unknown issue in using Retrofit 2

I am new to Android and I want to make an application based on REST API.

So I search and found the Retrofit 2.

I used a sample of retrofit that I found in Github to learn how it works.

This files are my Java classes :

MainActivity.java

public class MainActivity extends AppCompatActivity {
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        final TextView textv = findViewById(R.id.textv);
        GitHubClient gitHubClient;
        gitHubClient = ServiceGenerator.createService(GitHubClient.class);

        final Call<GitHubUser> call = gitHubClient.getFeed("google");
        call.enqueue(new Callback<GitHubUser>() {
            @Override
            public void onResponse(Call<GitHubUser> call, Response<GitHubUser> response) {
                GitHubUser gitModel = response.body();
                if (gitModel != null) {
                    textv.setText(getString(R.string.main_response_text,
                            gitModel.getName(),
                            gitModel.getBlog()));
                } else {
                    textv.setText("user doesn't exist");
                }
            }

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

        });

    }

}

GithubClient.java

public interface GitHubClient {
    @GET("users/{user}")
    Call<GitHubUser> getFeed(@Path("user") String user);
}

ServiceGenerator.java

public class ServiceGenerator {

    public static final String API_BASE_URL = "https://api.github.com/";

    private static HttpLoggingInterceptor interceptor = new HttpLoggingInterceptor();
    private static Interceptor logging = interceptor.setLevel(HttpLoggingInterceptor.Level.BASIC);

    private static OkHttpClient httpClient = new OkHttpClient.Builder()
            .addInterceptor(logging)
            .build();

    private static Retrofit.Builder builder =
            new Retrofit.Builder()
                    .baseUrl(API_BASE_URL)
                    .addConverterFactory(GsonConverterFactory.create());

    public static <S> S createService(Class<S> serviceClass) {
        Retrofit retrofit = builder.client(httpClient).build();
        return retrofit.create(serviceClass);
    }
}

GithubUser.java

public class GitHubUser {

    private String name;
    private String blog;

    public String getName() {
        return name;
    }

    public String getBlog() {
        return blog;
    }

}

JSON code from Github

{
  ...
  "name": "Google",
  "blog": "https://opensource.google.com/",
  ...
}

I want to get the name and blog URL

And when I run the program the output is:

javax.net.ssl.SSLProtocolException: SSL handshake aborted: ssl=0xb8952b10: Failure in SSL library, usually a protocol error
error:1407742E:SSL routines:SSL23_GET_SERVER_HELLO:tlsv1 alert protocol version(external/openssl/ssl/s23_clnt.c:741 0x83bc3770:0x00000000)

What's the problem and how can I fix?

The first step here would be to understand the error.

Try printing the error from your onFailure method like this:

@Override
public void onFailure(Call<GitHubUser> call, Throwable t) {
    textv.setText("unknown error"); // can also do textv.setText(t.getMessage()) to display error reason
    t.printStackTrace();
}

You should get in your logcat some more useful information about what went wrong, it will be easier to figure out how to solve it after that Also have you added internet access permission in your manifest?