ptolemy.domains.dde.kernel

Class NullToken

java.lang.Object

ptolemy.data.Token

ptolemy.domains.dde.kernel.NullToken

public class NullToken
extends Token

A NullToken is a marker class used to break deadlock in certain topologies of DDE models. A NullToken object does not represent modeled computation but serves as an indicator that an actor in a DDE model can safely advance time to be equal to the time stamp associated with the NullToken.

NullTokens are not part of the Ptolemy II type lattice. As such, a NullToken will violate type resolution if passed through a typed IO port. For this reason, Null Tokens are placed directly into receivers without using the send() method of TypedIOPort. This tactic circumvents type resolution constraints and affirms the notion that NullTokens do not represent computation and hence should never be incorporated into actor code. Note further, that the get() method of DDEReceiver does not return NullTokens but instead consumes them and then proceeds until a "real" token becomes available.

Since:

Ptolemy II 0.3

Version:

$Id$

Author:

John S. Davis II

See Also:

Token, DDEReceiver

Pt.AcceptedRating:

Red (cxh)

Pt.ProposedRating:

Red (davisj)

Field Summary

Fields inherited from class ptolemy.data.Token

NIL

Constructor Summary

Constructors

Constructor and Description
NullToken()

Method Summary

Methods inherited from class ptolemy.data.Token

add, addReverse, divide, divideReverse, getType, isCloseTo, isCloseTo, isEqualTo, isNil, modulo, moduloReverse, multiply, multiplyReverse, notSupportedConversionMessage, notSupportedIncomparableConversionMessage, notSupportedIncomparableMessage, notSupportedMessage, notSupportedNullNilStringMessage, one, pow, subtract, subtractReverse, toString, zero, zeroReturnType

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, wait, wait, wait

Constructor Detail

NullToken

public NullToken()