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: [EMAIL PROTECTED] > [mailto:[EMAIL PROTECTED] 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: [EMAIL PROTECTED] > > [mailto:[EMAIL PROTECTED] 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 Ĺoskot > > http://mateusz.loskot.net > > _______________________________________________ > > geos-devel mailing list > > geos-devel@geos.refractions.net > > http://geos.refractions.net/mailman/listinfo/geos-devel > > > _______________________________________________ > geos-devel mailing list > geos-devel@geos.refractions.net > http://geos.refractions.net/mailman/listinfo/geos-devel > _______________________________________________ geos-devel mailing list geos-devel@geos.refractions.net http://geos.refractions.net/mailman/listinfo/geos-devel
