[Android Studio] How to Call REST API with Retrofit

Retrofit is a REST Client library (Helper Library) used in Android and Java to create an HTTP request and also to process the HTTP response from a REST API.
We can also use retrofit to receive data structures other than JSON, for example SimpleXML and Jackson.

Advantages of using Retrofit

- It manages network communication systematically and efficiently.

- You can request data in certain format, and parse the response data.

- It is relatively easier to handle errors.

Here are the steps to call REST API using Retrofit in Android.

1. Add INTERNET permission in AndroidManifest.xml

<uses-permission android:name="android.permission.INTERNET" />

2. Open Gbuild.gradle (:app) and add the libraries inside the dependencies.

    implementation 'com.squareup.retrofit2:retrofit:2.1.0'
    implementation 'com.squareup.retrofit2:converter-gson:2.0.0'

3. Create an APIResponse class to parse the result data later.
    In my case, the response format is like
    {"status":200,"result":"success",detail":"this is the response"}.

import com.google.gson.annotations.Expose;
import com.google.gson.annotations.SerializedName;

public class APIResponse {

    @SerializedName("status")
    @Expose
    private int rstatus;

    @SerializedName("result")
    @Expose
    private String result;

    @SerializedName("detail")
    @Expose
    private String detail;

    public String getDetail() {
        return detail;
    }

    public String getResult() {
        return result
    }

    public int getStatus() {
        return rstatus;
    }
}

4. Create an interface to manage URL and API request part.
    - First one requests GET to "http://api.somehost.com/get_new_target" without any parameters.
    - Second one requests POST to "http://api.somehost.com/upload_record" with String parameter "name" and filedata.
Instead of default ResponseBody, we will use APIResponse class that we've created at 3.

import okhttp3.MultipartBody;
import retrofit2.Call;
import retrofit2.http.GET;
import retrofit2.http.Multipart;
import retrofit2.http.POST;
import retrofit2.http.Part;

public interface APIService {
    public static final String API_URL = "http://api.somehost.com";


    @GET("/get_new_target")
    Call<APIResponse>getTarget();

    @POST("upload_record")
    @Multipart
    Call<APIResponse>updateResult(
            @Part("name") String name,
            @Part MultipartBody.Part audiofile
    );

}

5. We have completed the interface. Now we want to use it to actually make a request.
    Create a Retrofit object and an APIService object as global variables.

public class MainActivity extends AppCompatActivity {

    APIService apiService;
    Retrofit retrofit;

        Call<APIResponse> target = apiService.getTarget();
        target.enqueue(new Callback<APIResponse>(){
            @Override
            public void onResponse(Call<APIResponse> cal, Response<APIResponse> response) {
                try{

                    //do something with response!


                } catch (Exception e){
                    e.printStackTrace();
                }
            }

            @Override
            public void onFailure(Call<APIResponse> call, Throwable t) {
                LogWrapper.e("APIError",t.toString());
            }

        });