Deserialize class extending TreeSet with custom comparator using GSON

Question

I have a class which extends the TreeSet class. Its constructor provides a custom comparator to the TreeSet :

public class SortedPersonSet extends TreeSet<Person> {
    public SortedPersonSet(Comparator<Person> comp) {
        super(comp);
    }
}

I want to serialize and deserialize that class using GSON for example:

SortedPersonSet personSet =
            new SortedPersonSet((p1, p2) -> Long.compare(p1.getAge(), p2.getAge()));
    personSet.add(new Person("Peter", 21));
    personSet.add(new Person("Klaus", 17));
    personSet.add(new Person("Martin", 27));
    personSet.add(new Person("John", 22));

// Serialize
Gson gson = new GsonBuilder().create();
String personSetAsString = gson.toJson(personSet);
System.out.println(personSetAsString);

// Deserialize
Gson gsonb = new GsonBuilder().create();
SortedPersonSet newPersons = gsonb.fromJson(personSetAsString, SortedPersonSet.class);
System.out.println(newPersons);

However, this version throws an exception because class Person does not implement Comparable .

So I have tried the approach which helped here by implementing a custom JsonDeserializer which returns a SortedPersonSet with a custom comparator:

 public class SortedSetDeserializer implements JsonDeserializer<SortedPersonSet> {

    @Override
    public SortedPersonSet deserialize(JsonElement json, Type typeOfT,
            JsonDeserializationContext context) throws JsonParseException {
        return new SortedPersonSet((p1, p2) -> Long.compare(p1.getAge(), p2.getAge()));
    }
}

// Deserialization in main
Gson gsonb = new GsonBuilder()
                .registerTypeAdapter(SortedPersonSet.class, new SortedSetDeserializer()).create();

SortedPersonSet newPersons = gsonb.fromJson(personSetAsString, SortedPersonSet.class);
System.out.println(newPersons);

Unfortunately, there is still a mistake in my code, because when I deserialize the JSON string via SortedPersonSet newPersons = gsonb.fromJson(personSetAsString, SortedPersonSet.class); , the resulting SortedPersonSet is empty. I hope you can point out where I made the mistake. Another (hopefully simpler) option would be appreciated as well.

Thanks in advance!

Answer 1

Result set is empty because in custom deserialiser you do not parse array; only creates new set and returns it. Simple implementation with real deserialising class should look like this:

class SortedSetDeserializer implements JsonDeserializer<SortedPersonSet> {

    @Override
    public SortedPersonSet deserialize(JsonElement json, Type typeOfT, JsonDeserializationContext context) throws JsonParseException {
        SortedPersonSet people = new SortedPersonSet(Comparator.comparingLong(Person::getAge));
        if (json.isJsonArray()) {
            JsonArray array = json.getAsJsonArray();
            array.forEach(e -> {
                people.add(context.deserialize(e, Person.class));
            });
        }
        return people;
    }
}

From other side, creating new subtype of TreeSet does not sound good. If your Person class will implement Comparable :

class Person implements Comparable<Person> {

    @Override
    public int compareTo(Person o) {
        return Long.compare(this.getAge(), o.getAge());
    }
}

You can easily deserialise this array to TreeSet :

Type type = new TypeToken<TreeSet<Person>>() {
}.getType();
TreeSet<Person> newPersons = gsonb.fromJson(personSetAsString, type);