[geos-devel]createEmptyGeometry()

[Martin Davis]

I thought about this, but on the principal of Occam's Razor decided just to use an empty GC.  This has the advantage that there are fewer cases to handle for both developers and clients.
 
 
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 Charlie Savage
	Sent: March 30, 2006 2:33 PM
	To: GEOS Development List
	Subject: Re: [geos-devel]createEmptyGeometry() createsinstance ofGeometryCollection
	
	
	Would it make sense to have a separate EmptyGeometryClass specifically for this purpose?
	
	Charlie
	
	Martin Davis wrote: 

		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
			
			    

		_______________________________________________
		geos-devel mailing list
		geos-devel at geos.refractions.net
		http://geos.refractions.net/mailman/listinfo/geos-devel
		
		
		  

-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://lists.osgeo.org/pipermail/geos-devel/attachments/20060330/ee4365a8/attachment.html