### Question:
PackedRatingMatrix implements RatingMatrix { @Override public LongCollection getUserIds() { return userIndex().getKeyList(); } PackedRatingMatrix(PackedRatingData data); @Override LongCollection getUserIds(); @Override LongCollection getItemIds(); @Override KeyIndex userIndex(); @Override KeyIndex itemIndex(); @Override List<RatingMatrixEntry> getRatings(); @Override Collection<RatingMatrixEntry> getUserRatings(long userId); @Override synchronized Long2DoubleMap getUserRatingVector(long userId); @PreDestroy void dispose();  }### Answer:
@Test public void testGetUserIds() { LongCollection users = snap.getUserIds(); assertTrue(users.contains(1)); assertTrue(users.contains(3)); assertTrue(users.contains(4)); assertTrue(users.contains(5)); assertTrue(users.contains(6)); assertTrue(users.contains(7)); }