This is expected behaviour.
There's an explanation in the FAQ here:
http://tsusiatsoftware.net/jts/jts-faq/jts-faq.html
under the heading "Why do overlay operations not obey the axioms of set
theory?"
Swan Jerry wrote:
Dear list members,
Given geometries g1 and g2, they don't appear to be contained in their
union. The code that exhibits this behaviour is below. Can someone
kindly point me to the likely cause?
Regards,
Jerry.
////////////////////////////////////////////////////////////////////////////////////////////
Geometry amalgGeom = geom1.union( geom2 );
MultiPolygon mp;
*if*( amalgGeom *instanceof* Polygon )
mp = gf.createMultiPolygon( *new* Polygon [] { (Polygon)amalgGeom } );
*else*
mp = (MultiPolygon)amalgGeom;
assert mp.contains( geom1 );
assert mp.contains( geom2 );
////////////////////////////////////////////////////////////////////////////////////////////
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