Grouping LocalDateTime objects in intervals using Java 8

I have a List in the following format and I want to group this List into minute intervals.

 List<Item> myObjList = Arrays.asList(
                new Item(LocalDateTime.parse("2020-09-22T00:13:36")), 
                new Item(LocalDateTime.parse("2020-09-22T00:17:20")),
                new Item(LocalDateTime.parse("2020-09-22T01:25:20")),
                new Item(LocalDateTime.parse("2020-09-18T00:17:20")),
                new Item(LocalDateTime.parse("2020-09-19T00:17:20")));

For example, given an interval of 10 minutes the first 2 objects of the list should be in the same group, the 3rd should be in a different group, etc.

Can this List be grouped into intervals using Java's 8 groupingBy function?

My solution is to compare every date in the list with all the other dates in the list and add the dates that differ X minutes in a new List. This seems to be very slow and 'hacky' workaround and I wonder if there is a more stable solution.

It is possible to use Collectors#groupingBy to group LocalDateTime objects into lists of 10-minute intervals. You'll have to adapt this snippet to work with your Item class, but the logic is the same.

List<LocalDateTime> myObjList = Arrays.asList(
    LocalDateTime.parse("2020-09-22T00:13:36"),
    LocalDateTime.parse("2020-09-22T00:17:20"),
    LocalDateTime.parse("2020-09-22T01:25:20"),
    LocalDateTime.parse("2020-09-18T00:17:20"),
    LocalDateTime.parse("2020-09-19T00:17:20")
);

System.out.println(myObjList.stream().collect(Collectors.groupingBy(time -> {
    // Store the minute-of-hour field.
    int minutes = time.getMinute();

    // Determine how many minutes we are above the nearest 10-minute interval.
    int minutesOver = minutes % 10;

    // Truncate the time to the minute field (zeroing out seconds and nanoseconds),
    // and force the number of minutes to be at a 10-minute interval.
    return time.truncatedTo(ChronoUnit.MINUTES).withMinute(minutes - minutesOver);
})));

Output

{
    2020-09-22T00:10=[2020-09-22T00:13:36, 2020-09-22T00:17:20],
    2020-09-19T00:10=[2020-09-19T00:17:20],
    2020-09-18T00:10=[2020-09-18T00:17:20],
    2020-09-22T01:20=[2020-09-22T01:25:20]
}

You didn't specify the key for the groups so I just used the quotient of (minutes/10)*10 to get the start of the range of minutes tagged onto the time truncated to hours.

List<Item> myObjList = Arrays.asList(
		new Item(LocalDateTime.parse("2020-09-22T00:13:36")),
		new Item(LocalDateTime.parse("2020-09-22T00:17:20")),
		new Item(LocalDateTime.parse("2020-09-22T01:25:20")),
		new Item(LocalDateTime.parse("2020-09-18T00:17:20")),
		new Item(LocalDateTime.parse("2020-09-19T00:17:20")));

Map<String, List<Item>> map = myObjList.stream()
		.collect(Collectors.groupingBy(item -> {
			int range =
					(item.getTime().getMinute() / 10) * 10;
			return item.getTime()
					.truncatedTo(ChronoUnit.HOURS).plusMinutes(range) +
					" - " + (range + 9) + ":59";
		}));

		
map.entrySet().forEach(System.out::println);

Prints

2020-09-22T00:10 - 19:59=[2020-09-22T00:13:36, 2020-09-22T00:17:20]
2020-09-19T00:10 - 19:59=[2020-09-19T00:17:20]
2020-09-18T00:10 - 19:59=[2020-09-18T00:17:20]
2020-09-22T01:20 - 29:59=[2020-09-22T01:25:20]

Here is the class I used.

class Item {
	LocalDateTime ldt;
	
	public Item(LocalDateTime ldt) {
		this.ldt = ldt;
	}
	
	public String toString() {
		return ldt.toString();
	}
	
	public LocalDateTime getTime() {
		return ldt;
	}
}

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