Json API (JavaEE 7) + Java 8: collection to JsonArray

JavaEE 7 brings an API for JSON arrays (the surprising JsonArray ;)), Java 8 brings a stream API and lambdas so now you can combine both to create a JsonArray from a Collection!

Side note: the snippet of this article have been tested with Apache Fleece as JSon Processing implementation.

The idea is to start from a collection, transform it with Java 8 stream API (in the example I took only the public id field of an object) and convert the produced stream to a JsonArray. For this last part we can simply use collect() method of Java 8 stream API and JsonBuilderFactory to create a new JsonArrayBuilder for the convertion:

@ApplicationScoped
public class ResourceService {
    @Inject // just for the sample
    private SomeCollectionProvider collections;

    @Inject // created from Json.createBuilderFactory(Collections.emptyMap())
    private JsonBuilderFactory builderFactory;

    public JsonArray collectionToJsonArray() {
        return collections.createACollection()
                .stream()
                .map(myObject -> myObject.id) // field is public
                .collect(
                        builderFactory::createArrayBuilder,
                        (a, s) -> a.add(s),
                        (b1, b2) -> b1.add(b2))
                .build();
    }
}

And that’s it! You got a JsonArray of your id from a collection without any loop or complicated code 🙂

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