Junit Test using Math.Random and strings involved

Im stuck on testing this little section of code and i have no idea how i would start with testing it? Can anyone help!?

TBH i dont even know if this is right?

  public int FindRandomNumber(){
          int index = (int) (Math.random() * (Bournemouth.bison.length / 3));
          String what = Bournemouth.bison[index * 3];
          String who = Bournemouth.bison[1 + index * 3];
           System.out.printf("%s said \"%s\"", who, what);

        return index;