java - Grouping items by date

Question

I have items in my list and the items have a field, which shows the creation date of the item.

and I need to group them based on a "compression", which the user gives. The options are Day, Week, Month and Year.

If the user selects day compression, I need to group my items as such that the items, which are created in the same day, will be groupped. In my example above, only item 1 and item 2 are created in the same day. The others are also groups but they will have only one item because at their day, only one item is created.

{{item1, item2}, {item3}, {item4}, {item5}, {item6}, {item7}}

If the user selects week:

{{item1, item2, item3, item4}, {item5}, {item6}, {item7}}

If the user selects month:

{{item1, item2, item3, item4, item5}, {item6}, {item7}}

If the user selects year:

{{item1, item2, item3, item4, item5, item6}, {item7}}

After groups are created, the date of the items are not important. I mean the key can be anything, as long as the groups are created.

In case of usage of Map, I thought as the keys as follow:

day = day of the year
week = week of the year
month = month of the year
year = year

What would be the best solution to this problem? I could not even start it an I cannot think of a solution other than iteration.

Answer 1

I would use Collectors.groupingBy with an adjusted LocalDate on the classifier, so that items with similar dates (according to the compression given by the user) are grouped together.

For this, first create the following Map:

static final Map<String, TemporalAdjuster> ADJUSTERS = new HashMap<>();

ADJUSTERS.put("day", TemporalAdjusters.ofDateAdjuster(d -> d)); // identity
ADJUSTERS.put("week", TemporalAdjusters.previousOrSame(DayOfWeek.of(1)));
ADJUSTERS.put("month", TemporalAdjusters.firstDayOfMonth());
ADJUSTERS.put("year", TemporalAdjusters.firstDayOfYear());

Note: for "day", a TemporalAdjuster that lets the date untouched is being used.

Next, use the compression given by the user to dynamically select how to group your list of items:

Map<LocalDate, List<Item>> result = list.stream()
    .collect(Collectors.groupingBy(item -> item.getCreationDate()
            .with(ADJUSTERS.get(compression))));

The LocalDate is adjusted by means of the LocalDate.with(TemporalAdjuster) method.