Improvement to JsonCollectors interface

[andrebreves]

The current signature of the collector JsonCollectors#toJsonObject​() is:

static Collector<Map.Entry<String, JsonValue>, JsonObjectBuilder, JsonObject> toJsonObject​()

That signature forces the following code:

Map<String, JsonArray> map = ...;
JsonObject obj = map.entrySet()
        .stream()
        .collect(toJsonObject());

To create another Map.Entry just to cast the value to JsonValue in order to work:

Map<String, JsonArray> map = ...;
JsonObject obj = map.entrySet()
        .stream()
        .map(e -> Map.entry(e.getKey(), (JsonValue) e.getValue()))
        .collect(toJsonObject());

My suggestion is to change the signature of JsonCollectors#toJsonObject​() to:

static Collector<Map.Entry<String, ? extends JsonValue>, JsonObjectBuilder, JsonObject> toJsonObject()

So Map of String to any JsonValue sub-type could be collected to JsonObject with little effort.

And I guess that modification should be applied to the other signatures that have JsonValue on the receiving side as well:

static Collector<? extends JsonValue, Map<String, JsonArrayBuilder>, JsonObject>
    groupingBy​(Function<? extends JsonValue, String> classifier)

static <T extends JsonArrayBuilder>
    Collector<? extends JsonValue, Map<String, T>, JsonObject>
    groupingBy​(Function<? extends JsonValue, String> classifier, Collector<? extends JsonValue, T, JsonArray> downstream)

static Collector<? extends JsonValue, JsonArrayBuilder, JsonArray> toJsonArray​()

static Collector<? extends JsonValue, JsonObjectBuilder, JsonObject>
        toJsonObject​(Function<? extends JsonValue, String> keyMapper, Function<? extends JsonValue, JsonValue> valueMapper)