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 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()
```