Introduction

Retrofit is one of the most popular HTTP libraries for Android. It is developed by Square and is used to connect Android applications with REST APIs.

Using Retrofit, you can easily:

• Fetch data from APIs

• Send data to servers

• Upload files

• Handle JSON responses

• Integrate RESTful services

In this tutorial, you will learn how to integrate Retrofit API in Android using Java.

What is Retrofit?

Retrofit is a type-safe HTTP client for Android.

Without Retrofit:

HttpURLConnection

With Retrofit:

Call<User> call = api.getUser();

Much cleaner and easier.

Prerequisites

Before starting:

• Android Studio

• Java Knowledge

• Internet Connection

• Android Project

Step 1: Add Internet Permission

AndroidManifest.xml

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

Step 2: Add Retrofit Dependency

app/build.gradle

implementation 'com.squareup.retrofit2:retrofit:2.9.0'
implementation 'com.squareup.retrofit2:converter-gson:2.9.0'

Sync Project.

API Example

We'll use:

https://jsonplaceholder.typicode.com/users

This is a free testing API.

Step 3: Create Model Class

User.java

public class User {

    private int id;
    private String name;
    private String email;

    public int getId() {
        return id;
    }

    public String getName() {
        return name;
    }

    public String getEmail() {
        return email;
    }

}

Step 4: Create API Interface

ApiService.java

import java.util.List;

import retrofit2.Call;
import retrofit2.http.GET;

public interface ApiService {

    @GET("users")
    Call<List<User>> getUsers();

}

Understanding Annotations

GET Request

@GET("users")

Fetch data from:

/users

POST Request

@POST("users")

Send data.

PUT Request

@PUT("users/{id}")

Update data.

DELETE Request

@DELETE("users/{id}")

Delete data.

Step 5: Create Retrofit Client

RetrofitClient.java

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

public class RetrofitClient {

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

    private static Retrofit retrofit;

    public static Retrofit getClient() {

        if(retrofit == null) {

            retrofit =
                    new Retrofit.Builder()
                            .baseUrl(BASE_URL)
                            .addConverterFactory(
                                    GsonConverterFactory.create()
                            )
                            .build();
        }

        return retrofit;
    }

}

Step 6: Create Layout

activity_main.xml

<LinearLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="vertical"
    android:padding="16dp"
    android:layout_width="match_parent"
    android:layout_height="match_parent">

    <Button
        android:id="@+id/btnLoad"
        android:text="Load Users"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"/>

</LinearLayout>

Step 7: Fetch API Data

MainActivity.java

import android.os.Bundle;
import android.util.Log;
import android.widget.Button;

import androidx.appcompat.app.AppCompatActivity;

import java.util.List;

import retrofit2.Call;
import retrofit2.Callback;
import retrofit2.Response;

public class MainActivity
        extends AppCompatActivity {

    Button btnLoad;

    @Override
    protected void onCreate(
            Bundle savedInstanceState) {

        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        btnLoad =
                findViewById(R.id.btnLoad);

        btnLoad.setOnClickListener(v -> {

            loadUsers();

        });

    }

    private void loadUsers() {

        ApiService apiService =
                RetrofitClient
                        .getClient()
                        .create(ApiService.class);

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

        call.enqueue(new Callback<List<User>>() {

            @Override
            public void onResponse(
                    Call<List<User>> call,
                    Response<List<User>> response) {

                if(response.isSuccessful()) {

                    List<User> users =
                            response.body();

                    for(User user : users) {

                        Log.d(
                                "USER",
                                user.getName()
                        );

                    }

                }

            }

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

                Log.d(
                        "ERROR",
                        t.getMessage()
                );

            }

        });

    }

}

How enqueue() Works

call.enqueue(...)

Runs API request asynchronously.

Benefits:

• No UI freeze

• Fast response

• Background thread execution

GET Request Example

@GET("users")
Call<List<User>> getUsers();

Used to fetch data.

POST Request Example

Create User:

@POST("users")
Call<User> createUser(
        @Body User user
);

Sending Data

User user =
        new User(
                "Anoj",
                "anoj@gmail.com"
        );

api.createUser(user);

Path Parameter Example

API:

/users/1

Interface:

@GET("users/{id}")
Call<User> getUser(
        @Path("id") int id
);

Usage:

api.getUser(1);

Query Parameter Example

API:

/users?id=1

Interface:

@GET("users")
Call<List<User>> getUser(
        @Query("id") int id
);

Response Handling

Success:

response.isSuccessful()

Status Code:

response.code()

Response Body:

response.body()

Error Body:

response.errorBody()

JSON Example

API Response:

[
  {
    "id": 1,
    "name": "Leanne Graham",
    "email": "leanne@gmail.com"
  }
]

Converted Automatically:

List<User>

using Gson.

Common Errors

Base URL Missing Slash

Wrong:

https://jsonplaceholder.typicode.com

Correct:

https://jsonplaceholder.typicode.com/

Internet Permission Missing

Error:

Unable to resolve host

Solution:

Add INTERNET permission.

Null Response

Cause:

response.body()

is null.

Solution:

Check:

response.isSuccessful()

first.

Best Practices

✅ Create separate RetrofitClient

✅ Use Model Classes

✅ Use Repository Pattern

✅ Handle API Errors

✅ Show ProgressBar

✅ Use RecyclerView for Data Display

Retrofit Architecture

API
 ↓
Retrofit
 ↓
Model Class
 ↓
Repository
 ↓
ViewModel
 ↓
UI

Used in modern Android apps.

Why Developers Prefer Retrofit?

• Easy to use

• Fast

• Clean code

• Automatic JSON parsing

• Supports GET, POST, PUT, DELETE

• Works perfectly with Firebase and MVVM

Conclusion

Congratulations! 🎉

You have successfully integrated Retrofit API in Android using Java.

You learned:

• Retrofit Setup

• GET Requests

• POST Requests

• Query Parameters

• Path Parameters

• JSON Parsing

• Error Handling

Retrofit is one of the most important libraries every Android developer should learn because almost every modern app communicates with APIs.

Next Tutorial

• RecyclerView in Android (Java)

• MVVM Architecture in Android

• Room Database CRUD

• Firebase Authentication

• Firebase Realtime Database