history.md

A brief history of futures in Java

Java 1.5 introduced the Future interface but it was a very limited - you could call get() and isDone() but there was no way to get notified of state changes. Instead you would need to do polling.

Google Guava introduced the ListenableFuture interface which extends Future but also adds addListener(Runnable, executor) - this one method enabled a more asynchronous development model. All other useful methods could now be implemented as utility functions on top of the primitives.

The Google Guava class Futures included some very useful methods:

• addCallback(callback) - convenience method on top of addListener
• transform(function) - return a new future after applying a function to the values
• transformAsync(function) - like transform, but the function should return a future instead. (Similar to Java 8 thenCompose)

Then with Java 8 we got CompletableFuture which had achieved feature parity with Google Guava futures, though with differences in:

• Fluent API - CompletableFuture has a fluent API unlike ListenableFuture (but Google later added FluentFuture too)
• Mutability - Google futures separates the producer side from the consumer side, while they are much more tightly coupled in CompletableFuture.
• Number of primitives - Google futures have a small set of primitives (transform, transformAsync, catching) that can be combined while CompletableFuture has a large set of methods for combinations of the underlying primitives. thenApply, thenRun and thenAccept could be reduced to a single promitive and likewise for handle, runAfter, runAsync, whenComplete. Almost all methods in CompletableFuture also exist in three variants: regular, async with default executor and async with custom executor.