Continuations in Java

Question

Is there a good implementation of continuations in Java?

If so, what is the overhead like? The JVM wasn\'t designed with these sort of things in mind, right? So is t

Answer 1

Since Java 8, there is now a CompletableFuture class which supports continuations and more functional / reactive programming approaches.

Consider the following example, where a Class offers a downloadAndResize method:

public CompletableFuture downloadAndResize(String imageUrl, int width, int height) {
    return CompletableFuture
        .supplyAsync(() -> downloadImage(imageUrl))
        .thenApplyAsync(x -> resizeImage(x, width, height));
}

private Image downloadImage(String url){
    // TODO Download the image from the given url...
}

private Image resizeImage(Image source, int width, int height){
    // TODO Resize the image to w / h
}

Usage of the above method could look like:

CompletableFuture imagePromise = downloadAndResize("http://some/url", 300, 200);

imagePromise.thenAccept(image -> {
    // Gets executed when the image task has successfully completed

    // do something with the image

});