Compare2a.java

package test;

import java.util.concurrent.*;
import java.util.concurrent.atomic.AtomicReference;

/*
 * Let's say we want to fetch the user's profile from one API call, 
 * and the user's friends from another API call, in parallel.
 */
public class Compare2Synchronous {
    // I/0 operations: blocking, synchronous
    String sendHttpGet(String url) { return null; }

    // the business logic: manual, explicit concurrency
    // accepting the executor as a parameter, so that it can be shared
    String runBusinessProcess(ExecutorService executor, long userId) throws InterruptedException {
        // the atomic references will store the results of both API calls
        AtomicReference<String> profileResult = new AtomicReference<String>(null);
        AtomicReference<String> friendsResult = new AtomicReference<String>(null);
        // the latch will reach 0 once both API calls complete
        CountDownLatch done = new CountDownLatch(2);

        // submitting tasks to send the API calls. If the executor has enough threads, 
        // these will run in parallel
        executor.execute(() -> {
            String result = sendHttpGet("http://profile_service/get/" + userId);
            profileResult.set(result);
            done.countDown();
        });
        executor.execute(() -> {
            String result = sendHttpGet("http://friends_service/get/" + userId);
            friendsResult.set(result);
            done.countDown();
        });

        // in the main thread, waiting until both computations are done
        done.await();

        return "Profile: " + profileResult.get() + ", friends: " + friendsResult.get();
    }
}