How to use Callable interface with Threads in Java?

Background

In one of the previous posts on Creating Threads with Executor service, we saw an interface called Callable. Executor has a method called submit that took Callable object and returned a future object which had the results. Unlike the Runnable interface, we can use Callable interface to return the result or throw checked Exceptions. In this post, we will see how to use Callable interface with plain Thread construct.

How to use Callable interface with Threads in Java?

We know that Thread takes Runnable interface only. No surprises there. So we cannot directly use objects implementing the Callable interface. Also to get the result we need Future object as we saw in Executors. Remember any design pattern that suits this use case? yes, it is Adapter Pattern. Java provides a class called FutureTask which implements Runnable and Future, combining both functionalities conveniently. Let's see an example of how we can do this -

package com.osfg;

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

public class Test {
 public static void main(String args[]) throws InterruptedException, ExecutionException {

  Callable<String> callable = () -> {
   Thread.sleep(5000);
   return "Hello!";
  };

  FutureTask<String> futureTask = new FutureTask<>(callable);
  Thread t = new Thread(futureTask);
  t.start();

  while (!futureTask.isDone()) {
   System.out.println("Task not done yet!");
   Thread.sleep(1000);
  }

  System.out.println(futureTask.get());
 }

}

You can see here that our Callable waits for 5 seconds and returns the results. And in the main thread, we do a check every 1 second to see if the result is available in the FutureTask. So if you run above code you will get -

Task not done yet!

Task not done yet!

Task not done yet!

Task not done yet!

Task not done yet!

Hello!

FutureTask has following methods -

• public boolean cancel(boolean mayInterrupt): Used to stop the task. It stops the task if it has not started. If it has started, it interrupts the task only if mayInterrupt is true.
• public Object get() throws InterruptedException, ExecutionException: Used to get the result of the task. If the task is complete, it returns the result immediately, otherwise, it waits until the task is complete and then returns the result.
• public boolean isDone(): Returns true if the task is complete and false otherwise

Related Links

• Creating Threads with Executor service(OSFG)
• Adapter and Facade Design Patterns in Java(OSFG)