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())));

Answer 2

Also, you can use the merger function option of the Collectors.toMap function Collectors.toMap(keyMapper,valueMapper,mergeFunction) as follows:

final Map<String, String> m = as.stream()
                                .collect(Collectors.toMap(
                                          x -> x.name, 
                                          x -> x.property,
                                          (property1, property2) -> property1+";"+property2);

Answer 3

@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

Answer 4

Same Same But Different

Map<String, Set<String>> m = new HashMap<>();

as.forEach(a -> {
    m.computeIfAbsent(a.name, v -> new HashSet<>())
            .add(a.property);
    });