how to get the intersection of a list of lists in Java using asynchronous and using multi-thread

Question

I have a Person class like this:

public class Person {
    public String name;
    public List<String> hobbies;


    public Person(String name, List<String> hobbies) {
        this.name = name;
        this.hobbies = hobbies;
    } 
    //setter and getter
}

Now suppose I have a list of Person objects I want to get the intersection of hobbies asynchronously. I know how to get the intersection of two lists normally but I want to do this asynchronous and multi-threaded. I

Answer 1

you can do this:

List<Person> persons = new ArrayList<>();
persons.add(new Person("hatef", Arrays.asList("a", "b", "c", "t")));
persons.add(new Person("hatef", Arrays.asList("a", "b", "t")));
persons.add(new Person("hatef", Arrays.asList("t", "a", "b")));
persons.add(new Person("hatef", Arrays.asList("a", "b", "f", "t")));
persons.add(new Person("hatef", Arrays.asList("a", "t", "b", "z")));

then use CompletableFuture

Map<String, Integer> all = new ConcurrentHashMap<>();
ExecutorService executorService = Executors.newFixedThreadPool(Math.max(10, persons.size()));
persons
        .stream()
        .map(person -> CompletableFuture.supplyAsync(() -> person, executorService))
        .map(p -> p.thenApply(Person::getHobbies))
        .map(p -> p.thenAccept(hobbies -> hobbies.forEach(hobby -> all.compute(hobby, (key, value) -> value == null ? 1 : ++value))))
        .map(CompletableFuture::join)
        .forEach(x -> {}); // just dummy consumer to make stream triggers

int n = persons.size();
Set<String> result = all
            .entrySet()
            .stream()
            .filter(entry -> entry.getValue() == n)
            .map(Map.Entry::getKey)
            .collect(Collectors.toSet());
System.out.println(result);

which results in:

[a, b, t]

if you have order in hobby list I think you can use the idea of Longest Common Prefix problem and it is efficient

I agree with @JoachimSauer's comment