@Test
public void testIsolatedUser() throws Exception {

  DataModel dataModel = getDataModel(
          new long[] {1, 2, 3, 4},
          new Double[][] {
                  {0.1, 0.2},
                  {0.2, 0.3, 0.3, 0.6},
                  {0.4, 0.4, 0.5, 0.9},
                  {null, null, null, null, 1.0},
          });
  UserSimilarity similarity = new PearsonCorrelationSimilarity(dataModel);
  UserNeighborhood neighborhood = new NearestNUserNeighborhood(3, similarity, dataModel);
  UserBasedRecommender recommender = new GenericUserBasedRecommender(dataModel, neighborhood, similarity);
  long[] mostSimilar = recommender.mostSimilarUserIDs(4, 3);
  assertNotNull(mostSimilar);
  assertEquals(0, mostSimilar.length);
}