Can you provide some background, why you want to have a result like this?

Actually if you want Dummy.name + List the most easiest way is to select the Dummyentities:

// Repo is a SpringData repository interface
Set<Dummy> result = repo.findAll();
final Map<String, List<SubEntity>> collect = result.stream().collect(Collectors.toMap(Dummy::getName, Dummy::getSubs));

In case you want to use criteria queries for filtering by subentity-property, just add this to the findAll, might look like this:

 // Not really tested...            
 Set<Dummy> result = repo.findAll( (root, query, cb) -> 
    query.where(cb.equal(root.get(Dummy_.SUBS).get(Sub_.SOMEPROP), cb.literal("some-value"));

final Map<String, List<SubEntity>> collect = result.stream().collect(Collectors.toMap(Dummy::getName, Dummy::getSubs));

Alternatively use entity-manager + create query etc. to const