Java Concurrency - Futures and Callables


java.util.concurrent.Callable object can return the computed result done by a thread in contrast to runnable interface which can only run the thread. The Callable object returns Future object which provides methods to monitor the progress of a task being executed by a thread. Future object can be used to check the status of a Callable and then retrieve the result from the Callable once the thread is done. It also provides timeout functionality.


//submit the callable using ThreadExecutor
//and get the result as a Future object
Future<Long> result10 = executor.submit(new FactorialService(10));
//get the result using get method of the Future object
//get method waits till the thread execution and then return the result of the execution.
Long factorial10 = result10.get();


The following TestThread program shows usage of Futures and Callables in thread based environment.

Live Demo
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;

public class TestThread {

   public static void main(final String[] arguments) throws InterruptedException,
      ExecutionException {

      ExecutorService executor = Executors.newSingleThreadExecutor();

      System.out.println("Factorial Service called for 10!");
      Future<Long> result10 = executor.submit(new FactorialService(10));

      System.out.println("Factorial Service called for 20!");
      Future<Long> result20 = executor.submit(new FactorialService(20));

      Long factorial10 = result10.get();
      System.out.println("10! = " + factorial10);

      Long factorial20 = result20.get();
      System.out.println("20! = " + factorial20);


   static class FactorialService implements Callable<Long> {
      private int number;

      public FactorialService(int number) {
         this.number = number;

      public Long call() throws Exception {
         return factorial();

      private Long factorial() throws InterruptedException {
         long result = 1; 
         while (number != 0) { 
            result = number * result; 
         return result;	

This will produce the following result.


Factorial Service called for 10!
Factorial Service called for 20!
10! = 3628800
20! = 2432902008176640000