@Override
    public int hashCode() {
        return underlying().hashCode();
    }
@Test
    public void testHashCode() {
        final HashPMap<Object, Object> mock = mock(HashPMap.class);
        assertEquals(mock.hashCode(), new PCollectionsImmutableMap<>(mock).hashCode());
        final HashPMap<Object, Object> someOtherMock = mock(HashPMap.class);
        assertNotEquals(mock.hashCode(), new PCollectionsImmutableMap<>(someOtherMock).hashCode());
    }