google books api retrofit

Question

I am trying to use the google books API with Retrofit, it is returning an empty result.

this is the url:
https://www.googleapis.com/books/v1/volumes?q=9781451648546

In Retrofit I have an interface:

public interface IServiceEndPoint {
  @GET("https://www.googleapis.com/books/v1/volumes?q=9781451648546")
  Call<BookList> getBooks();
}

in my webservice class I have the following method:

public void getBooks(final Callback<BookList> callback){
  IServiceEndPoint endPoint = mRetrofit.create(IServiceEndPoint.class);
  Call<BookList> call = endPoint.getBooks();
  call.enqueue(callback);
}

in the activity class I have the method:

private void getBooks(){
  WebserviceHelper.getmInstance().getBooks(new Callback<BookList>() {
    @Override
    public void onResponse(Call<BookList> call, Response<BookList> response) {
      mBooks = response.body().getResults();
      mBookAdapter.update(mBooks);
    }

    @Override
    public void onFailure(Call<BookList> call, Throwable t) {

    }
  });
}

I have a Java class Book and BookList.

public class Book implements Serializable {

    @SerializedName("id")
    private String id;
    @SerializedName("title")
    private String title;
public String getId() {
        return id;
    }

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

    public String getTitle() {
        return title;
    }

    public void setTitle(String title) {
        this.title = title;
    }
}

public class BookList extends Book implements Serializable {
    @SerializedName("results")
    private List<Book> results;

    public List<Book> getResults() {
        return results;
    }

    public void setResults(List<Book> results) {
        this.results = results;
    }
}

In the manifest file I added

uses-permission android:name="android.permission.INTERNET

mBooks is returning null value, how could I solve this?
Thank you.

EDIT: shuvro's answer helped me correcting the problem. I also forgot to include the volumeInfo in my Book class. My book class looks as followed now:

public class Book implements Serializable {

    @SerializedName("id")
    private String id;

    private VolumeInfo volumeInfo;

    public VolumeInfo getVolumeInfo() {
        return volumeInfo;
    }

    public void setVolumeInfo(VolumeInfo volumeInfo) {
        this.volumeInfo = volumeInfo;
    }

    public String getId() {
        return id;
    }

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

}

Additionally I created the class volumeInfo:

public class VolumeInfo {
    private String title;
    private String subtitle;
    private String publisher;
    private String description;

    public String getSubtitle() {
        return subtitle;
    }

    public void setSubtitle(String subtitle) {
        this.subtitle = subtitle;
    }

    public String getDescription() {
        return description;
    }

    public void setDescription(String description) {
        this.description = description;
    }

    public String getPublisher() {
        return publisher;
    }

    public void setPublisher(String publisher) {
        this.publisher = publisher;
    }

    public String getTitle() {
        return title;
    }

    public void setTitle(String title) {
        this.title = title;
    }
}

Thanks all for the help!

Answer 1

Add the two dependency in your gradle file .

compile 'com.squareup.retrofit2:retrofit:2.1.0'
compile 'com.squareup.retrofit2:converter-gson:2.1.0'

Create a class , lets sats ServiceGenerator , your class should be like this

public class ServiceGenerator {


    private static OkHttpClient.Builder httpClient = new OkHttpClient.Builder();

    private static Retrofit.Builder builder =
            new Retrofit.Builder()
                    .baseUrl("https://www.googleapis.com/books/v1/")
                    .addConverterFactory(GsonConverterFactory.create());

    public static <S> S createService(Class<S> serviceClass) {
        Retrofit retrofit = builder.client(httpClient.build()).build();
        return retrofit.create(serviceClass);
    }
}

Now your declare your interface like this

public interface IServiceEndPoint {
  @GET("volumes")
  Call<BookList> getBooks(@Query("q") String id);
}

Now in activity or in fragment , use retrofit in this way

IServiceEndPoint serviceEndPoint = ServiceGenerator.createService(IServiceEndPoint.class)

Call<BookList> call = serviceEndPoint.getBooks("9781451648546");

 call.enqueue(new Callback<BookList>() {
        @Override
        public void onResponse(Call<BookList> call, Response<BookList> response) {
         //do whatever you want to do 
        }

        @Override
        public void onFailure(Call<BookList> call, Throwable t) {

        }
    });

Answer 2

You BookList POJO class has nothing to do with JSON response. It should be something like that:

public class BookList { 
    @SerializedName("items")
    private List<Item> items = new ArrayList<Item>();
}

You can find all POJO classes for that response here.