code: private static @Nullable
      User getOrCreateById(@Nonnull String id, @Nonnull String fullName, boolean create) {
          User u = AllUsers.get(id);
          if (u == null && (create || UserIdMapper.getInstance().isMapped(id))) {
              u = new User(id, fullName);
              AllUsers.put(id, u);
              if (!id.equals(fullName) && !UserIdMapper.getInstance().isMapped(id)) {
                  try {
                      u.save();
                  } catch (IOException x) {
                      LOGGER.log(Level.WARNING, "Failed to save user configuration for " + id, x);
                  }
              }
          }
          return u;
      }
docs: Retrieve a user by its ID, and create a new one if requested.
  
  @return An existing or created user. May be {@code null} if a user does not exist and
  {@code create} is false.