I can't find right now a way to find if there's an object (and which one) at given x, y, z coordinates.
If you really want to check for an exact Vector3f(x,y,z) you could do something link that:
// get all spatials from rootNode
List<Spatial> allSpatials = rootNode.descendantMatches(Spatial.class);
for (Spatial sp : allSpatials)
do whatever you want
But that is really only the case if the center of the object is at that location. Otherwise you will have to create a boundingvolume and then have to check this boundingvolume (e.g. boundingsphere) with
instead of the equals part in the if-condition