Jakarta EE 8: JsonCollectors And The OOP Alternative

With Java EE 8 Jakarta EE 8 there comes a new version of the JSON-P (JSON Processing) specification, namely version 1.1. One of the new things this version brings is the utility class JsonCollectors.

This class is OK, it provides some useful collectors to manipulate collections of JsonValues and turn them into JsonObjects or JsonArrays. Let’s see how it works and dive into a more OOP alternative (I’m going to examine JsonArray, but this applies to JsonObject as well and probably to the others too).

Straight to the point, with JSON-P 1.1, you can turn a collection of JsonValue into a JsonArray like this:

List<JsonValue> values = ...;
JsonArray array = values.stream().collect(
  JsonCollectors.toJsonArray()
);

If you work with Java EE 7 and maybe Java 7 instead of Java 8, you would have to do the following:

List<JsonValue> values = ...;
JsonArrayBuilder builder = Json.createArrayBuilder();
for(JsonValue value : values) {
    builder = builder.add(value);
}
JsonArray array = builder.build();

Very beautiful and handy. Actually, I’m not the only one writing these examples, you can already find a few at a first Google search.

What I didn’t see, however, is an OOP approach. The snippets above answer the question “How do we put these values into an array?”, instead of answering the question “How do we make these values act as an array?”.

It’s a very important difference. In the current approach, you have a few issues:

• Code duplication: everywhere you need an array, you will see those few lines of code/chain of method calls;
• Static methods: if you don’t want to duplicate those lines of code, you have to put them in a static method somewhere, which probably won’t be unit-tested;
• Coupling: if Java EE 9 comes and changes the JsonCollectors API, or any of the methods stream() or collect(...) changes in the future, you will have to find all the usages and edit your code;

You’ll say that this is a trivial example and it’s nothing to worry about, because there’s very little chance for something to break in the future. And you are right, but this is a classic case of “let’s turn X into Y”, rather than “let’s add a new implementation of Y, based on X”, which would be the correct object-oriented way!

So, I ask myself, why didn’t the JSON-P guys think of this? Why did they struggle to implement Collectors and other rocket stuff, instead of simply providing the following implementation of JsonArray?

package javax.json;

/**
 * JsonArray from a collection of JsonValues.
 * This is an alternative to having a static method somewhere, which calls the builder
 * and returns a JsonArray.
 *
 * Instead of having the following in your code:
 *
 * JsonArray arr = buildArray(value1, value2, value3);
 *
 * you will have the following
 *
 * JsonArray arr = new CollectedJsonArray(value1, value2, value3);
 */
public final class CollectedJsonArray implements JsonArray {

    /**
     * Collected array.
     */
    private final JsonArray collected;

    public CollectedJsonArray(JsonValue... values) {
        JsonArrayBuilder builder = Json.createArrayBuilder();
        for(JsonValue value : values) {
            builder = builder.add(value);
        }
        this.collected = builder.build();
    }

    @Override
    public JsonObject getJsonObject(int i) {
        return this.collected.getJsonObject(i);
    }

    @Override
    public JsonArray getJsonArray(int i) {
        return this.collected.getJsonArray(i);
    }

    //the other methods of JsonArray, delegated to this.collected.

}

I wrote this class myself. It is simple, API compliant and doesn’t care who is the JSON-P provider. Yet it is painful to write, because you have to implement all those methods and delegate the work to this.collected. Then, there should be some unit tests for it. I believe that this class should have been shipped with JSON-P 1.1. It is a mere wrapper, it has just a few lines in the ctor, but it spares our application of coupling, static methods and untested code.

What do you think? How would you have implemented it?