Using streams to group Map attributes from inner objects?

Question

I'm learning Java 8 - Java 11 and I got a code that I'm converting to java-streams. I have the following classes:

class Resource {
   List<Capability> capabilities;
}

class Capability {
   String namespace;
   Map<String, Object> attributes;
}

I have a stream of Resources and I want to extract all its capabilities attributes from two different namespaces ("a", "b") to a Map<Resource, Map<String, Object>> that I have sure that do not have duplicates keys.

I did many attempts using map, flatMap but with those, I can't keep a reference of the main resource object. Using the new feature of java9 I could progress, but I'm stuck on the code below where I was able to return all attributes, but in a set. I was not able yet to filter by a capability namespace and also put them in a map:

Map<Resource, Set<Object>> result = pResolved.stream()
    .collect(groupingBy(t -> t, flatMapping(
            resource -> resource.getCapabilities(null).stream(),
            flatMapping(
                    cap -> cap.getAttributes().entrySet().stream(),
                    toSet()))));

Seems that I'm on the right path.

Answer 1

There is a way using only java-8 methods as well:

Map<String, Set<Object>> result = pResolved.stream()                        
    .map(Resource::getCapabilities)                         // Stream<List<Capability>>
    .flatMap(List::stream)                                  // Stream<Capability>
    .collect(Collectors.toMap(                              // Map<String, Set<Object>>
        c -> c.getNamespace(),                              // Key: String (namespace)
        i -> new HashSet<>(i.getAttributes().values())));   // Value: Set of Map values

Let's assume the sample input is:

Resource [capabilities=[
    Capability [namespace=a, attributes={a1=aa1, a2=aa2, a3=aa3}]]]
Resource [capabilities=[
    Capability [namespace=b, attributes={b2=bb2, b3=bb3, b1=bb1}], 
    Capability [namespace=c, attributes={c3=cc3, c1=cc1, c2=cc2}]]]

Then the code above would result in:

a: [aa1, aa3, aa2]
b: [bb1, bb3, bb2]
c: [cc1, cc3, cc2]

Answer 2

You could instead use Collectors.toMap as the downstream :

Map<Resource, Map<String, Object>> result = pResolved
        .stream()
        .collect(groupingBy(Function.identity(),
                flatMapping(resource -> resource.getCapabilities().stream(),
                        flatMapping(cap -> cap.getAttributes().entrySet().stream(),
                                toMap(Map.Entry::getKey, Map.Entry::getValue)))));