code: public void setIdentity(@NonNull String userId, @Nullable BranchReferralInitListener
              callback) {
          ServerRequest req = new ServerRequestIdentifyUserRequest(context_, callback, userId);
          if (!req.constructError_ && !req.handleErrors(context_)) {
              handleNewRequest(req);
          } else {
              if (((ServerRequestIdentifyUserRequest) req).isExistingID()) {
                  ((ServerRequestIdentifyUserRequest) req).handleUserExist(branchReferral_);
              }
          }
      }
docs: <p>Identifies the current user to the Branch API by supplying a unique identifier as a
  {@link String} value, with a callback specified to perform a defined action upon successful
  response to request.</p>
  
  @param userId   A {@link String} value containing the unique identifier of the user.
  @param callback A {@link BranchReferralInitListener} callback instance that will return
  the data associated with the user id being assigned, if available.