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

groupingBy does exactly what you want:

import static java.util.stream.Collectors.*;
...
as.stream().collect(groupingBy((x) -> x.name, mapping((x) -> x.property, toSet())));