Future Interface in Java

The Future interface is a part of java.util.concurrent package, introduced in Java 5. It represents the result of an asynchronous computation, a value that will be available in the future after the task completes.

Future interface used to check the status of a task (completed, running, or cancelled).
It allows retrieving the result of a Callable once it’s done.
Supports cancellation of tasks that are still running.

Declaration

public interface Future<V>

Here, V -> The type of result returned by the asynchronous computation.

Java

import java.util.concurrent.*;

public class GFG{

    public static void main(String[] args) throws Exception{

        // Create an ExecutorService
        ExecutorService executor
            = Executors.newSingleThreadExecutor();

        // Define a Callable task
        Callable<Integer> task = () ->
        {
            System.out.println("Processing...");
            Thread.sleep(1000);
            return 10 * 5;
        };

        // Submit the task and get a Future object
        Future<Integer> future = executor.submit(task);

        // Check if task is done
        System.out.println("Task completed? "
                           + future.isDone());

        // Retrieve the result
        System.out.println("Result: " + future.get());

        // Check again
        System.out.println("Task completed? "
                           + future.isDone());

        // Shutdown the executor
        executor.shutdown();
    }
}

Output:

Explanation:

A single-threaded executor is created to run a task.
The task sleeps for 1 second and returns 50.
Before completion, isDone() returns false.
After the result is retrieved using get(), it returns true.
Finally, the executor is shut down.

Methods of Future Interface

cancel(boolean mayInterruptIfRunning): Cancels the execution of the task if possible.
isCancelled(): Returns true if the task was cancelled before completion.
isDone(): Returns true if the task is completed or cancelled.
V get(): Waits (if needed) and returns the computed result.
V get(long timeout, TimeUnit unit): Waits up to the given time and returns the result if available.

Example: Canceling a Task