How listener collision??

How listener or handle events when a collision happens??? I readed that the materiales can do that, but without materiales how can i do it??

you have to use materials. all objects use the "default" material by... default... so if you want to get a callback on ALL collisions, create a ContactCallback class, and register it with a MaterialPair of (default vs. default).

ok, i created the ContactCallback, how can set it to MaterialPair???