Future and Callable

Definition

A Callable is a task that returns a value (a Runnable does not). When you submit it to an executor you get a Future: a receipt you can use later to get the result with get(), which waits if it is not ready, or to see that the task failed.

Example code

Submit tasks, then collect their results

import java.util.List;
import java.util.concurrent.Callable;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Future;

class Demo {
    public static void main(String[] args) throws Exception {
        ExecutorService pool = Executors.newFixedThreadPool(2);

        Callable<Integer> square = () -> 7 * 7;
        Future<Integer> answer = pool.submit(square);
        System.out.println(answer.get());                         // waits for the result

        Future<Integer> failing = pool.submit(() -> { throw new IllegalStateException("boom"); });
        try {
            failing.get();
        } catch (ExecutionException e) {
            System.out.println("task failed with: " + e.getCause().getMessage());
        }

        List<Future<Integer>> all = pool.invokeAll(List.of(() -> 1, () -> 2, () -> 3));   // run several, wait for all
        int total = 0;
        for (Future<Integer> f : all) total += f.get();
        System.out.println(total);
        pool.shutdown();
    }
}

// Output:
// 49
// task failed with: boom
// 6