Java Threads : Callable and Future


Whenever it comes to creating a thread in Java we think of Runnable interface. Runnable has a method run with no parameters and no return value. But in case we expect some value to be returned after completion of a thread, we have to look for some workarounds. Callable is an interface introduced in Java 5 to take care of this specific requirement. Callable is similar to Runnable, but it returns a value.  A Future interface holds the result of asynchronous computation and it is used in conjunction with Callable. FutureTask is a wrapper class which implements both Future and Runnable interfaces and it provides a convenient way to convert a Callable into both a Future and a Runnable.

You can refer the online documentation of Java SE 5 to know more about Callable, Future and FutureTask.

Let us see a simple example demonstrating the use of Callable:


To demonstrate how to use Callable and Future.


Create a StringGenerator thread which is supposed to return a random string to the caller. Create a StringGeneratorTest class which spawns ten StringGenerator thread and displays the concatenation of the string returned from all the threads.

The Code:

The StringGenerator Class:

package com.technicalmusings.samples.thread;

import java.util.concurrent.Callable;
import static java.lang.Math.random;

public class StringGenerator implements Callable<String> {

	public String call() throws Exception {
		String [] allStrings =
                {"Apple", "Ball", "Cat", "Dog", "Elephant",
                "Flower", "Grape", "Horse", "Ink","Joker"};
		int index = (int)(random()*100)/10;

		//Let's wait for sometime
		return allStrings[index];

The StringGeneratorTest class:

package com.technicalmusings.samples.thread;

import java.util.ArrayList;
import java.util.concurrent.Callable;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.Future;
import java.util.concurrent.FutureTask;

public class StringGeneratorTest {

	public static void main(String[] args) {

		//Create an array to store the future objects.
		ArrayList<Future<String>> results = new ArrayList<Future<String>>();
		for (int i=0; i<10; i++){
			//Create the instance of the Callable task
                        Callable<String> stringGenerator = new StringGenerator();

			//create the object of FutureTask
			FutureTask<String> task =
                                new FutureTask<String>(stringGenerator);

			//Add to the list

			//Create a thread object using the task object created
			Thread t = new Thread(task);

			//Start the thread as usual


		//Compute the results now.
		StringBuilder resultStr = new StringBuilder();

		long start = System.currentTimeMillis();

		for(Future<String> result: results){
			try {
				//The blocking get call
				resultStr.append(" ");

			} catch (InterruptedException e) {
			} catch (ExecutionException e) {

		long end = System.currentTimeMillis();

		System.out.println("The returned string is:\n"+resultStr);
		System.out.println("Execution time:"+(end - start));


The returned string is:
Dog Dog Joker Elephant Flower Ball Grape Elephant Elephant Horse
Execution time:1014

The Executors class is also worth looking at as it contains utility methods to convert from other common forms to Callable.

Enjoy Learning,



11 thoughts on “Java Threads : Callable and Future

  1. Pingback: Mocks et Multithreading « Choses à faire
    • The English Translation: MultithreadingPremièrement it is important to understand the functioning of the Callable interface in Java, an interface similar to Runnable, unlike almost a thread Callable.

    • Hi Kamal,

      Normally one of the best usage of threads is for background processes.
      For example, suppose you have an order processing website and you want to log the information entered by the user in database or to a remote web service, you can use threads. The moment user submit the order, you can instantiate a thread for logging.


  2. Hi Kamesh,

    Thanks a lot, it a very good article and new thing for me.
    But wondering how Runnable task not returning anything, after converted to Callable objects using Executors, returns result? Will explore this.

    Best regards,

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