Stream-Only Custom Gson

FetchResultsSerializer.java

import com.google.gson.*;
import org.apache.commons.lang3.tuple.Pair;

import java.lang.reflect.Type;
import java.util.List;
import java.util.stream.Collector;

//
// Converts a List of Pair<String, Long> into a JSON array of objects where the key of the Pair is
// field name "url" and the value is "fileSize"
//
public class FetchResultsSerializer implements JsonSerializer<List<Pair<String, Long>>> {

    @Override
    public JsonElement serialize(List<Pair<String, Long>> listOfPairs, Type type, JsonSerializationContext jsonSerializationContext) {

        return listOfPairs.stream().map(
                p -> {
                    JsonElement key = new JsonPrimitive(p.getKey());
                    JsonElement value = new JsonPrimitive(p.getValue());
                    JsonObject obj = new JsonObject();
                    obj.add("url", key);
                    obj.add( "fileSize", value);
                    return obj;
                }
        ).collect(
                Collector.of(
                        () -> new JsonArray(),
                        (a, obj) -> a.add(obj),
                        (a, d) -> a
                )
        );
    }
}