Android: Design Pattern - Code to interface

I am learning design patterns from Head First Design Patterns and the first pattern i came across is to "Code to interface".Now i want to use that pattern in my android app.My app includes lots of api calls.I am using Retrofit for for network calls.So i have used that design pattern considering some scenario something like this:
1. Suppose in future i need to shift from Retrofit to volley or any other networking library.
2. So i created an interface with a method to getData from server and created a class which implements that interface.The implemented method includes the Retrofit code to getData from server.
3. Now in future if i want to use volley i'll create a new class implementing the interface and with volley code to retrieve data from server.
The code goes like this

Interface

public interface NetworkCallsApi {
    String getDataFromServer(Activity activity,String url,String Callback);
}

Retrofit Implementation

public class NetworkCallsRetrofitImpl implements NetworkCallsApi{

private Retrofit retrofit;

@Override
public String getDataFromServer(Activity activity, String url, String Callback) {

    retrofit = new Retrofit.Builder()
            .baseUrl(StringConstants.ROOT_URL)
            .addConverterFactory(GsonConverterFactory.create())
            .build();

    RetrofitApi apiCalls = retrofit.create(RetrofitApi.class);

    Call<ResponseBody> call = apiCalls.getDataFromServer(url);

    call.enqueue(new Callback<ResponseBody>() {
        @Override
        public void onResponse(Call<ResponseBody> call, Response<ResponseBody> response) {

        }

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

        }
    });
    return null;
}
}

ServiceCalls.java

public class ServiceCalls {

public static ServiceCalls serviceCalls;
public NetworkCallsApi networkCallsimpl;

public static ServiceCalls getInstance(){

    if(serviceCalls == null){
        serviceCalls = new ServiceCalls();
    }
    return serviceCalls;
}

public void setNetworkCallsimpl(NetworkCallsApi networkCallsimpl) {
    this.networkCallsimpl = networkCallsimpl;
}

public String getDataFromServer(Activity activity, String url, String callback){
    networkCallsimpl.getDataFromServer(activity,url,callback);
    return null;
}
}

MainFragment Code:

public class StatusFragment extends Fragment {    

@Override
public void onCreate(@Nullable Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    //Setting the implementation
    ServiceCalls.getInstance().setNetworkCallsimpl(new NetworkCallsRetrofitImpl());
}

@Nullable
@Override
public View onCreateView(LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {

    view = inflater.inflate(R.layout.fragment_pnr_status,container,false);

    try{
        init();
    }catch(Exception e){

    }
    return view;
}

private void init() {

    ok.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {

            setUrlKeysAndValues();
            ServiceCalls.getInstance().getDataFromServer(getActivity()
                    ,Utils.getInstance().buildUrl(urlKeys,urlValues),"onResponse");
        }
    });

}
}

Now my question is "have i correctly implemented that design pattern".If not,what changes i need to do.In onCreate of MainFragment Code i am hardCoding the implementation.How Code to interface design pattern could be better applied in this scenario.Thankyou.