uml - Understanding Communication diagram -

consider following diagrams:

i want transform these diagrams java code, find difficult figure communication diagram. searched lot in web couldn't find explain flow of it(how literally read it), "what meaning of it".

so far, class diagram ,i know class b sub-class of a, , b , c knowing each other (and each 1 of them have private member of other), but, meaning of arrows , index ? class b call op3 of own c? when, why, how?

follow numbers:

1. create instance of class b (which becomes :b)
2. :b creates new instance of c
3. :b calls method op3() in :c
   1. :c calls op2() in :b
      1. :b calls op1() (from superclass a) in itself