This file isn't part of the runtime product. The Runtime isn't based on ArcObjects as it's a new product build from the ground up.
The arcobjects.jar I believe is part of the ArcGIS Engine SDK which can be downloaded via your EDN account.
If you are about to start on a new project, I would however encourage you to consider using the ArcGIS Runtime SDK instead which is where the main focus of our development work is happening for desktop / mobile based applications.
Let me know what you are trying to do and I'll see if I can point you in the right direction.
Mark