Lecture Notes: An Introduction to Retrofit: Efficient Networking in Android


Hello everyone, and welcome to today's lecture on Retrofit, a powerful and popular networking library for Android applications.
Retrofit simplifies the process of making network requests and handling responses, allowing developers to focus on building feature-rich applications. In this lecture, we will cover the basics of Retrofit, how to set it up in your Android project, and how to make HTTP requests using this library.

I. What is Retrofit?

Retrofit is a type-safe HTTP client for Android and Java applications. Developed by Square, it simplifies the process of making network requests by converting your API into a Java interface.
Retrofit uses annotations to describe the API's endpoints, request methods, and parameters. It also supports various serialization libraries, such as Gson, Jackson, and Moshi, to convert JSON data into Java objects and vice versa.

II. Setting Up Retrofit in Your Android Project

To start using Retrofit in your Android project, you need to add the necessary dependencies to your build.gradle file:

dependencies {
// Retrofit core library
implementation 'com.squareup.retrofit2:retrofit:2.9.0'

// Gson converter
implementation 'com.squareup.retrofit2:converter-gson:2.9.0'

After adding these dependencies, remember to sync your project with the updated build configuration.

III. Creating an API Interface

To begin using Retrofit, you need to create an interface representing your API. This interface will define your API's endpoints, request methods, and parameters. For example, let's assume we are working with a simple API that retrieves a list of users:

import retrofit2.Call;
import retrofit2.http.GET;
import java.util.List;

public interface API {
Call<List<User>> getUsers();

In this example, we define a single endpoint, users, which returns a list of User objects. The @GET annotation specifies the HTTP request method and the relative URL.

IV. Building the Retrofit Instance

Next, we need to create a Retrofit instance, which will handle the network requests for our API.
Typically, you would create a single instance for your entire application. Here is an example of how to create a Retrofit instance:

import retrofit2.Retrofit;
import retrofit2.converter.gson.GsonConverterFactory;

public class RetrofitClient {
private static final String BASE_URL = "";
private static Retrofit retrofit = null;

public static Retrofit getClient() {
if (retrofit == null) {
retrofit = new Retrofit.Builder()
return retrofit;

In this example, we define a RetrofitClient class with a getClient() method that returns a singleton Retrofit instance. We provide the base URL for the API and add a Gson converter factory to handle JSON serialization. (gets us the goal being able to move between JSON and Java OBJECTS).

V. Making Requests with Retrofit (remember the goal of network programming here in KOTLIN is being able to access a network resource and get a package of data wrapped in a box called JSON, or GSON)

Now that we have our API interface and Retrofit instance, we can start making network requests.
To do this, simply create an instance of your API interface using the Retrofit instance:
API api = RetrofitClient.getClient().create(API.class);

You can then use this API instance to make network requests.
For example, to fetch a list of users, you would call the getUsers() method:

Call<List<User>> call = api.getUsers();

To execute the network request and handle the response, use the enqueue() method:

call.enqueue(new Callback<List<User>>() {
public void onResponse(Call<List<User>> call, Response<List<User>> response) {
if (response.isSuccessful()) {
List<User> users = response.body();
// Handle the list of users
} else {
// Handle the error

public void onFailure(Call<List<User>> call, Throwable t) {
// Handle the failure


In this lecture, we have covered the basics of Retrofit, including setting up the library in an Android project, creating an API interface, building a Retrofit instance, and making network requests.
Retrofit simplifies networking tasks, allowing developers to build efficient and robust Android applications.
By leveraging Retrofit's features and adopting best practices, you can create a seamless and responsive user experience in your Android apps.
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