Sample ApiModule https://hackernoon.com/yet-another-mvp-article-part-3-calling-apis-using-retrofit-23757f4eee05

ApiModule.java

package com.mirhoseini.marvel.domain;

/*...*/

@Module
public class ApiModule {

    @Provides
    @Singleton
    public MarvelApi provideMarvelApi(Retrofit retrofit) {
        return retrofit.create(MarvelApi.class);
    }

    @Provides
    @Singleton
    public Retrofit provideRetrofit(HttpUrl baseUrl, Converter.Factory converterFactory, CallAdapter.Factory callAdapterFactory, OkHttpClient okHttpClient) {
        return new Retrofit.Builder()
                .baseUrl(baseUrl)
                .addConverterFactory(converterFactory)
                .addCallAdapterFactory(callAdapterFactory)
                .client(okHttpClient)
                .build();
    }

    @Provides
    @Singleton
    public Converter.Factory provideGsonConverterFactory(Gson gson) {
        return GsonConverterFactory.create(gson);
    }

    @Singleton
    @Provides
    public Gson provideGson() {
        return new Gson();
    }

    @Provides
    @Singleton
    public CallAdapter.Factory provideRxJavaCallAdapterFactory() {
        return RxJavaCallAdapterFactory.create();
    }

}