title: how to use the libgdx contactlistener
body: <p>I've just began to work with the Libgdx's Box2d Engine but i simply do not understand when the methods of the <em>Contactlistener</em> should be called. There is on the one hand "begin contact" and on the other "end contact". Where should i call them, to get the Number of of a certain fixture touching others? And how do I implement the <em>Contactlistener</em>? 
  A redirec' to a Tutorial would answer my Question. I didn't find anything while searching google.
  This <a href="http://www.iforce2d.net/b2dtut/jumpability">one</a> helped me a lot but it is written for C++ and does not refer to the implementation into a main-gamecircle.
  Thx for helping me ;) </p>