IntIndexToIndexMultiMap implements IndexToIndexMultiMap { @Override public String toString() { return "IntIndexToIndexMultiMap{" + "keys=" + map.size() + '}'; } IntIndexToIndexMultiMap(
            @NotNull
            final Collection<? extends Collection<Integer>> map); @Override long getSizeInBytes(); @Override void writeTo(
            @NotNull
            final OutputStream os); @Override String toString();  }
@Test public void string() { final TreeMultimap<Integer, Integer> elements = TreeMultimap.create(); final int documents = 10; for (int i = 0; i < documents; i++) elements.put(i / 2, i); final IndexToIndexMultiMap set = new IntIndexToIndexMultiMap( elements.asMap().values()); final String text = set.toString(); assertTrue(text.contains(Integer.toString(documents / 2))); }