Retrofit is a library for Java and Android that allows making HTTP requests to REST APIs. It uses OkHttp for networking functions and can convert HTTP responses to Java objects using converters like Gson. To use Retrofit, add it and converter dependencies to the build file, create a Retrofit instance with the base URL and converter, define an interface for API requests, and call those methods to make asynchronous network requests that return response data.

1. Retrofit Library in Android
Retrofit is REST API Client for Java.
It is developed by Square Inc. It uses OkHttp library for HTTP Request. It is a simple
library that is used for network transaction.
It is a very easy and fast library to retrieve and upload the data via Rest based web
service.
Adding Dependency in Build.gradle-
dependencies {
implementation 'com.squareup.retrofit2:retrofit:2.1.0'
implementation 'com.google.code.gson:gson:2.6.2'
implementation 'com.squareup.retrofit2:converter-gson:2.1.0'
}
Adding Internet Permission in Manifest-
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
package="com.example.kripashankar.countryappusingretrofit">
<uses-permission android:name="android.permission.INTERNET" />
Retrofit mainly need three things which are following-
1. Retrofit Instance-

2. You can create Retrofit instance by Retrofit.Builder().
You have to specify base url and converter factory at the time of Retrofit instance
creation as in the below example.
2. Interface -
public interface ApiCallInterface
{
@POST("api_name") // use @POST if api is post.
Call<CountryResponse> getResponseData();
}
3. Model Class-
Retrofit need a model class (POJO) for sending and receiving request.
Retrofit use the model class for parsing the server response by using convertors like
Gson, Jackson.
Example -
In below example, we are going to display country name and country code in
RecyclerView.
For which as in createRetroFitBuilder() method of example first we set header using
OkHttpClient class because API is authenticate api (set header only if api is
authenticate). Then we create Retrofit instance and set base url and converter factory
and call interface method which interact with API on background thread and
success of API can be get in onResponse() method of enqueue call back.Failure of
API can be get in onFailure() method of enqueue call back.After that pass the list of
country at adapter as in method setUpAdapterView() of example.
public class MainActivity extends AppCompatActivity {
private RecyclerView recyclerViewCountry;
private CountryListAdapter countryListAdapter;
String BASE_URL = "http://example/retrofit/api/";
CountryResponse countryResponse;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
initializeView();
createRetroFitBuilder();
}
private void initializeView()

3. {
recyclerViewCountry = (RecyclerView)
findViewById(R.id.recyclerViewCountry);
}
private void createRetroFitBuilder() {
// set header through OkHttpClient if API is authenticate API.
OkHttpClient.Builder httpClient = new OkHttpClient.Builder();
httpClient.addInterceptor(new Interceptor() {
@Override
public okhttp3.Response intercept(Chain chain) throws
IOException {
Request request = chain.request().newBuilder()
.addHeader("timestamp", "153138130")
.addHeader("authentication_key",
"QJTpP/7rai7D7KF2RcNK=")
.build();
return chain.proceed(request);
}
});
// creating retrofit object and set base url , Converter factory
Retrofit retrofit = new Retrofit.Builder()
.baseUrl(BASE_URL)
.addConverterFactory(GsonConverterFactory.create())
.client(httpClient.build())
.build();
// calling of interface method which interact with API and give
response in onResponse().
ApiCallInterface apiCallInterface =
retrofit.create(ApiCallInterface.class);
Call<CountryResponse> call =
apiCallInterface.getResponseData();
call.enqueue(new Callback<CountryResponse>() {
@Override
public void onResponse(Call<CountryResponse> call,
Response<CountryResponse> response)
{
countryResponse = response.body();
setUpAdapterView();
}

4. @Override
public void onFailure(Call<CountryResponse> call, Throwable
t) {
Toast.makeText(MainActivity.this,t.getMessage(),Toast.LENGTH_SHORT).s
how();
}
});
}
private void setUpAdapterView() {
if(countryResponse != null)
{
countryListAdapter = new
CountryListAdapter(countryResponse,this);
LinearLayoutManager mLayoutManager = new
LinearLayoutManager(this);
mLayoutManager.setOrientation(LinearLayoutManager.VERTICAL);
recyclerViewCountry.setLayoutManager(mLayoutManager);
recyclerViewCountry.setAdapter(countryListAdapter);
}
}
}