    @Test
    public void groupByAge() throws Exception {

        expected = new HashMap<>();
        expected.put(23, new AtomicInteger(2));
        expected.put(25, new AtomicInteger(3));
        expected.put(26, new AtomicInteger(2));
        people = Arrays.asList(
                new PartitionSortRepeats.Person(25, "Rita"),
                new PartitionSortRepeats.Person(23, "Felipe"),
                new PartitionSortRepeats.Person(25, "Vera"),
                new PartitionSortRepeats.Person(25, "Nathan"),
                new PartitionSortRepeats.Person(26, "Daniel"),
                new PartitionSortRepeats.Person(23, "Zach"),
                new PartitionSortRepeats.Person(26, "Tom")
        );

        test(expected, people);
    }