Using Callable to Return Results From Runnables

by John Zukowski

The Runnable interface has been around since the
beginning of time for the Java platform. It allows you to define a
task to be completed by a thread. As most people probably know
already, it offers a single method run() that accepts
no arguments and returns no values, nor can it throw any checked
exceptions. If you need to get a value back from the now-completed
task, you must use a method outside the interface and wait for some
kind of notification message that the task completed. For example,
the following demonstrates what you might do for just such a
scenario:

  Runnable runnable = ...;
  Thread t = new Thread(runnable);
  t.start();
  t.join();
  String value = someMethodtoGetSavedValue()

Nothing is inherently wrong with this code, but it can be done
differently now, thanks to the Callable interface
introduced in J2SE 5.0. Instead of having a run()
method, the Callable interface offers a
call() method, which can return an Object or, more
specifically, any type that is introduced in the genericized form:

  public interface Callable<V> {
     V call() throws Exception;
  }

Because you cannot pass a Callable into a
Thread to execute, you instead use the
ExecutorService to execute the Callable
object. The service accepts Callable objects to run by
way of the submit() method:

  <T> Future<T> submit(Callable<T> task)

As the method definition shows, submitting a Callable object to
the ExecutorService returns a Future
object. The get() method of Future will
then block until the task is completed. This is the equivalent of
the join() call in the first example. Actually, it is
the equivalent of both the join() call and the get
value call as get() returns the value calculated by the
Callable instance.

To demonstrate, the following example creates separate
Callable instances for each word passed in on the
command line and sums up their length. Each Callable
will just calculate the sum of its individual word. The set of
Future objects are saved to acquire the calculated
value from each. If the order of the returned values needed to be
preserved, a List could be used instead.

import java.util.*;
import java.util.concurrent.*;

public class CallableExample {

  public static class WordLengthCallable
        implements Callable {
    private String word;
    public WordLengthCallable(String word) {
      this.word = word;
    }
    public Integer call() {
      return Integer.valueOf(word.length());
    }
  }

  public static void main(String args[]) throws Exception {
    ExecutorService pool = Executors.newFixedThreadPool(3);
    Set<Future<Integer>> set = new HashSet<Future≶Integer>>();
    for (String word: args) {
      Callable<Integer> callable = new WordLengthCallable(word);
      Future<Integer> future = pool.submit(callable);
      set.add(future);
    }
    int sum = 0;
    for (Future<Integer> future : set) {
      sum += future.get();
    }
    System.out.printf("The sum of lengths is %s%n", sum);
    System.exit(sum);
  }
}

The WordLengthCallable saves each word and uses the
word's length as the value returned by the call()
method. This value could take some time to generate but in this case
is known immediately. The only requirement of call() is
the value is returned at the end of the call. When the
get() method of Future is later called,
the Future will either have the value immediately if
the task runs quickly, as in this case, or will wait until the value
is done generating. Multiple calls to get() will not
cause the task to be rerun in the thread.

Because the goal of the program is to calculate the sum of all
word lengths, it doesn't matter in which order the
Callable tasks finish. It is perfectly OK if the last
task completes before the first three. The first get()
call to Future will just wait for the first task in the
Set to complete. This does not block other tasks from
running to completion separately. It is just waiting for that one
thread or task to complete.

This particular example uses a fixed-size thread pool for the
ExecutorService, but any available service will do.

For more information on the use of executors and thread pools,
see the Executors lesson [1] in the Java Tutorial. The
SwingWorker class is another example of a
Runnable object that works with a Future,
though in a slightly different way. See the
Worker Threads and SwingWorker lesson [2] for more information on
that.

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