[geos-devel] createEmptyGeometry()createsinstance ofGeometryCollection

[Martin Davis]

Here's another example of the use of an empty GeometryCollection: it's what Point.getBoundary() returns.  It would definitely be misleading to return something of more definite type in this case, IMO.

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 
> Martin Davis
> Sent: March 30, 2006 2:24 PM
> To: GEOS Development List
> Subject: RE: [geos-devel] 
> createEmptyGeometry()createsinstance ofGeometryCollection
> 
> 
> 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 
> > _______________________________________________
> > geos-devel mailing list
> > geos-devel at geos.refractions.net 
> > http://geos.refractions.net/mailman/listinfo/geos-devel
> > 
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