@CheckForNull File getDirectory(String userId) {
        String directoryName = idToDirectoryNameMap.get(getIdStrategy().keyFor(userId));
        return directoryName == null ? null : new File(usersDirectory, directoryName);
    }
@Test
    public void testEmptyGet() throws IOException {
        UserIdMapper mapper = createUserIdMapper(IdStrategy.CASE_INSENSITIVE);
        assertThat(mapper.getDirectory("anything"), nullValue());
    }