In our previous article, we explored the world of ListenableFuture. Now, let's dive deeper into more advanced techniques, specifically focusing on transformations and chaining. Imagine we have a ListenableFuture obtained from an asynchronous service. Additionally, we have a simple method:

Document parse(String xml) {//...

Instead of working with the String itself, we need the Document. One approach would be to resolve the Future and process the String. However, a more elegant solution is to apply a transformation once the results are available, making our method behave as if it always returned ListenableFuture. This is remarkably straightforward:

final ListenableFuture future = //...
 
final ListenableFuture documentFuture = Futures.transform(future, new Function() {
    @override    public Document apply(String contents) {
        return parse(contents);
    }
});

Or, for better readability:

final Function parseFun = new Function() {
    @override    public Document apply(String contents) {
        return parse(contents);
    }
};
 
final ListenableFuture future = //...
 
final ListenableFuture documentFuture = Futures.transform(future, parseFun);

While Java syntax may have its limitations, let's focus on what we've achieved. Futures.transform() doesn't wait for the underlying ListenableFuture to apply the parse() transformation. Instead, it registers a callback, ready to be notified whenever the given future completes. This transformation is applied dynamically and transparently for us at the right moment. We still have a Future, but this time wrapping a Document.

Now, let's take it a step further. We also have an asynchronous, possibly long-running method that calculates the relevance (whatever that means in this context) of a given Document:

ListenableFuture calculateRelevance(Document pageContents) {//...

Can we somehow chain it with the ListenableFuture we already possess? First attempt:

final Function> relevanceFun = new Function>() {
    @override    public ListenableFuture apply(Document input) {
        return calculateRelevance(input);
    }
};
 
final ListenableFuture future = //...
final ListenableFuture documentFuture = Futures.transform(future, parseFun);
final ListenableFuture> relevanceFuture = Futures.transform(documentFuture, relevanceFun);

For more information on enhanced asynchronous programming possibilities, visit this link.