Java 8 Parallel Stream Concurrent Grouping

user3665053 :

Suppose I have a class as

Class Person {
  String name;
  String uid;
  String phone;
}

I am trying to group by all the fields of the class. How do i use parallel streams in JAVA 8 to convert a

List<Person> into Map<String,Set<Person>>

where the key of the map is the value of each field in the class . JAVA 8 the following example groups by a single field, how can i do it for all fields of a class into a single Map?

ConcurrentMap<Person.Sex, List<Person>> byGender =
roster
    .parallelStream()
    .collect(
        Collectors.groupingByConcurrent(Person::getGender));

Lino :

You can do that by using the of static factory method from Collector:

Map<String, Set<Person>> groupBy = persons.parallelStream()
    .collect(Collector.of(
        ConcurrentHashMap::new,
        ( map, person ) -> {
            map.computeIfAbsent(person.name, k -> new HashSet<>()).add(person);
            map.computeIfAbsent(person.uid, k -> new HashSet<>()).add(person);
            map.computeIfAbsent(person.phone, k -> new HashSet<>()).add(person);
        },
        ( a, b ) -> {
            b.forEach(( key, set ) -> a.computeIfAbsent(key, k -> new HashSet<>()).addAll(set));
            return a;
        }
    ));

As Holger in the comments suggested, following approach can be preferred over the above one:

Map<String, Set<Person>> groupBy = persons.parallelStream()
     .collect(HashMap::new, (m, p) -> { 
         m.computeIfAbsent(p.name, k -> new HashSet<>()).add(p); 
         m.computeIfAbsent(p.uid, k -> new HashSet<>()).add(p); 
         m.computeIfAbsent(p.phone, k -> new HashSet<>()).add(p); 
     }, (a, b) -> b.forEach((key, set) -> {
         a.computeIfAbsent(key, k -> new HashSet<>()).addAll(set));
     });

It uses the overloaded collect method which acts identical to my suggested statement above.