[Geotools-devel] NullFilter and constant Expression

[Jody G. <jga...@re...>]

Justin has been bugging me to make sense of Expressions ....

In an XML file we got:
- <LiteralExpression><LiteralExpression>
- <LiteralExpression/>

Question is which one maps to null and which one to ""?

I would guess that:
- <LiteralExpression><LiteralExpression> maps to ""
- <LiteralExpression/> maps to null

And I would be wrong - a parser cannot tell the difference -- and 
NullFilter only returns true on an null.
So should we fix NullFilter to return true on "" ? Or should we consider 
a null check on a Literal to be a flawed idea?

This differences is coming up as we review the default SLD situation, 
given that we are recording an Expression getColor() should we use null 
or "" to indicate that we lack information? Or should we give up and use 
null - or NullExpression (a constant NullObject) that cannot be 
visited/encoded?

Jody