Retrofit: how to parse a JSON array that combines an array and an object?

Question

I am working on an Android app that uses Retrofit+OkHttp to connect to a REST API and consume JSON data. I\'m fairly new to Retrofit, so I\'m still learning how it works, bu

Answer 1

You seem to have not use List<MyObject> because you have to create at least two kinds of objects: "single" and "multiple" where the latter must implement List<E> to satisfy the array-like interface.

For example. JsonResponse is simpler than what jsonschema2pojo generates:

final class JsonResponse {

    final boolean success = Boolean.valueOf(false);
    final String message = null;
    final List<MyObject> data = null;

}

Now, MyObject can look like this:

abstract class MyObject
        implements Parcelable {

    private MyObject() {
    }

    static <E> MyObject multiple(final List<E> list) {
        return new MultipleObjects<>(list);
    }

    static final class SingleObject
            extends MyObject {

        private SingleObject() {
        }

        final String info = null;
        final String info2 = null;
        final String info3 = null;

    }

    static final class MultipleObjects<E>
            extends MyObject
            implements List<E> {

        private final List<E> list;

        private MultipleObjects(final List<E> list) {
            this.list = list;
        }

        // @formatter:off
        @Override public int size() { return list.size(); }
        @Override public boolean isEmpty() { return list.isEmpty(); }
        @Override public boolean contains(final Object o) { return list.contains(o); }
        @Override public Iterator<E> iterator() { return list.iterator(); }
        @Override public Object[] toArray() { return list.toArray(); }
        @Override public <T> T[] toArray(final T[] a) { return list.toArray(a); }
        @Override public boolean add(final E e) { return list.add(e); }
        @Override public boolean remove(final Object o) { return list.remove(o); }
        @Override public boolean containsAll(final Collection<?> c) { return list.containsAll(c); }
        @Override public boolean addAll(final Collection<? extends E> c) { return list.addAll(c); }
        @Override public boolean addAll(final int index, final Collection<? extends E> c) { return list.addAll(index, c); }
        @Override public boolean removeAll(final Collection<?> c) { return list.removeAll(c); }
        @Override public boolean retainAll(final Collection<?> c) { return list.retainAll(c); }
        @Override public void clear() { list.clear(); }
        @Override public E get(final int index) { return list.get(index); }
        @Override public E set(final int index, final E element) { return list.set(index, element); }
        @Override public void add(final int index, final E element) { list.add(index, element); }
        @Override public E remove(final int index) { return list.remove(index); }
        @Override public int indexOf(final Object o) { return list.indexOf(o); }
        @Override public int lastIndexOf(final Object o) { return list.lastIndexOf(o); }
        @Override public ListIterator<E> listIterator() { return list.listIterator(); }
        @Override public ListIterator<E> listIterator(final int index) { return list.listIterator(index); }
        @Override public List<E> subList(final int fromIndex, final int toIndex) { return list.subList(fromIndex, toIndex); }
        // @formatter:on

    }

}

The class above implements an abstract class that can be implemented in two ways. Note that no public constructors are exposed by design: SingleObject can be deserialized using Gson really easy using the reflective strategy, whilst MultipleObjects is an array-like object that requires some manual construction.

The deserialization part:

final class MyObjectJsonDeserializer
        implements JsonDeserializer<MyObject> {

    private static final JsonDeserializer<MyObject> myObjectJsonDeserializer = new MyObjectJsonDeserializer();

    // You have to detect it more accurately yourself   
    private static final Type genericListType = new TypeToken<List<Object>>() {
    }.getType();

    private MyObjectJsonDeserializer() {
    }

    static JsonDeserializer<MyObject> getMyObjectJsonDeserializer() {
        return myObjectJsonDeserializer;
    }

    @Override
    public MyObject deserialize(final JsonElement jsonElement, final Type type, final JsonDeserializationContext context)
            throws JsonParseException {
        if ( jsonElement.isJsonNull() ) {
            return null;
        }
        if ( jsonElement.isJsonObject() ) {
            // Note that the deserialization is implemented using context,
            // because it makes sure that you are using the Gson instance configuration
            // Simply speaking: do not create gson instances in 99,9% cases
            return context.deserialize(jsonElement, MyObject.SingleObject.class);
        }
        if ( jsonElement.isJsonArray() ) {
            return multiple(context.deserialize(jsonElement, genericListType));
        }
        // Or create a more sophisticated detector... Or redesign your mappigns
        if ( jsonElement.isJsonPrimitive() ) {
            throw new JsonParseException("Cannot parse primitives");
        }
        throw new AssertionError(jsonElement);
    }

}

Example use:

private static final Gson gson = new GsonBuilder()
        .registerTypeAdapter(MyObject.class, getMyObjectJsonDeserializer())
        .create();

public static void main(final String... args)
        throws IOException {
    try ( final JsonReader jsonReader = getPackageResourceJsonReader(Q43946453.class, "polymorphic.json") ) {
        final JsonResponse response = gson.fromJson(jsonReader, JsonResponse.class);
        for ( final MyObject datum : response.data ) {
            System.out.println(datum.getClass().getSimpleName());
        }
    }
}

Output:

MultipleObjects
SingleObject

Answer 2

There is a way.

Add a custom JsonDeserializer and handle that field manually.

Examples

public static class Deserializer implements JsonDeserializer<JsonResponse> {

    private final Gson gson = new GsonBuilder().create();

    @Override
    public JsonResponse deserialize(JsonElement json, Type typeOfT,
                                      JsonDeserializationContext context)
            throws JsonParseException {
        JsonResponse response = gson.fromJson(json, typeOfT);

        JsonObject jObject = json.getAsJsonObject();
        //Handle jObject here, and parse each object of data array 
        //accordingly to its type - JsonObject/JsonArray
        return response;
    }
}

You need to register it in your global Gson instance, like this:

new GsonBuilder()
            .registerTypeAdapter(JsonResponse.class, new JsonResponse.Deserializer())
            .create();