How to call a method and run it asynchronously?

Is there a way to run getUsers() asynchronously only from main() in Java 8?

Running getUsers() on the main thread would take 300 seconds. I wish to make it in less than 180 seconds with 4 cores.

public class Main {

  // Getting users (with sleep)
  public List<User> getUsers() {

    
    List<User> users = new ArrayList();
    for(int i = 0; i < 100; i++) {
      users.add(new User("someName", "someAge"));
      TimeUnit.MILLISECONDS.sleep(3000);
    }
    return users;
  }

  public static void main(String[] args) {
    
    List<User> users = getUsers();
  }  
}

Without modifying getUsers() (the objective), running the following is still taking 300 seconds:

  public static void main(String[] args) {
    
    List<User> users =
    CompletableFuture.supplyAsync(() -> getUsers()).join();
  }  

Answer

You can divide your loop of users creation into smaller loops, and run them in parallel. Consider this example, which runs 4 times faster :

public static void main(String[] args) {
    // create a pool executor of 4 threads, increase or decrease it if need
    Executor executor = Executors.newFixedThreadPool(4); 

    CompletableFuture<List<User>> future1 = CompletableFuture.supplyAsync(() -> getUsers(25), executor);
    CompletableFuture<List<User>> future2 = CompletableFuture.supplyAsync(() -> getUsers(25), executor);
    CompletableFuture<List<User>> future3 = CompletableFuture.supplyAsync(() -> getUsers(25), executor);
    CompletableFuture<List<User>> future4 = CompletableFuture.supplyAsync(() -> getUsers(25), executor);

    List<User> allUsers = Stream.of(future1, future2, future3, future4)
                    .map(CompletableFuture::join)
                    .flatMap(List::stream)
                    .collect(Collectors.toList());
}


public static List<User> getUsers(int count) {
    List<User> users = new ArrayList<>();
    for (int i = 0; i < count; i++) {
        users.add(new User("someName", "someAge"));
        try {
            TimeUnit.MILLISECONDS.sleep(3000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
    return users;
}