Use a callable object to retrieve values from tasks executed in a thread pool.

CallableFuture.java

import java.util.concurrent.*;

/**
 * Use a callable to retrieve values from tasks executed in a thread pool.
 */
public class CallableFuture {

    public static void main(String[] args) {

        Callable<Long> task = new Factorial(20);

        ExecutorService service = Executors.newSingleThreadExecutor();

        Future<Long> result = service.submit(task);

        String threadName = Thread.currentThread().getName();
        System.out.printf("%s waiting for result...%n", threadName);
        try {
            System.out.printf("%s got result: %d%n", threadName, result.get());
        } catch (InterruptedException e) {
            System.err.printf("%s interrupted while waiting for result!%n", threadName);
        } catch (ExecutionException e) {
            System.err.printf("%s got exception while executing task: %s%n", threadName, e.getMessage());
        }

        service.shutdown();
    }
}

class Factorial implements Callable<Long> {

    private long n;

    public Factorial(long n) {
        System.out.printf("%s created callable Factorial with n: %d%n", Thread.currentThread().getName(), n);
        this.n = n;
    }

    @Override
    public Long call() throws Exception {
        String threadName = Thread.currentThread().getName();
        System.out.printf("%s calculating factorial...%n", threadName);
        long factorial = 1;
        for (long j = 1; j <= n; j++) {
            System.out.printf("%s factorial %d*%d=", threadName, factorial, j);
            factorial *= j;
            System.out.printf("%d%n", factorial);
        }
        return factorial;
    }
}