In section 12.4.2 [JTS1.4 ] states: "The convex hull of a Geometry is the smallest convex Polygon that contains all the points in the Geometry. If the convex hull contains fewer than 3 points, a lower dimension Geometry is returned, specified as follows:
Points in Convex Hull
Geometry of Result
0
empty GeometryCollection
1
Point
2
LineString
3 or more
Polygon
JTS will return a Geometry with the minimal number of points needed to represent the convex hull. In particular, no more than two consecutive points will be collinear."