Summary - Concurrent API

Combine ExecutorService and CompletableFuture to write a parallel downloader.

Summary - Concurrent API

In this group you saw the core tools of java.util.concurrent: a pool with ExecutorService, results with Callable and Future, chains with CompletableFuture, a safe map with ConcurrentHashMap, and flexible locking with Lock.

Now you will combine these into a mini task: building a ParallelSum that runs tasks in parallel.

Exercise Sum two tasks in parallel with ParallelSum

Write main in a ParallelSum class; create a pool with Executors.newFixedThreadPool(2). Define two Callable<Integer> tasks: () -> 10 and () -> 20. Submit both; fetch results with get(); sum and print. Call shutdown().

Show example solution

import java.util.concurrent.*;

class ParallelSum {
    public static void main(String[] args) throws Exception {
        ExecutorService pool = Executors.newFixedThreadPool(2);
        Future<Integer> a = pool.submit(() -> 10);
        Future<Integer> b = pool.submit(() -> 20);
        System.out.println(a.get() + b.get());
        pool.shutdown();
    }
}

Exercise A chain with CompletableFuture

Write main in a new ChainDemo class; start with CompletableFuture.supplyAsync(() -> "hi"), transform with .thenApply(s -> s.toUpperCase()) to upper case and print with System.out.println(f.join()).

Show example solution

import java.util.concurrent.*;

class ChainDemo {
    public static void main(String[] args) {
        CompletableFuture<String> f =
            CompletableFuture.supplyAsync(() -> "hi").thenApply(s -> s.toUpperCase());
        System.out.println(f.join());
    }
}

Test yourself

What is the best method to wait for a task's result?
- Thread.sleep()
- Object.wait()
- Thread.yield()
- Future.get()
Show answer

Future.get() — Future.get() waits until the task finishes.
Match the Concurrent API summary

ExecutorService

Manages a pool
Future

Waits for result
CompletableFuture

Builds a chain
Lock

Flexible lock

Next lesson: Path

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