CODEWITHSUNDEEP
javacollectionsgrouping
sadrutdin.zainukov
sadrutdin.zainukov

Reputation: 49

How to group List of Object by date?

My Class:

public class Proposal {
  Date createDate;
  // ...
}

We have two objects java.util.Date and know for sure that the objects of this collection are between these two dates. How to divide these objects by day between these dates, so that I can get a list of these objects by date?

Upvotes: 0

Views: 3653

Answers (3)

user11595728
user11595728

Reputation:

As of Java 8 this can be accomplished seamlessly with Collectors.groupingBy:

Map<Date, List<Proposal>> groupedByDate = proposals.stream()
    .collect(Collectors.groupingBy(Proposal::getDate));

Upvotes: 1

mavriksc
mavriksc

Reputation: 1132

Something like this to get one of the lists. List<Object> inDateList = list.stream().filter(o-> startDate< o.createDate && o.createDate< endDate).collect(Collectors.toList());

then List<Object> outDateList = new ArrayList<>(list); outDateList.removeAll(inDateList);

EDIT Just to clarify on my note above. 

public Map<String, Set<Proposal>> groupProposals(Iterable<Proposal> proposals) {
SimpleDateFormat sdf = new SimpleDateFormat("yyyyMMdd");
return proposals.stream()
//GroupingBy creates the Map<Key, Collection<Something>>
.collect(Collectors.groupingBy(p->sdf.format(p.getCreateDate()),//Creates the Key and buckets
        Collectors.mapping(i-> i, Collectors.toSet()))); //what kind of buckets do you want. 
}

Upvotes: 0

Jeff I
Jeff I

Reputation: 394

Something like this?

public Map<String, Set<Proposal>> groupProposals(Iterable<Proposal> proposals) {
    Map<String, Set<Proposal>> map = new HashMap<>();
    SimpleDateFormat sdf = new SimpleDateFormat("yyyyMMdd");
    for (Proposal p : proposals) {
        String key = sdf.format(p.getCreateDate());
        if (!map.containsKey(key)) {
            map.put(key, new HashSet<>());
        }
        map.get(key).add(p);
    }
    return map;
}

Upvotes: 3

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