[Java] – How to create threads that return values?

We can no longer use the Runnable interface to do this. We must use the callable interface that was implemented in version 1.5 of java.

Example :

public class MyTask implements Callable<String> {
    public String call() throws Exception {
        // Here your implementation
        return new String("Result");

The Callable interface solves two problems: the thread can return something, defined by the generic parameter <String>, and the method call throws a Exception to not need the try-catch.

The tasks need to be performed by Callable ExecutorService, ie through the thread pool. The Thread class can not get a Callable:

ExecutorService executor = Executors.newCachedThreadPool();
executor.submit(new MyTask());

The question still is: where does the return Callable? The answer is in the submit method ExecutorService.

Understanding the return type Future

If we are using a Runnable we should use the execute method of ExecutorService, but when we use a Callable we should use the submit method. The difference is that the submit has a return, it is the Future type.

The submit method does not block the execution and can submit as many tasks we want.

public void test() throws Exception{
    ExecutorService executor = Executors.newCachedThreadPool();
    Future<String> futureCall = executor.submit(new MyTask());
    String result = futureCall.get(); // Here the thread will be blocked 
    // until the result came back.

How to limit the thread response time?

String result = futureCall.get(10,TimeUnit.SECONDS);
// Will waiting for ten seconds the result.

How to cancel the response time and unlock the thread?


Conclusions and Review

  • To set a task of a thread use the Callable interface; .
  • A Callable can return something and throw any exception;
  • We must submit Callable through a ExecutorService;
  • The submit method returns a Future ExecutorService;
  • The Future will receive the return of a Callable Implementation Class;


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