I Google a lot and I keep only getting an answer for Pro Desktop version of creating a fish net grid for a polygon. If someone know if its possible to implement something similar or exactly that with runtime Java. I would really appreciate the help.
Thanks in Advance
There are a couple of approaches you could take to this. One way would be to use Local Server to run a geoprocessing tool to create it for you. I can see that fishnet is supported here Local Server geoprocessing tools support—ArcGIS Runtime SDK for Java | ArcGIS for Developers
If you didn't want to follow the Local Server / Geoprocessing approach, it wouldn't be hard to to take the bounding box of your geometry and write some of your own code to create a fishnet. You could display this in a graphic layer.
I'm not sure of your workflow or what you need the fishnet for, but do either of these ideas help?