Java 8 reverse of grouping by

Question

I'm trying to do a simple thing but I'm a bit stuck. I have the following :

Map<Date, Collection<String>>

So, for one Date I have a list of String (not a unique list). What I need is to transform this to the following format:

List<Object<Date,String>>

Each element of the first list could be present several times

(Ex of my main list: (date1, string1), (date1, string1), (date2, string2), (date2, string3))

I'm trying to do so with java stream but is not so obvious.

Any idea ?

Thanks.

EDIT: Here is what I have so far

    Map<Date, Collection<MyObject>> result = new HashMap<>();
    ......adding elements to the map...

                result.entrySet().stream().
                collect(Collectors.toMap(
                        e -> e.getKey(),
                        v -> v.getValue().stream().map( p ->  aggregateProductService.findProduct(customer.getPublicId(), p, setId)).
                                                   filter(Optional::isPresent).
                                                   map(Optional::get).
                                                   collect(toList())
                        )
                ).entrySet().stream().sorted(Map.Entry.comparingByKey()).flatMap(e -> e.getValue().stream().map(s -> Pair.of(e.getKey(), s))).
limit(10).
                           collect(toList()).stream().
                          collect(Collectors.groupingBy(Pair::getLeft, Collectors.toList()));

This piece of code does not compile. I have the following error:

Pair::getLeft "non static method cannot be referenced from static context"

It is not clear, Can you provide a constructive example??

bananas
– bananas

2017-09-29 10:10:21 +00:00
Commented Sep 29, 2017 at 10:10 — bananas
– bananas, Commented Sep 29, 2017 at 10:10

Eugene · Accepted Answer · 2017-09-29 10:11:07Z

4

If I understood correctly:

map.entrySet()
   .stream()
   .flatmap(e -> e.getValue().stream().map(s -> Pair.of(e.getKey(), s)))
   .collect(Collectors.toList());

answered Sep 29, 2017 at 10:11

Eugene

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5 Comments

CC. Over a year ago

Great. Do you know how to do the reverse after that ? Actually I need to flatten in order to limit and after that group then like initially. I'm struggling with the groupBy but no success.

Eugene Over a year ago

@CC. probably something along the lines of limit(10).collect(Collectors.groupingBy(Pair::getLeft, Collectors.toList()))

CC. Over a year ago

Yes, this is what I've tried but I keep having this error message for Pair::getLeft "non static method cannot be referenced from static context"

Eugene Over a year ago

@CC. well without actual code, it hard to say exactly what does not work

Eugene Over a year ago

@CC. don't get me wrong, but this is a different question right now... you could create a new one and get the desired answer.

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