Q:

modify java3d SceneGraph dynamically

I recently started working on java3D. Currently I am working on a java3D racing game. 
I just wanted to know is it possible to add objects (i.e. scenegraph nodes nodes) e.g. spehere, box, etc.  to a scene graph which has been compiled.  
If i want to add an object onto a graph on a keyPress event, or on a timing event how can it be done? 

A:

Yes, you can add and remove BranchGroups from the universe scene graph dynamically. 
Typically each BranchGroup is compiled into a single mesh internally for faster rendering. 
So you can make your sphere, car mesh, etc a child of a BranchGroup, and then add that BranchGroup as a child of the main BranchGroup. 
You may also need to set 
MyBranchGroup.setCapability( Group. ALLOW_CHILDREN_EXTEND );
to allow adding children.