How to create a complete future in java

Question

What is the best way to build a complete future in Java? I performed my own CompletedFuture below, but was hoping something similar already existed.

 public class CompletedFuture<T> implements Future<T> { private final T result; public CompletedFuture(final T result) { this.result = result; } @Override public boolean cancel(final boolean b) { return false; } @Override public boolean isCancelled() { return false; } @Override public boolean isDone() { return true; } @Override public T get() throws InterruptedException, ExecutionException { return this.result; } @Override public T get(final long l, final TimeUnit timeUnit) throws InterruptedException, ExecutionException, TimeoutException { return get(); } } 

Answer 1

Apache Commons Lang defines a similar implementation called ConstantFuture, you can get it by calling:

 Future<T> future = ConcurrentUtils.constantFuture(T myValue); 

Answer 2

In Java 8, you can use the built-in CompletableFuture file:

  Future future = CompletableFuture.completedFuture(value); 

Answer 3

Guava defines Futures.immediateFuture(value) , which does the job.

Answer 4

 FutureTask<String> ft = new FutureTask<>(() -> "foo"); ft.run(); System.out.println(ft.get()); 

will output "foo";

You may also have Future, which throws an exception when calling get ():

 FutureTask<String> ft = new FutureTask<>(() -> {throw new RuntimeException("exception!");}); ft.run(); 

Answer 5

I found a very similar class for your in Java rt.jar

com.sun.xml.internal.ws.util.CompletedFuture

It also allows you to specify an exception that can be thrown when the get () method is called. Just set it to null if you don't want to throw an exception.

Answer 6

In Java 6, you can use the following:

 Promise<T> p = new Promise<T>(); p.resolve(value); return p.getFuture();