Java Stream Collectors.toMap value is a Set

Question

I want to use a Java Stream to run over a List of POJOs, such as the list List below, and transform it into a Map Map>

Answer 1

@Nevay 's answer is definitely the right way to go by using groupingBy, but it is also achievable by toMap by adding a mergeFunction as the third parameter:

as.stream().collect(Collectors.toMap(x -> x.name, 
    x -> new HashSet<>(Arrays.asList(x.property)), 
    (x,y)->{x.addAll(y);return x;} ));

This code maps the array to a Map with a key as x.name and a value as HashSet with one value as x.property. When there is duplicate key/value, the third parameter merger function is then called to merge the two HashSet.

PS. If you use Apache Common library, you can also use their SetUtils::union as the merger