Android Retrofit 2.1  HTTP client :

In this example i have divided in to 5 steps :

1. Add gradle dependency

2. Create models for accessing the final data (in this example, the api response json converting to java object this object class is called model )

3. Create interface to define api end point

4. Create a service class for interacting our java code with retrofit.

5. Make the api calls wherever you required

1. Add Gradle dependency (ref: http://square.github.io/retrofit/ )

compile 'com.squareup.retrofit2:retrofit:2.1.0'
compile 'com.squareup.retrofit2:converter-gson:2.1.0'

2. Create models 

User.java

public class User {
    public int id;
    public String name;
    public String email;
    public String phone;
}

3. Create interface

ApiInterface.java

import java.util.List;

import retrofit2.Call;
import retrofit2.http.Body;
import retrofit2.http.GET;
import retrofit2.http.POST;
import retrofit2.http.Path;
import retrofit2.http.Query;
import samples.bm.com.firebaselogin2.models.User;

public interface ApiInterface {

    @GET("get/users")
    Call<List<User>> getAllUsers(); //List<User> is the response from api

    @GET("get/user")
    Call<User> getUser( @Query("id") int id); //using query param (get/user?id=17479666). User is the response from the api

    //@GET("get/user/{id}")
    //Call<User> getUser( @Query("id") int id); //using path param (get/user/17479666)

    @POST("register/user")
    Call<User> registerUser(@Body User user);

    @FormUrlEncoded
    @POST("update/{id}/user")
    Call<User> updateUser(@Field("name") String name, @Field("phone") String phone, @Path("id") String id);

}

expected api response

get/users

[
    {
	"id": 17479666,
	"name": "name1",
	"email": "[email protected]",
	"phone": "3435353535434"
    }, 
    {
	"id": 17479668,
	"name": "name3",
	"email": "[email protected]",
	"phone": "3435353343434"
    }
]

get/user

{
    "id": 17479666,
    "name": "name1",
    "email": "[email protected]",
    "phone": "3435353535434"
}

4. Create a service class

ApiService.java

import com.google.gson.Gson;
import com.google.gson.GsonBuilder;

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

public class ApiService {

    private static ApiInterface service;
    private static String BASE_URL = "https://api.domain.com/"; //your api domain

    public static ApiInterface getInstance() {

        if (service == null) {

            Gson gson = new GsonBuilder()
                    .setDateFormat("yyyy-MM-dd'T'HH:mm:ssZ")
                    .create();

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

            service = retrofit.create(ApiInterface.class);
        }

        return service;
    }
}

5. Make the api calls

Retrofit supports synchronous and asynchronous request execution.

 Asynchronous Requests

GET calls

ApiService.getInstance().getAllUsers().enqueue(new Callback<List<User>>() {
    @Override
    public void onResponse(Call<List<User>> call, Response<List<User>> response) {
        //api success
        List<User> users = response.body();
    }

    @Override
    public void onFailure(Call<List<User>> call, Throwable t) {
        Log.e("Apicall", t.getMessage());
    }
});

GET call with query string/param OR GET call with path string/param

int id=17479666;
ApiService.getInstance().getUser(id).enqueue(new Callback<User>() {
    @Override
    public void onResponse(Call<User> call, Response<User> response) {
        //api success
        User user = response.body();
    }

    @Override
    public void onFailure(Call<User> call, Throwable t) {
        Log.e("Apicall", t.getMessage());
    }
});

Post call

POST call with body param

User user = new User("samoe", "[email protected]", "234242424");

ApiService.getInstance().registerUser(user).enqueue(new Callback<User>() {
    @Override
    public void onResponse(Call<User> call, Response<User> response) {
        //api success
        User user = response.body();
    }

    @Override
    public void onFailure(Call<User> call, Throwable t) {
        Log.e("Apicall", t.getMessage());
    }
});

POST call with form encoding

    String name = "some name";
    String phone = "54686868";
    String id="17479666";

    ApiService.getInstance().updateUser(name, phone, id).enqueue(new Callback<User>() {
        @Override
        public void onResponse(Call<User> call, Response<User> response) {
            //api success
            User user = response.body();
        }

        @Override
        public void onFailure(Call<User> call, Throwable t) {
            Log.e("Apicall", t.getMessage());
        }
    });
}

Synchronous request

You can also call the apis synchronously.

try {            
     List<User> users = ApiService.getInstance().getAllUsers().execute().body();
} catch (IOException e) {
     e.printStackTrace();
}

You cannot run this code in main tread you will get exception like ( java.lang.RuntimeException:Unable to start activity ComponentInfo{  …. } android.os.NetworkOnMainThreadException). So better to use synchronous request in services or asyncTask.

How to create AsyncTask : https://wiki.workassis.com/android-asynctask

How to create Service : https://wiki.workassis.com/android-intentservice-example

Ref : http://square.github.io/retrofit/