code: public static BoxDeveloperEditionAPIConnection userClient(String userId) {
          if (userId == null) { //   session data has expired
              return null;
          }
          System.out.format("userClient called with userId %s \n\n", userId);
          try {
              BoxDeveloperEditionAPIConnection userClient = BoxDeveloperEditionAPIConnection.getAppUserConnection(
                      userId, CLIENT_ID, CLIENT_SECRET, jwtEncryptionPreferences, accessTokenCache);
              return userClient;
          } catch (BoxAPIException apiException) {
              apiException.printStackTrace();
              throw apiException;
          }
      }
docs: Get a BoxDeveloperEditionAPIConnection that can be used by an App user to access Box.
  
  @param userId The UserId of a valid Box App User
  @return BoxDeveloperEditionAPIConnection new Box Developer Edition connection with App User token