Stream groupingBy: reducing to first element of list [duplicate]

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.