Lots of people named John choose to use another name. 'John' is common enough that having to negotiation which John on a team or group of friends uses the name is a common problem. I prefer to either use 'Jack' (a common alternate form of John, similar to Richard/Dick), or simply my last name by itself. I've also seen many Johns use their middle name, or 'John [LastName]' pronounced almost as a single word. Back home my family calls me by my middle name, since my father, grandfather and cousin are also named John.