This gist demos the working of a known number of Async tasks handled with completeable futures and hashmap

java

package com.example.futures;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;

import java.util.HashMap;
import java.util.concurrent.CompletableFuture;

@SpringBootApplication
public class FuturesDemoApplication {

    public static void main(String[] args) {

        SpringApplication.run(FuturesDemoApplication.class, args);


        CompletableFuture<HashMap<String, String>> hashMapCompletableFuture = getCompletableFutures();
        hashMapCompletableFuture.thenApplyAsync((t) -> {
            System.out.println(t.get("2"));
            System.out.println("+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-");
            System.out.println(t.get("1"));
            return null;
        });
    }

    public static CompletableFuture<HashMap<String, String>> getCompletableFutures() {
        final HashMap<String, String> myHashMap = new HashMap<>();
        CompletableFuture<String> firstAsync =
            CompletableFuture.supplyAsync(() -> {
                try {
                    Thread.sleep(1000);
                    return "First Async Resolved";
                } catch (InterruptedException e) {
                    throw new RuntimeException(e);
                }
            });
        CompletableFuture<String> secondAsync =
            CompletableFuture.supplyAsync(() -> {
                try {
                    Thread.sleep(300);
                    return "Second Async Resolved";
                } catch (InterruptedException e) {
                    throw new RuntimeException(e);
                }
            });
        return CompletableFuture.allOf(
                firstAsync,
                secondAsync
            )
            .thenApplyAsync(ignored -> {
                myHashMap.put("1", firstAsync.join());
                myHashMap.put("2", secondAsync.join());
                return myHashMap;
            });
    }
}