public KeySet keySet()
    {
        if (null == keySet)
        {
            keySet = new KeySet();
        }

        return keySet;
    }
@Test
    void removeIfOnKeySet()
    {
        final Predicate<Integer> filter = (key) -> true;

        final UnsupportedOperationException exception =
            assertThrowsExactly(UnsupportedOperationException.class, () -> cache.keySet().removeIf(filter));
        assertEquals("Cannot remove from KeySet", exception.getMessage());
    }