Making REST API Calls in Android Using Retrofit

In this tutorial, we'll learn how to make REST API calls in Android using Retrofit, a powerful HTTP client library.

Setting Up Retrofit

First, we need to add Retrofit to our project dependencies. Add the following lines to your `build.gradle` file:

gradle 

implementation 'com.squareup.retrofit2:retrofit:2.9.0' 
implementation 'com.squareup.retrofit2:converter-gson:2.9.0' // For JSON serialization/deserialization

Creating Retrofit Interface and Model Class

We define a Retrofit interface to represent the API endpoints and a model class to represent the data returned by the API:

Retrofit Interface : 

import retrofit2.Call;
import retrofit2.http.GET;
import retrofit2.http.Path;
public interface ApiService {
    @GET("posts/{id}")
    Call<Post> getPost(@Path("id") int postId);
}

Model Class 

public class Post {
    private int userId;
    private int id;
    private String title;
    private String text;
    // Getters and setters
}

Making API Calls

We create a Retrofit instance, make API calls, and handle the response in our Android activity:

Create Retrofit Instance

import retrofit2.Retrofit;

import retrofit2.converter.gson.GsonConverterFactory;

public class RetrofitClient {

    private static Retrofit retrofit;

    private static final String BASE_URL = "https://jsonplaceholder.typicode.com/";

    public static Retrofit getRetrofitInstance() {

        if (retrofit == null) {

            retrofit = new Retrofit.Builder()

                    .baseUrl(BASE_URL)

                    .addConverterFactory(GsonConverterFactory.create())

                    .build();

        }

        return retrofit;

    }

}

Make API Calls

Use the Retrofit interface to make API calls in your activity or fragment.

import androidx.appcompat.app.AppCompatActivity;
import android.os.Bundle;
import android.util.Log;
import retrofit2.Call;
import retrofit2.Callback;
import retrofit2.Response;
public class MainActivity extends AppCompatActivity {
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        ApiService apiService = RetrofitClient.getRetrofitInstance().create(ApiService.class);
        Call<Post> call = apiService.getPost(1); // Example call for getting post with ID 1
        call.enqueue(new Callback<Post>() {
            @Override
            public void onResponse(Call<Post> call, Response<Post> response) {
                if (response.isSuccessful()) {
                    Post post = response.body();
                    // Do something with the post object
                    Log.d("API_RESPONSE", "Title: " + post.getTitle());
                } else {
                    Log.e("API_ERROR", "Failed to fetch post");
                }
            }
            @Override
            public void onFailure(Call<Post> call, Throwable t) {
                Log.e("API_FAILURE", "Error: " + t.getMessage());
            }
        });
    }
}

Conclusion

In this tutorial, we've covered the basics of making REST API calls in Android using Retrofit. Retrofit simplifies the process of interacting with RESTful APIs and is widely used in Android development.

Happy coding!