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<A> below, and transform it into a Map Map<String, Set<String>>.

For example, class A is:

class A {
    public String name;
    public String property;
}

I wrote the code below that collects the values into a map Map<String, String>:

final List<A> as = new ArrayList<>();
// the list as is populated ...

// works if there are no duplicates for name
final Map<String, String> m = as.stream().collect(Collectors.toMap(x -> x.name, x -> x.property));

However, because there might be multiple POJOs with the same name, I want the value of the map be a Set. All property Strings for the same key name should go into the same set.

How can this be done?

// how do i create a stream such that all properties of the same name get into a set under the key name
final Map<String, Set<String>> m = ???

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

@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