Thomas Second Brain

Future Class

2 min read

Definition

Future<V> is a Java interface representing the result of a computation that may become available later.

Strictly speaking, Future<V> is an interface, not a concrete class.

Main idea

When a task is submitted to ExecutorService, the caller may not want to wait immediately.

Instead, it can receive a Future<V> as a placeholder for the result:

Future<Integer> future = pool.submit(() -> 42);

Later, the program can ask whether the task is done, wait for the result, or cancel it.

Common methods

  • get(): wait until the result is ready, then return it
  • get(timeout, unit): wait only up to a time limit
  • isDone(): check whether the computation has finished
  • cancel(boolean mayInterruptIfRunning): try to cancel the task
  • isCancelled(): check whether cancellation succeeded

Example

import java.util.concurrent.*;
 
ExecutorService pool = Executors.newFixedThreadPool(2);
Future<Integer> future = pool.submit(() -> 6 * 7);
 
if (!future.isDone()) {
    System.out.println("still running...");
}
 
int result = future.get();
pool.shutdown();

What blocks and what does not?

  • creating the Future<V> is usually immediate
  • the submitted task runs asynchronously
  • future.get() is the operation that typically blocks

This makes Future<V> useful when we want to start work now and collect the result later.

Relationship with Runnable and Callable

  • Runnable: no return value
  • Callable<V>: returns a value of type V
  • Future<V>: handle for the eventual result of a submitted task

A common pattern is:

  1. create a Callable<V>
  2. submit it to an ExecutorService
  3. receive a Future<V>
  4. call get() later when the result is needed

Limitations

Future<V> is useful, but somewhat limited:

  • get() is blocking
  • combining multiple futures is awkward
  • callbacks and pipelines are not built in

For richer asynchronous composition, Java provides CompletableFuture.

Important

Future<V> is best thought of as a handle to an asynchronous result, not as the result itself.

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