assertEquals(ImmutableMap.of("four", 4), filtered);

    unfiltered.clear();
    assertEquals(ImmutableMap.of(), unfiltered);
    assertEquals(ImmutableMap.of(), filtered);
  }

  public void testFilteredValuesIllegalPut() {
    Map<String, Integer> unfiltered = createUnfiltered();
    Map<String, Integer> filtered = Maps.filterValues(unfiltered, EVEN);
    filtered.put("a", 2);
    unfiltered.put("b", 4);