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.

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

import java.util.concurrent.*; public class GFG {  public static void main(String[] args) throws Exception{  ExecutorService executor  = Executors.newSingleThreadExecutor();  Callable<String> task = () ->  {  Thread.sleep(2000);  return "Task Completed";  };  Future<String> future = executor.submit(task);  // Cancel the task before it finishes  boolean cancelled = future.cancel(true);  System.out.println("Task cancelled? " + cancelled);  executor.shutdown();  } } 

Output:

Callable vs Future