Part 1 – Simple GET Request

[embedyt] https://www.youtube.com/watch?v=4JGvDUlfk7Y[/embedyt]

In this video series we will learn how to use Retrofit, which is a type-safe HTTP client for Android and Java.
Retrofit allows easy communication with a web service by abstracting the HTTP API into a Java interface.

In part 1 we will set up Retrofit in a new Android Studio project by adding the necessary Gradle dependencies, and then already do our first GET request.
We create an interface with 1 method that we annotate with @GET and the relative URL to the API endpoint of the REST API we want to query. We then create a Retrofit instance, define the baseUrl, add GSON as the converter by passing GsonConverterFactory to the addConverterFactory method, and let Retrofit create the implementation of our API interface at compile time.
The Call object that our GET method returns encapsulates a single request + response. With enqueue we can execute this request asynchcronously on a background thread and get our result back in the onResponse callback. OnFailure will be called if something in the process of communicating with the server or processing the response went wrong. With isSuccessFull we check if our response code is between 200 and 300 and if that’s the case we can retrieve our result from the response body.
GSON will parse the array of JSON objects into a List of the Java model class that we prepared beforehand.

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Links & DependenciesPost.javaactivity_main.xmlJsonPlaceHolderApi.javaMainActivity.javaAndroidManifest.xml

Retrofit + converter dependencies:

square.github.io/retrofit/

JSONPlaceHolder API:

jsonplaceholder.typicode.com/

package com.codinginflow.retrofitexample;

import com.google.gson.annotations.SerializedName;

public class Post {

    private int userId;

    private int id;

    private String title;

    @SerializedName("body")
    private String text;

    public int getUserId() {
        return userId;
    }

    public int getId() {
        return id;
    }

    public String getTitle() {
        return title;
    }

    public String getText() {
        return text;
    }
}

<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:padding="8dp"
    tools:context=".MainActivity">

    <android.support.v4.widget.NestedScrollView
        android:layout_width="match_parent"
        android:layout_height="match_parent">

        <TextView
            android:id="@+id/text_view_result"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:textColor="#000" />

    </android.support.v4.widget.NestedScrollView>

</android.support.constraint.ConstraintLayout>

package com.codinginflow.retrofitexample;

import java.util.List;

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

public interface JsonPlaceHolderApi {

    @GET("posts")
    Call<List<Post>> getPosts();
}

package com.codinginflow.retrofitexample;

import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.widget.TextView;

import java.util.List;

import retrofit2.Call;
import retrofit2.Callback;
import retrofit2.Response;
import retrofit2.Retrofit;
import retrofit2.converter.gson.GsonConverterFactory;

public class MainActivity extends AppCompatActivity {
    private TextView textViewResult;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        textViewResult = findViewById(R.id.text_view_result);

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

        JsonPlaceHolderApi jsonPlaceHolderApi = retrofit.create(JsonPlaceHolderApi.class);

        Call<List<Post>> call = jsonPlaceHolderApi.getPosts();

        call.enqueue(new Callback<List<Post>>() {
            @Override
            public void onResponse(Call<List<Post>> call, Response<List<Post>> response) {

                if (!response.isSuccessful()) {
                    textViewResult.setText("Code: " + response.code());
                    return;
                }

                List<Post> posts = response.body();

                for (Post post : posts) {
                    String content = "";
                    content += "ID: " + post.getId() + "\n";
                    content += "User ID: " + post.getUserId() + "\n";
                    content += "Title: " + post.getTitle() + "\n";
                    content += "Text: " + post.getText() + "\n\n";

                    textViewResult.append(content);
                }
            }

            @Override
            public void onFailure(Call<List<Post>> call, Throwable t) {
                textViewResult.setText(t.getMessage());
            }
        });
    }
}

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.codinginflow.retrofitexample">

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

    <application
        android:allowBackup="true"
        android:icon="@mipmap/ic_launcher"
        android:label="@string/app_name"
        android:roundIcon="@mipmap/ic_launcher_round"
        android:supportsRtl="true"
        android:theme="@style/AppTheme">
        <activity android:name=".MainActivity">
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
    </application>

</manifest>