[geos-devel] createEmptyGeometry() createsinstance ofGeometryCollection

[Martin Davis]

This isn't a question of what Geometry.isEmpty() returns.  In fact, isEmpty behaves exactly the way you mention - it checks whether there are any points in the Geometry, and returns false if not.

The issue is: what should a method return when it needs to return an empty geometry, but there is no guide about what type of geometry to return?  For instance, if you run line.intersect(polygon), and they don't actually intersect, what should be returned?  JTS adopts the convention that an empty GeometryCollection is returned.  Otherwise, it would have to choose a particular type to return, and this might be misleading.

Actually reviewing this has exposed a minor but probably bad design decision in JTS: empty buffers currently return an empty GC, whereas they should return an empty Polygon (to maintain the exit condition that buffer always returns a polygonal object).

Martin Davis, Senior Technical Architect
Vivid Solutions Inc.      www.vividsolutions.com
Suite #1A-2328 Government Street Victoria, B.C. V8T 5G5
Phone: (250) 385 6040 - Local 308 Fax: (250) 385 6046


> -----Original Message-----
> From: geos-devel-bounces at geos.refractions.net 
> [mailto:geos-devel-bounces at geos.refractions.net] On Behalf Of 
> Mateusz Loskot
> Sent: March 30, 2006 2:05 PM
> To: GEOS Development List
> Subject: Re: [geos-devel] createEmptyGeometry() 
> createsinstance ofGeometryCollection
> 
> 
> Martin Davis wrote:
> > Yes.  The spec doesn't cover this, but I chose to use an empty
> > GeometryCollection as a "typeless" empty geometry, for 
> methods which 
> > need to return one.  The createEmptyGeometry method is just a 
> > convenience method for this convention.
> 
> OK, I understant it.
> 
> > Anyway, what else could that method return that would make sense
> > (given that it has no type information)?
> 
> It depends on the idea behind "empty geometry" or "null 
> geometry" [1]. As I know, OGC Simple Feature Spec. explains 
> that empty geometry represents empty point set (empty set of 
> coordinates). According to my understanding: Point is empty 
> if it contains "uninitialized" coordinates MultiPoint is 
> empty if it contains no points in its set. etc.
> 
> There is also a distinction between empty and null.
> NULL means something undefined, uninitialized - something 
> unsafe. EMPTY means something well constructor but with empty 
> set of points. In some case, NULL object can be compared to 
> EMPTY object with true result.
> 
> So, I think isEmpty() function should check the state of the 
> coordinates set of Geometry return appropriate value (true/false).
> 
> [1] Saying "null geometry" I think about logical meaning but 
> not the physical representation of instance of geometry: null 
> reference/null pointer.
> 
> Cheers
> -- 
> Mateusz Łoskot
> http://mateusz.loskot.net 
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