public SkypeBuilder withChat(String id){
  if (!id.startsWith("19:"))   throw new IllegalArgumentException("Invalid chat id");
  if (password != null)   throw new IllegalArgumentException("Not guest account");
  this.chatId=id;
  return this;
}

public SkypeBuilder withChat ( String id ) { if ( ! id . startsWith ( "19:" ) ) throw new IllegalArgumentException ( "Invalid chat id" ) ; if ( password != null ) throw new IllegalArgumentException ( "Not guest account" ) ; this . chatId = id ; return this ; }
join a particular chat as a guest . will have no effect if a password is specified
