY
026 */
027package ptolemy.data;
028
029///////////////////////////////////////////////////////////////////
030//// AbsentToken
031
032/**
033 A placeholder token that represents the absence of a token.
034
035 @author Paul Whitaker
036 @version $Id$
037 @since Ptolemy II 8.0
038 @Pt.ProposedRating Red (pwhitake)
039 @Pt.AcceptedRating Red (pwhitake)
040 */
041public class AbsentToken extends Token {
042    // The AbsentToken class is primarily used in the
043    // Synchronous/Reactive (SR) domain.  The AbsentToken class is in
044    // the actor package so that other classes may use it, such as
045    // actor.lib.vhdl.RegisterSR
046
047    /** Construct a token with value false.
048     */
049    public AbsentToken() {
050    }
051
052    ///////////////////////////////////////////////////////////////////
053    ////                         public variables                  ////
054
055    /** An instance of an absent token. */
056    public static final AbsentToken ABSENT = new AbsentToken();
057}