Call me!

Today you will have a great occasion to call someone you would never expect to call... a thread. We talked already about the concurrency on the blog: Threads part 1: The basics and Threads part 2: functions overview. But the problem with the previous solution to achieve simultaneous computation is the lack of explicit mechanism to retrieve the result from a thread. As luck would have it, since Java 1.5 we have an access to a new interface – Callable. In comparison to Runnable there are a few significant differences:

• with Callable you can easily return a result of another execution thread.
• with Callable you can throw checked exceptions.
• with Callable you have to use a thread executor.

So far so good. Now, to get more familiar with it, let's have a look at some code:

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 Computation implements Callable<Integer> {

@Override
public Integer call() throws Exception {
    int result = 0;
    for(int i = 0; i < 10000; i++) {
        for(int j = 0; j < 100000; j++) {
            // Some taught computation
            result += (i * j) - (i * j) + 1;
        }
    }
    return result;
}

}

public class CallableExample {
public static void main(String[] args) {
    ExecutorService pool = Executors.newFixedThreadPool(5);
    Future<Integer> f1 = pool.submit(new Computation());
    for(int i = 0; i < 10; i++) {
        System.out.println(i);
    }
    try {
        System.out.println(f1.get());
    } catch (InterruptedException e) {
        e.printStackTrace();
    } catch (ExecutionException e) {
        e.printStackTrace();
    }
    pool.shutdown();
}
}

Ok, there are a few new elements regarding Callable. First of all, if we implement the Callable interface, we have to implement T call() method. This is where the action takes place, it's a method corresponding to the run() method from Runnable interface. And the other important thing – it's the place where we can throw an exception (Exception or any subclass).
When the class implementing Callable interface is ready, we can instantiate it and run through a submit() method of a thread executor what happens in line 27. The submit() method will return some implementation of a Future interface. It represents the thread's computation and provides a few useful methods, among others:
boolean isDone();
which returns true if the task is completed.
T get()
which returns a result of a task of type T, or if the task is not completed yet it waits until it's finished. Actually this could be achieved with a wait() method using a Runnable interface and calling it on the instance of a thread from the main execution thread.

And that's it. I'm sure that the above information and the sample code will let you use the Callable interface in appropriate way.