How to use Retrofit

Posted on August 24, 2016 • 3 minutes • 469 words

Table of contents

Two weeks ago i wrote an Android app with connection to a REST-API. As yet i used Volley for requests. But i finally took a look at Retrofit and i think i like it. In this post i want to give a short introduction on how to use it.

What is Retrofit?

Retrofit is type-safe HTTP client for Android and Java by Square, Inc. You can use it to simplify your API calls.

1) Create Project

Add internet permission to your AndroidManifest.xml:

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

Add Retrofit in your build.gradle:

dependencies {
compile 'com.squareup.retrofit2:retrofit:2.1.0'
compile 'com.google.code.gson:gson:2.6.2'
compile 'com.squareup.retrofit2:converter-gson:2.0.2'
}

In this example i will use Gson for JSON deserialization.

2) Create a new interface

public interface SampleRetrofitInterface {

    @GET("/posts")
    Call<List<Post>> getPostList();

    @GET("/posts/{id}")
    Call<Post> getSinglePost(@Path("id") int postId);

    @GET("/posts/")
    Call<Post> getSinglePostQuery(@Query("userId") int userId);

    @PUT("/posts/{id}")
    Call<Post> putSinglePost(@Path("id") int postId);

    @DELETE("/posts/{id}")
    Call<String> deletePost(@Path("id") int postId);

    @POST("users/new")
    Call<Post> createPost(@Body Post post);


    Retrofit retrofit = new Retrofit.Builder()
            .addConverterFactory(GsonConverterFactory.create())
            .baseUrl("https://jsonplaceholder.typicode.com")
            .build();

}

In this example the baseUrl is “https://jsonplaceholder.typicode.com/" With the annotations you can set the standard HTTP methods like (GET, POST, PUT, DELETE, PATCH, and HEAD).

For example when you call getSinglePostQuery(3) Retrofit will connect to https://jsonplaceholder.typicode.com/posts?userId=3 When you call getSinglePost(3) it will be https://jsonplaceholder.typicode.com/posts/3

3) Create Java class for parsing

This is how a typical response from https://jsonplaceholder.typicode.com/posts looks like.

[

    {
        "userId": 1,
        "id": 1,
        "title": "sunt aut facere repellat provident occaecati excepturi optio reprehenderit",
        "body": "quia et suscipit\nsuscipit recusandae consequuntur expedita et cum\nreprehenderit molestiae ut ut quas totam\nnostrum rerum est autem sunt rem eveniet architecto"
    },
    {
        "userId": 1,
        "id": 2,
        "title": "qui est esse",
        "body": "est rerum tempore vitae\nsequi sint nihil reprehenderit dolor beatae ea dolores neque\nfugiat blanditiis voluptate porro vel nihil molestiae ut reiciendis\nqui aperiam non debitis possimus qui neque nisi nulla"
    }
]

And this is my class that Gson will use to create objects from the JSON response:

public class Post {

    int userId;
    int id;
    String title;
    String body;

    public Post(int userId, int id, String title, String body) {
        this.userId = userId;
        this.id = id;
        this.title = title;
        this.body = body;
    }

    public int getUserId() {
        return userId;
    }

    public int getId() {
        return id;
    }

    public String getTitle() {
        return title;
    }

    public String getBody() {
        return body;
    }
}

4) Use the interface

Now you can use the methods from your network interface anywhere you want.

SampleRetrofitInterface api = SampleRetrofitInterface.retrofit.create(SampleRetrofitInterface.class);
        Call<List<Post>> call = api.getPostList();

        call.enqueue(new Callback<List<Post>>() {
        @Override
        public void onResponse(Call<List<Post>>call, Response<List<Post>> response) {
                Log.i("INFO", String.valueOf(response.body().size()));
        
        
                }
        
        @Override
        public void onFailure(Call<List<Post>>call, Throwable t) {
                // Log error here since request failed
        
                }
        });

The Response in onResponse will return a List of Posts. I think Retrofit is a easy way to connect to an REST-API.

Example Project

You can find my example project on GitHub

BTW: I used a similar library (Refit) in a Xamarin Project