using CompletionStage instead of CompletableFuture

Question

given the following method:

private static String getChuckNorrisJoke () {
    try {
        HttpURLConnection con = (HttpURLConnection) new
        URL( "http://api.icndb.com/jokes/random" ).openConnection();
        BufferedReader in = new BufferedReader( new InputStreamReader(con.getInputStream()));
        StringBuilder response = new StringBuilder();
        String line;
        while ((line = in.readLine()) != null ) {
            response.append(line);
        }
        in.close();
        return response.toString();
    } catch (IOException e) {
        throw new IllegalStateException( "Something is wrong: " , e);
    }
}

the following statement can be used to run the method in an asynchronous fashion.

final CompletableFuture<String> jokeAsync = CompletableFuture.supplyAsync(() -> getChuckNorrisJoke());

although I think that I understand CompletionStage relation to CompletableFuture, I am not sure how I can use CompletionStage to accomplish same task.

final CompletionStage<String> jokeAsync = ?

also, I am not sure about "combining stages"

Didier L · Accepted Answer · 2018-09-25T20:19:48.610

6

CompletionStage is the interface implemented by CompletableFuture, so you can just declare jokeAsync as a CompletionStage and it will work:

final CompletionStage<String> jokeAsync = CompletableFuture.supplyAsync(() -> getChuckNorrisJoke());

If you have several stages, you can combine them in different ways, like:

thenCombine() to combine the result of 2 stages
applyToEither() to process the result of the first one that completes and return a new stage with the result
acceptEither() to consume the result of either without returning a new result
runAfterBoth() and runAfterEither() to run an operation after both or either stage is completed.

The CompletionStage API does not offer some operations that are only provided by CompletableFuture:

submitting tasks, e.g. with supplyAsync()
combining many stages with allOf() and anyOf()
programmatically completing stages or creating already completed stages
waiting for a stage's result with join() or get()

But the toCompletableFuture() allows to convert any stage and thus bridge the gap.

edited Sep 25 '18 at 20:19

answered Sep 25 '18 at 19:37

Didier L

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like this? final CompletionStage firstJokeAsync = CompletableFuture.supplyAsync(() -> getChuckNorrisJoke()); final CompletionStage secondJokeAsync = CompletableFuture.supplyAsync(() -> getChuckNorrisJoke()); return firstJokeAsync.thenCombineAsync(secondJokeAsync, service); – Ahmad Abdullah Sep 25 '18 at 22:26
If `service` is a `BiFunction` that accepts 2 `String` parameters and returns some result, then yes. – Didier L Sep 25 '18 at 22:33
BiFunction service = (x, y) -> {return x + y; };..... yes, this works fine. it concatenates the 2 Strings together. but is there a nice way to get the output of (.thenCombineAsync) to match this type (CompletionStage>) – Ahmad Abdullah Sep 26 '18 at 10:14
Just make a list out of it: `BiFunction> service = (x, y) -> Arrays.asList(x, y);` – Didier L Sep 26 '18 at 14:39

using CompletionStage instead of CompletableFuture

1 Answers1