Create an observable that fetches smaller batches of users based on prefix

UserLister.java

class UserLister {
  ...
  public Observable<User> streamAll(int batchSize) {
      final int prefixLength = estimatePrefixLength(batchSize);
      Preconditions.checkState(prefixLength > 0);
      log.debug("Estimated prefix length: " + prefixLength);

      return Observable.from(RepositoryUtils.hexPrefixes(prefixLength)).flatMapIterable(new Function1<String, List<User>>() {
        public List<User> call(String prefix) {
          repository.listByIdPrefix(useridPrefix);
        }
      });
  }
}