GSON - How can I parse two JSONArrays with the same name, but different parameters?

139

October 06, 2018, at 09:10 AM

In the Reddit JSON API, comments can contain two different types of JSONArrays, both called "children".

"children" is usually an array of Objects containing a String "kind" and Object "data":

"children": [ { "kind": "t3", "data": {} } ...]

I've been handling these fine. My problem is that, sometimes, children will be a simple String array:

"children": [ "e78i3mq", "e78hees", "e78jq6q" ]

When parsing these, GSON throws an exception like the following:

Caused by: java.lang.IllegalStateException: Expected BEGIN_OBJECT but was STRING at line 1 column 3780 path $[1].data.children[0].data.replies.data.children[0].data.replies.data.children[0].data.replies.data.children[0].data.children[0]

How can I handle these String array cases?

Answer 1

If the same endpoint is returning a different type in some instances I suggest wrapping that part in an object and using a deserializer to check the type and assign accordingly. You can do something like this:

public Parent serialize(String jsonString) {
    GsonBuilder builder = new GsonBuilder();
    builder.registerTypeAdapter(WrappedChild.class, new ChildDeserializer());
    Gson gson = builder.create();
    return gson.fromJson(jsonString, Parent.class);
}
class Parent {
    public List<WrappedChild> children;
}
class ObjectChild {
    public String body;
}
class WrappedChild {
    public ObjectChild objectChild;
    public String stringChild;
}
class ChildDeserializer implements JsonDeserializer<WrappedChild> {
    private Gson gson = new Gson();
    @Override
    public WrappedChild deserialize(JsonElement json, Type typeOfT, JsonDeserializationContext context) throws JsonParseException {
        if (json != null) {
            if (json.isJsonObject()) {
                WrappedChild result = new WrappedChild();
                result.objectChild = gson.fromJson(json, ObjectChild.class);
                return result;
            } else if (json.isJsonPrimitive()) {
                WrappedChild result = new WrappedChild();
                result.stringChild = json.getAsString();
                return result;
            }
        }
        return null; // Or throw new Exception("Unknown child type");
    }
}

If you are using retrofit just pass the Gson created by the builder to GsonConverterFactory.create as a parameter when creating your service.