@Override
    public boolean isEmpty() {
        return underlyingMap.isEmpty();
    }
@Test
    public void testIsEmptyAndSize() {
        Map<String, Integer> underlying = new HashMap<>();
        TranslatedValueMapView<String, String, Integer> view =
            new TranslatedValueMapView<>(underlying, v -> v.toString());
        assertTrue(view.isEmpty());
        assertEquals(0, view.size());
        underlying.put("quux", 101);
        assertFalse(view.isEmpty());
        assertEquals(1, view.size());
    }