Evaluating a FlatField (and VisADTimeSlider?)

Hi,

I'm trying to do something fairly similar to
"Simple.java" in the Developers Guide. Simple.java
uses an Integer1DSet as the domain, and
"field.evaluate" works fine. However, I want to
represent time as a Gridded1DDoubleSet in the domain.
When I do this (see attached code) I get the exception
below. 

So should I be able to do this, or do I need to go
about it a different way?

And a supplementary question... has anyone done a
VisADSlider or similar specifically to deal with time
(ie format it nicely). It probably wouldn't be much
extra code, but if you've got one please let me know.

Thanks in advance,

James

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C:\Program Files\aifs\data>java SimpleTime
Time = (1970-01-01 00:00:01Z)
Exception in thread "main"
java.lang.NullPointerException
        at
visad.Gridded1DDoubleSet.valueToInterp(Gridded1DDoubleSet.java:263)
        at
visad.FlatField.resample(FlatField.java:3450)
        at
visad.FunctionImpl.evaluate(FunctionImpl.java:169)
        at
visad.FunctionImpl.evaluate(FunctionImpl.java:146)
        at SimpleTime.<init>(SimpleTime.java:46)
        at SimpleTime.main(SimpleTime.java:50)
 

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Attachment: SimpleTime.java
Description: SimpleTime.java