Parallelism in Java — A 360° Perspective from Threads to Project Loom

Can you use Callable without ExecutorService in Java?

1.4 Using Runnable with ExecutorService

Runnable vs Callable

2. Thread Pools & ExecutorService

Benefits of Thread Pools

What is ExecutorService?

Thread Pool Methods

How to use ExecutorService?

ExecutorService executor = Executors.newFixedThreadPool(3);

executor.submit(() -> {
    System.out.println("Task executed by: " + Thread.currentThread().getName());
});

executor.shutdown();  // Initiates orderly shutdown after submitted tasks complete

Example: Fixed Thread Pool

Methods of ExecutorService

3. Fork/Join Framework

Core Classes and Interfaces

How It Works

Computing Sum of an Array Using RecursiveTask

Fork/Join Framework vs ExecutorService

4. Parallel Streams (parallelStream())

List<Integer> numbers = Arrays.asList(1, 2, 3, 4, 5, 6, 7, 8, 9, 10);

int sum = numbers.parallelStream()
                 .mapToInt(n -> n * 2)
                 .sum();

System.out.println("Sum: " + sum);

5. CompletableFuture (Async Programming)

CompletableFuture<String> future = CompletableFuture.supplyAsync(() -> {
    // Simulate some long-running task
    try { Thread.sleep(1000); } catch (InterruptedException e) {}
    return "Hello, World!";
});

System.out.println("Doing something else...");

// Get the result (waits if not yet complete)
String result = future.join();
System.out.println("Result: " + result);

Chaining Example

CompletableFuture.supplyAsync(() -> "Hello")
    .thenApply(greeting -> greeting + " World")
    .thenAccept(System.out::println);  // Prints: Hello World

Combining Multiple Tasks

CompletableFuture<Integer> future1 = CompletableFuture.supplyAsync(() -> 10);
CompletableFuture<Integer> future2 = CompletableFuture.supplyAsync(() -> 20);

CompletableFuture<Integer> combined = future1.thenCombine(future2, Integer::sum);
System.out.println("Sum: " + combined.join()); // Output: Sum: 30

Error Handling

CompletableFuture<Integer> future = CompletableFuture.supplyAsync(() -> {
    if (true) throw new RuntimeException("Oops!");
    return 10;
}).exceptionally(ex -> {
    System.out.println("Caught exception: " + ex.getMessage());
    return 0;
});

System.out.println("Result: " + future.join()); // Output: 0

6. Virtual Threads (Java 21+)

How to Create Virtual Threads (Java 21+)

Using Thread.ofVirtual() API

Runnable task = () -> System.out.println("Running in: " + Thread.currentThread());

Thread vThread = Thread.ofVirtual().start(task);

Using Executor with Virtual Threads

ExecutorService executor = Executors.newVirtualThreadPerTaskExecutor();

executor.submit(() -> {
    System.out.println("Virtual thread: " + Thread.currentThread());
});
executor.shutdown();

7. Reactive Programming (Project Reactor, RxJava, Akka)

Popular Reactive Frameworks in Java

Project Reactor (Spring WebFlux)

Flux<Integer> numbers = Flux.range(1, 5)
    .map(i -> i * 2);

numbers.subscribe(System.out::println); // Outputs 2, 4, 6, 8, 10

RxJava

Observable.just("A", "B", "C")
    .map(String::toLowerCase)
    .subscribe(System.out::println); // Outputs a, b, c

Akka (Actor Model)

public class HelloActor extends AbstractActor {
    public Receive createReceive() {
        return receiveBuilder()
            .match(String.class, msg -> System.out.println("Received: " + msg))
            .build();
    }
}

// Sending a message to the actor
actorRef.tell("Hello, Akka!", ActorRef.noSender());

Conclusion