code: private void update(Collection<ResolverTuple> underlyingTuples, Throwable t) {
          if (t != null)
              applyException(t);
          else if (underlyingTuples != null)
              applyUpdate(underlyingTuples);
          else
              applyException(null);
      }
docs: Update callback, called when the completable future completes.