Is there a package (or extension) for computing isochrone lines or areas, that is the area which can be reached from a point in less than X minutes by travelling on a transport network ?
Not currently, you would need to use the
gt-graph package to compute the time taken to each node in your graph and then contour those times.
As far as I know the only contouring code in GeoTools is a process that calls JAI-Tools on a raster (coverage) as in this demo. But there is a
DelaunayTrianglator in the graph package that you could apply the marching squares algorithm to (scroll down to the triangle implementation).
If you do implement the vector algorithm we would be delighted to add the code to GeoTools.