### Question:
PackedRatingMatrix implements RatingMatrix { @Override public LongCollection getItemIds() { return itemIndex().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 testGetItemIds() { LongCollection items = snap.getItemIds(); assertEquals(5, items.size()); assertTrue(items.contains(7)); assertTrue(items.contains(8)); assertTrue(items.contains(9)); assertTrue(items.contains(10)); assertTrue(items.contains(11)); }