stream of list groupinngBy to map

Question

the example code I've been given is

public Map<String, List<Bier>> opzettenOverzichtBierenPerSoort() {
   //TODO
    
    return bieren.stream().collect(Collectors.groupingBy(Bier::getSoort, TreeMap::new, Collectors.toList()));
}

input is a list of beer objects and it return a map of the kind of beer with all the beers in it.

now my question. wat are the second and third arguments in the groupingBy? I get the first one which states what it's grouped by...but the second and third seem a bit random.

Have you read the documentation on oracle's site? They have javadocs that explain them. — Barracuda, Jan 03 '22 at 12:10

score 2 · Accepted Answer · answered Jan 03 '22 at 12:14

The second argument is a Supplier<M>, which is used to produce a Map instance.

The third argument is a downstream Collector, which specifies what to do with the Bier elements which belong to a single group.

If you run the single argument variant:

return bieren.stream().collect(Collectors.groupingBy(Bier::getSoort));

It will still collect the elements of each group into a List (that's the default behavior), but you don't have control over the type of Map that will map the String keys into the corresponding Lists.

In your 3 argument example, you request that the Map will be a TreeMap, which means the keys will be sorted.

The current implementation of the single argument variant:

return bieren.stream().collect(Collectors.groupingBy(Bier::getSoort));

is equivalent to:

return bieren.stream().collect(Collectors.groupingBy(Bier::getSoort, HashMap::new, Collectors.toList()));

which means the keys of the Map will not be sorted.

stream of list groupinngBy to map

1 Answers1