Mixed type/ type wildcard/ polymorphism

[ericntd]

My server search API returns a list of object of various types e.g. "products", "brands" etc.

I have Product and Brand model both extending BaseModel:

public class BaseModel {
    @Id
    protected String id;

    public String getId() {
        return id;
    }

    public void setId(String id) {
        this.id = id;
    }
}

I've tried the following:

public interface SearchService {
    SearchService serviceJasminb = MyRESTClient.retrofitJasminb.create(SearchService.class);

    @GET("search")
    Call<JSONAPIDocument<ArrayList<? extends BaseModel>>> search(@Query("query") String query);

but I got the following exception:

java.lang.IllegalArgumentException: Method return type must not include a type variable or wildcard: retrofit2.Call<com.github.jasminb.jsonapi.JSONAPIDocument<java.util.ArrayList<? extends com.xxx.yyy.models.BaseModel>>>
                                                     for method SearchService.retrieveSearch
                                                     at retrofit2.ServiceMethod$Builder.methodError(ServiceMethod.java:720)
                                                     at retrofit2.ServiceMethod$Builder.methodError(ServiceMethod.java:711)
                                                     at retrofit2.ServiceMethod$Builder.createCallAdapter(ServiceMethod.java:224)
                                                     at retrofit2.ServiceMethod$Builder.build(ServiceMethod.java:160)
                                                     at retrofit2.Retrofit.loadServiceMethod(Retrofit.java:166)
                                                     at retrofit2.Retrofit$1.invoke(Retrofit.java:145)
                                                     at java.lang.reflect.Proxy.invoke(Proxy.java:393)
                                                     at $Proxy16.retrieveSearch(Unknown Source)
                                                     at com.xxx.yyy.caches.SearchCache.fetchSearch(SearchCache.java:187)
                                                     at com.xxx.yyy.views.activities.SearchActivity.run(SearchActivity.java:352)
                                                     at android.os.Handler.handleCallback(Handler.java:739)
                                                     at android.os.Handler.dispatchMessage(Handler.java:95)
                                                     at android.os.Looper.loop(Looper.java:148)
                                                     at android.app.ActivityThread.main(ActivityThread.java:5417)
                                                     at java.lang.reflect.Method.invoke(Native Method)
                                                     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)
                                                     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)

[jasminb]

Hey @ericn37, error you got is related to retrofit. Why not use BaseType as type instead of extends?

You will get a collection of different objects depending on the actual type attribute in the parsed response.

[emetsger]

I don't have much to add to this conversation, except to add a "me too".
I'm working with a JSON API that has relationships that can point to
objects of different types.

For example, an object may have a relationship named somerel with a
related link to http://example.org/resource/1, and the JSON
representation at http://example.org/resource/1 may have a type of Baz.

A different object may express a relationship named somerel with a
related link to http://example.org/resource/2, and the JSON
representation http://example.org/resource/2 may have a type of Biz.

The semantics of the relationship named somerel hold for these two
example objects, even though dereferencing the link returns objects of
different types.

This becomes problematic when you are dereferencing somerel: you may
retrieve objects typed as Biz or objects typed as Baz:

@Type("foo")
public class Foo {
  @Relationship(value = "somerel", resolve = true)
  private Biz field;

  // ....
}

This class can't be used to deserialize somerel where the type is Baz;
it only works for type Biz.

One option is to have Baz and Biz share a super-type, and update the
java class as follows:
Baz -- extends --> BaseObject
Biz -- extends --> BaseObject

@Type("foo")
public class Foo {
  @Relationship(value = "somerel", resolve = true)
  private BaseObject field;

  // ....
}

However, that does not work either. You must deserialize a concrete type
(either Baz or Biz) which fails, before assigning to a base class.

I am not sure if this is a Retrofit problem or a jsonapi-converter problem,
or what. But currently the semantics of a relationship are coupled to the
type of object returned by that relationship, which is unfortunate.

On Thu, Oct 6, 2016 at 11:51 AM, Jasmin Begic notifications@github.com
wrote:

Hey @ericn37 https://github.com/ericn37, error you got is related to
retrofit. Why not use BaseType only?

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[Servus7]

@emetsger It's an jsonapi-converter problem. Other libraries like moshi-jsonapi are able to convert different types.

[ericntd]

https://github.com/faogustavo/JSONApi is also able to parse multiple types.
It used to be my favourite.
Their drawback is that they are are not be to handle circular reference properly 😭

However, is it a tradeoff?
Multiple types/ polymorphism ==> no circular reference
Circular reference (this libary) ==> no multiple types/ polymorphism

[ericntd]

@jasminb I've tried what you suggested, however, I only got BaseModel object, not the subclass.
When I try to cast, I got ClassCastException

Process: com.xxx.yyy, PID: 10629
                                                 java.lang.ClassCastException: com.xxx.yyy.models.BaseModel cannot be cast to com.xxx.yyy.models.LandingPagePromoList
                                                     at com.xxx.yyy.tasks.LandingPageTask$1.onResponse(LandingPageTask.java:35)
                                                     at retrofit2.ExecutorCallAdapterFactory$ExecutorCallbackCall$1$1.run(ExecutorCallAdapterFactory.java:68)
                                                     at android.os.Handler.handleCallback(Handler.java:739)
                                                     at android.os.Handler.dispatchMessage(Handler.java:95)
                                                     at android.os.Looper.loop(Looper.java:148)
                                                     at android.app.ActivityThread.main(ActivityThread.java:5417)
                                                     at java.lang.reflect.Method.invoke(Native Method)
                                                     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)
                                                     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)

My Retrofit code:

Call<JSONAPIDocument<BaseModel>> obj;
        obj = MyService.service.fetchLandingPage(objectId);
        obj.enqueue(new Callback<JSONAPIDocument<BaseModel>>() {
            @Override
            public void onResponse(Call<JSONAPIDocument<BaseModel>> call,
                                   Response<JSONAPIDocument<BaseModel>> response) {
                if (response.isSuccessful()) {
                    MyModel tmp = (MyModel) response.body().get();
                    listener.handleLandingPage(tmp);
                } else {
                    listener.handleError(response.errorBody().toString());
                }
            }

            @Override
            public void onFailure(Call<JSONAPIDocument<BaseModel>> call, Throwable t) {
                Log.e(TAG, "onFailure", t);
                listener.handleError(null);
            }
        });

BaseModel:

@JsonIgnoreProperties(ignoreUnknown = true)
@Type("base")
public class BaseModel {
    @Id
    protected String id;

    public BaseModel() {
    }

    public String getId() {
        return id;
    }

    public void setId(String id) {
        this.id = id;
    }
}

MyModel:

@JsonIgnoreProperties(ignoreUnknown = true)
@Type("mymodel")
public class MyModel extends BaseModel implements Parcelable {
    @Relationship("objectbs")
    private ArrayList<ObjectB> promos;

    @JsonProperty("slug-url")
    private String slug;

    public MyModel() {
    }

    public ArrayList<ObjectB> getPromos() {
        return promos;
    }

    public void setPromos(ArrayList<ObjectB> promos) {
        this.promos = promos;
    }

    public String getSlug() {
        return slug;
    }

    public void setSlug(String slug) {
        this.slug = slug;
    }

    @Override
    public int describeContents() {
        return 0;
    }

    @Override
    public void writeToParcel(Parcel dest, int flags) {
        dest.writeTypedList(this.promos);
        dest.writeString(this.slug);
    }

    protected MyModel(Parcel in) {
        this.promos = in.createTypedArrayList(ObjectB.CREATOR);
        this.slug = in.readString();
    }

    public static final Creator<MyModel> CREATOR = new Creator<MyModel>() {
        @Override
        public MyModel createFromParcel(Parcel source) {
            return new MyModel(source);
        }

        @Override
        public MyModel[] newArray(int size) {
            return new MyModel[size];
        }
    };
}

[jasminb]

Hey,

What version of the lib you are using. Support for providing base type is SNAPSHOT only and not yet part of any release. Until new release is out, please use latest snapshot (example in the README).

[ericntd]

Hi @jasminb , I'm using 0.4. Will try the snapshot. Thanks :)

[jasminb]

Hey @ericn37, did 0.5 solve the issue for you?

[ericntd]

Yes, indeed 0.5 did solve the problem, thanks @jasminb