How to get the result of a Future in Java

What is Future in Java?

The executor service’s submit() method submits a task to a thread for execution. However, it does not know when the task’s outcome will be available. As a result, it returns a Future. This is a reference that can be used to retrieve the task’s outcome when it becomes available.

In other languages, such as JavaScript, the idea of a Future is similar to a Promise. It reflects the outcome of a calculation that will be completed at a later date.

Hence, a Future is a placeholder that stores the result of an asynchronous computation.

The get() method

The get() method is a blocking call. It is used to wait for the processing of the task to finish and retrieve the result.

Syntax

V get()

Parameters

The method has no parameters.

Return value

This method returns the result of the computation.

Code

import java.util.concurrent.*;
public class Main {
    public static void main(String[] args) throws InterruptedException, ExecutionException {
        ExecutorService executorService = Executors.newSingleThreadExecutor();
        Callable<String> stringCallable = () -> {
            Thread.sleep(1000);
            return "hello edpresso";
        };
        Future<String> stringFuture = executorService.submit(stringCallable);
        while(!stringFuture.isDone() && !stringFuture.isCancelled()) {
            Thread.sleep(200);
            System.out.println("Waiting for task completion...");
        }
        String result = stringFuture.get();
        System.out.println("Retrieved result from the task - " + result);
        executorService.shutdown();
    }
}

Explanation

  • Line 1: We import the relevant classes.
  • Line 7: We define a single-threaded executor service.
  • Lines 9 to 11: We define a callable that sleeps for one second and returns a string.
  • Line 14: We get a future object as a result of submitting the callable to the executor service.
  • Lines 16 to 19: We wait for the future to complete in a while loop.
  • Line 21: We get the result of the future using the get() method.
  • Line 23: The result is printed to the console.
  • Line 25: The executor service is shut down.

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