How to Group Objects in a List into other Lists by Attribute using streams & Java 8?

Question

I want to group a List of Objects containing a time attribute into 5-Minute intervals, preferably using streams and collectors.

The only possible solution I found on StackOverflow is to calculate how many intervals (sublists) I need, add every object to every one of these Lists and filter out the ones that dont fit into the respective timeframe, which is not exactly a nice solution.

(You can find the Thread here: How to group elements of a List by elements of another in Java 8)

I thought of something similar to this:

List<MyObject> list = new ArrayList<MyObject>();
.......
List<List<MyObject>> grouped = list.stream().collect(Collectors.groupingBy(obj -> obj.getTime() / intervalLength));

This of course doenst work.

I hope that, with Java 8 & its features being used more and more, we can find a suting solution for such a Problem.

Regards, Claas M.

EDIT: In the End, I used @Harsh Poddar's solution and converted the Map into a List of Lists using

for(Map.Entry<Integer, List<MyObject>> entry:map.entrySet())
  list.add(entry.getValue());

Note: The List had to be sorted afterwards.

Answer 1

This works totally fine:

Map<Long,<List<MyObject>> grouped = ticks.stream().collect(Collectors.groupingBy(obj -> obj.getTime() / intervalLength));

This method will give you a map on which you may then call .values() to get the list of lists. First element will contain elements with obj.getTime() from 0 to intervalLength provided there were such elements. The second will contain from intervalLength to intervalLength*2 and so on

Answer 2

Use a function to map a time to a bucket:

// assume time is in seconds
int bucketNumber(int baseTime, int thisTime) {
    return (thisTime - baseTime) / 300;
}

Then collect by bucket number:

ticks.stream().collect(groupingBy(e -> bucketNumber(baseTime, e.getTime()));

Now you have a Map<Integer, List<Tick>> where the key is the index of the five-minute bucket starting at baseTime, and a list of ticks in that bucket. If you just want a histogram, use a downstream collector:

ticks.stream().collect(groupingBy(e -> bucketNumber(baseTime, e.getTime()), 
                                  counting());