Stream groupingBy: reducing to first element of list

Question

I have a List<Valuta> which can be represented (simplified) JSON-style:

[ { codice=EUR, description=Euro, ratio=1 }, { codice=USD, description=Dollars, ratio=1.1 } ]

I want to transform that in a Map<String, Valuta> like this:

{ EUR={ codice=EUR, description=Euro, ratio=1 }, USD={ codice=USD, description=Dollars, ratio=1.1 }}

I wrote this one-liner:

getValute().stream().collect(Collectors.groupingBy(Valuta::getCodice));

but this returns a Map<String, List<Valuta>> instead of what I need.

I suppose mapping() function would work for me, but don't know how.

Answer 1

Actually, you need to use Collectors.toMap here instead of Collectors.groupingBy:

Map<String, Valuta> map = 
    getValute().stream()
               .collect(Collectors.toMap(Valuta::getCodice, Function.identity()));

groupingBy is used to group elements of a Stream based on a grouping function. 2 Stream elements that will have the same result with the grouping function will be collected into a List by default.

toMap will collect the elements into a Map where the key is the result of applying a given key mapper and the value is the result of applying a value mapper. Note that toMap, by default, will throw an exception if a duplicate is encountered.