[embedyt]https://www.youtube.com/watch?v=R2c5Pv5cXc0[/embedyt]
Links & DependenciesComment.javaPost.javaactivity_main.xmlJsonPlaceHolderApi.javaMainActivity.javaAndroidManifest.xml

Retrofit + converter dependencies:

square.github.io/retrofit/

 

OkHttp GitHub page with HttpLoggingInterceptor dependency:

github.com/square/okhttp

 

JSONPlaceHolder API:

jsonplaceholder.typicode.com/

package com.codinginflow.retrofitexample;

import com.google.gson.annotations.SerializedName;

public class Comment {

    private int postId;

    private int id;

    private String name;

    private String email;

    @SerializedName("body")
    private String text;

    public int getPostId() {
        return postId;
    }

    public int getId() {
        return id;
    }

    public String getName() {
        return name;
    }

    public String getEmail() {
        return email;
    }

    public String getText() {
        return text;
    }
}
package com.codinginflow.retrofitexample;

import com.google.gson.annotations.SerializedName;

public class Post {

    private int userId;

    private Integer id;

    private String title;

    @SerializedName("body")
    private String text;

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

    public int getUserId() {
        return userId;
    }

    public Integer 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 java.util.Map;

import retrofit2.Call;
import retrofit2.http.Body;
import retrofit2.http.DELETE;
import retrofit2.http.Field;
import retrofit2.http.FieldMap;
import retrofit2.http.FormUrlEncoded;
import retrofit2.http.GET;
import retrofit2.http.PATCH;
import retrofit2.http.POST;
import retrofit2.http.PUT;
import retrofit2.http.Path;
import retrofit2.http.Query;
import retrofit2.http.QueryMap;
import retrofit2.http.Url;

public interface JsonPlaceHolderApi {

    @GET("posts")
    Call<List<Post>> getPosts(
            @Query("userId") Integer[] userId,
            @Query("_sort") String sort,
            @Query("_order") String order
    );

    @GET("posts")
    Call<List<Post>> getPosts(@QueryMap Map<String, String> parameters);

    @GET("posts/{id}/comments")
    Call<List<Comment>> getComments(@Path("id") int postId);

    @GET
    Call<List<Comment>> getComments(@Url String url);

    @POST("posts")
    Call<Post> createPost(@Body Post post);

    @FormUrlEncoded
    @POST("posts")
    Call<Post> createPost(
            @Field("userId") int userId,
            @Field("title") String title,
            @Field("body") String text
    );

    @FormUrlEncoded
    @POST("posts")
    Call<Post> createPost(@FieldMap Map<String, String> fields);

    @PUT("posts/{id}")
    Call<Post> putPost(@Path("id") int id, @Body Post post);

    @PATCH("posts/{id}")
    Call<Post> patchPost(@Path("id") int id, @Body Post post);

    @DELETE("posts/{id}")
    Call<Void> deletePost(@Path("id") int id);
}
package com.codinginflow.retrofitexample;

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

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

import java.util.HashMap;
import java.util.List;
import java.util.Map;

import okhttp3.OkHttpClient;
import okhttp3.logging.HttpLoggingInterceptor;
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;

    private JsonPlaceHolderApi jsonPlaceHolderApi;

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

        textViewResult = findViewById(R.id.text_view_result);

        Gson gson = new GsonBuilder().serializeNulls().create();

        HttpLoggingInterceptor loggingInterceptor = new HttpLoggingInterceptor();
        loggingInterceptor.setLevel(HttpLoggingInterceptor.Level.BODY);

        OkHttpClient okHttpClient = new OkHttpClient.Builder()
                .addInterceptor(loggingInterceptor)
                .build();

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

        jsonPlaceHolderApi = retrofit.create(JsonPlaceHolderApi.class);

        //getPosts();
        //getComments();
        //createPost();
        updatePost();
        //deletePost();
    }

    private void getPosts() {
        Map<String, String> parameters = new HashMap<>();
        parameters.put("userId", "1");
        parameters.put("_sort", "id");
        parameters.put("_order", "desc");

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

        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());
            }
        });
    }

    private void getComments() {
        Call<List<Comment>> call = jsonPlaceHolderApi
                .getComments("https://jsonplaceholder.typicode.com/posts/3/comments");

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

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

                List<Comment> comments = response.body();

                for (Comment comment : comments) {
                    String content = "";
                    content += "ID: " + comment.getId() + "\n";
                    content += "Post ID: " + comment.getPostId() + "\n";
                    content += "Name: " + comment.getName() + "\n";
                    content += "Email: " + comment.getEmail() + "\n";
                    content += "Text: " + comment.getText() + "\n\n";

                    textViewResult.append(content);
                }
            }

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

    private void createPost() {
        Post post = new Post(23, "New Title", "New Text");

        Map<String, String> fields = new HashMap<>();
        fields.put("userId", "25");
        fields.put("title", "New Title");

        Call<Post> call = jsonPlaceHolderApi.createPost(fields);

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

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

                Post postResponse = response.body();

                String content = "";
                content += "Code: " + response.code() + "\n";
                content += "ID: " + postResponse.getId() + "\n";
                content += "User ID: " + postResponse.getUserId() + "\n";
                content += "Title: " + postResponse.getTitle() + "\n";
                content += "Text: " + postResponse.getText() + "\n\n";

                textViewResult.setText(content);
            }

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

    private void updatePost() {
        Post post = new Post(12, null, "New Text");

        Call<Post> call = jsonPlaceHolderApi.putPost(5, post);

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

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

                Post postResponse = response.body();

                String content = "";
                content += "Code: " + response.code() + "\n";
                content += "ID: " + postResponse.getId() + "\n";
                content += "User ID: " + postResponse.getUserId() + "\n";
                content += "Title: " + postResponse.getTitle() + "\n";
                content += "Text: " + postResponse.getText() + "\n\n";

                textViewResult.setText(content);
            }

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

    private void deletePost() {
        Call<Void> call = jsonPlaceHolderApi.deletePost(5);

        call.enqueue(new Callback<Void>() {
            @Override
            public void onResponse(Call<Void> call, Response<Void> response) {
                textViewResult.setText("Code: " + response.code());
            }

            @Override
            public void onFailure(Call<Void> 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>