    @Test
    public void groupUnique() {

        Map<Integer, Integer> expected = new HashMap<>();
        expected.put(2, 1);
        expected.put(4, 2);
        expected.put(6, 3);

        final Map<Integer, Integer> result =
                GROUPING_HELPER.groupUnique(number -> number * 2);
        Assert.assertEquals("Unexpected map content", expected, result);
    }