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.
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.