Android Simple Login with Retrofit2

activity_login.xml

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:gravity="center_horizontal"
    android:orientation="vertical"
    android:paddingBottom="@dimen/activity_vertical_margin"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    tools:context="com.umairadil.androidlogin.LoginActivity">

    <!-- Login progress -->
    <ProgressBar
        android:id="@+id/login_progress"
        style="?android:attr/progressBarStyleLarge"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginBottom="8dp"
        android:visibility="gone"
        android:layout_gravity="center"/>

    <android.support.v4.widget.NestedScrollView
        android:id="@+id/login_form"
        android:layout_width="match_parent"
        android:layout_height="match_parent">

        <LinearLayout
            android:id="@+id/email_login_form"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:orientation="vertical">

            <android.support.design.widget.TextInputLayout
                android:id="@+id/input_email"
                android:layout_width="match_parent"
                android:layout_height="wrap_content"
                app:errorEnabled="true">

                <android.support.v7.widget.AppCompatEditText
                    android:id="@+id/email"
                    android:layout_width="match_parent"
                    android:layout_height="wrap_content"
                    android:hint="@string/prompt_email"
                    android:inputType="textEmailAddress"
                    android:maxLines="1"
                    android:singleLine="true" />

            </android.support.design.widget.TextInputLayout>

            <android.support.design.widget.TextInputLayout
                android:id="@+id/input_password"
                android:layout_width="match_parent"
                android:layout_height="wrap_content"
                app:errorEnabled="true">

                <android.support.v7.widget.AppCompatEditText
                    android:id="@+id/password"
                    android:layout_width="match_parent"
                    android:layout_height="wrap_content"
                    android:hint="@string/prompt_password"
                    android:imeActionId="6"
                    android:imeActionLabel="@string/action_sign_in_short"
                    android:imeOptions="actionUnspecified"
                    android:inputType="textPassword"
                    android:maxLines="1"
                    android:singleLine="true" />

            </android.support.design.widget.TextInputLayout>

            <android.support.v7.widget.AppCompatButton
                android:id="@+id/email_sign_in_button"
                style="?android:textAppearanceSmall"
                android:layout_width="match_parent"
                android:layout_height="wrap_content"
                android:layout_marginTop="16dp"
                android:text="@string/action_sign_in"
                android:textStyle="bold" />

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

build.gradle

dependencies {
    implementation fileTree(dir: 'libs', include: ['*.jar'])
    implementation 'com.android.support:appcompat-v7:26.1.0'
    implementation 'com.android.support:design:26.1.0'
    implementation 'com.android.support.constraint:constraint-layout:1.0.2'
    testImplementation 'junit:junit:4.12'
    androidTestImplementation 'com.android.support.test:runner:1.0.1'
    androidTestImplementation 'com.android.support.test.espresso:espresso-core:3.0.1'


    //Retrofit & OkHttp for HTTP Calls
    implementation 'com.squareup.retrofit2:retrofit:2.3.0'
    implementation 'com.squareup.retrofit2:converter-gson:2.3.0'
    implementation 'com.jakewharton.retrofit:retrofit2-rxjava2-adapter:1.0.0'
    implementation 'com.squareup.okhttp3:okhttp:3.9.0'
    implementation 'com.squareup.okhttp3:logging-interceptor:3.9.0'
}

LoginActivity.java

import android.os.Bundle;
import android.support.annotation.NonNull;
import android.support.design.widget.TextInputLayout;
import android.support.v4.widget.NestedScrollView;
import android.support.v7.app.AppCompatActivity;
import android.support.v7.widget.AppCompatButton;
import android.support.v7.widget.AppCompatEditText;
import android.text.TextUtils;
import android.util.Log;
import android.view.View;
import android.widget.ProgressBar;

import com.umairadil.androidlogin.models.LoginResponse;
import com.umairadil.androidlogin.restclient.RestClient;
import com.umairadil.androidlogin.restclient.RestMethods;

import java.util.regex.Matcher;
import java.util.regex.Pattern;

import retrofit2.Call;
import retrofit2.Callback;
import retrofit2.Response;

public class LoginActivity extends AppCompatActivity {

    private String TAG = LoginActivity.class.getSimpleName();

    AppCompatEditText email;
    AppCompatEditText password;
    AppCompatButton loginButton;

    TextInputLayout emailInput;
    TextInputLayout passwordInput;

    ProgressBar progressBar;
    NestedScrollView loginForm;

    RestMethods restMethods;

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

        //Builds HTTP Client for API Calls
        restMethods = RestClient.buildHTTPClient();

        //Binds UI to Activity
        setContent();

        loginButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                
                //Will validate & call Login API
                doLogin();
            }
        });
    }

    void setContent() {
        loginButton = findViewById(R.id.email_sign_in_button);
        email = findViewById(R.id.email);
        password = findViewById(R.id.password);
        progressBar = findViewById(R.id.login_progress);
        loginForm = findViewById(R.id.login_form);
        emailInput = findViewById(R.id.input_email);
        passwordInput = findViewById(R.id.input_password);
    }

    /**
     * Do login.
     * <p>
     * <p>This will validate inputs and then do Login.</p>
     */
    void doLogin() {

        //Check email feild is not empty
        if (TextUtils.isEmpty(email.getText().toString())) {
            emailInput.setError("No Email Provided!");
            return;
        }

        //Check email format is valid
        if (!isValidEmail(email.getText().toString())) {
            emailInput.setError("Email is invalid!");
            return;
        }

        //Check password feild is not empty
        if (TextUtils.isEmpty(password.getText().toString())) {
            passwordInput.setError("No Password Entered!");
            return;
        }
        
        //TODO Add more validations

        //Show loading and call HTTP client's login method
        showLoading();
        restMethods.login(email.getText().toString(), password.getText().toString()).enqueue(new Callback<LoginResponse>() {
            @Override
            public void onResponse(@NonNull Call<LoginResponse> call, @NonNull Response<LoginResponse> response) {

                //Response was successfull
                Log.i(TAG, "Response: " + response.body());

                hideLoading();
            }

            @Override
            public void onFailure(@NonNull Call<LoginResponse> call, @NonNull Throwable t) {
                t.printStackTrace();

                //Response failed
                Log.e(TAG, "Response: " + t.getMessage());

                hideLoading();
            }
        });
    }

    /**
     * validate your email address format.
     */
    public boolean isValidEmail(String email) {
        Pattern pattern;
        Matcher matcher;
        final String EMAIL_PATTERN = "^[_A-Za-z0-9-]+(\\.[_A-Za-z0-9-]+)*@[A-Za-z0-9]+(\\.[A-Za-z0-9]+)*(\\.[A-Za-z]{2,})$";
        pattern = Pattern.compile(EMAIL_PATTERN);
        matcher = pattern.matcher(email);
        return matcher.matches();
    }

    private void showLoading() {
        progressBar.setVisibility(View.VISIBLE);
        loginForm.setVisibility(View.GONE);
    }

    private void hideLoading() {
        progressBar.setVisibility(View.GONE);
        loginForm.setVisibility(View.VISIBLE);
    }


}

LoginResponse.java

/**
*<p>Modify this class according to your Login API's response JSON.</p>
**/
public class LoginResponse {

    String message;
    String userName;
    String email;

    public String getMessage() {
        return message;
    }

    public void setMessage(String message) {
        this.message = message;
    }

    public String getUserName() {
        return userName;
    }

    public void setUserName(String userName) {
        this.userName = userName;
    }

    public String getEmail() {
        return email;
    }

    public void setEmail(String email) {
        this.email = email;
    }
}

RestClient.java

import android.support.annotation.NonNull;
import android.util.Log;

import com.jakewharton.retrofit2.adapter.rxjava2.RxJava2CallAdapterFactory;

import okhttp3.OkHttpClient;
import okhttp3.logging.HttpLoggingInterceptor;
import retrofit2.Retrofit;
import retrofit2.converter.gson.GsonConverterFactory;

import static okhttp3.logging.HttpLoggingInterceptor.Level.BODY;

class RestClient {

    public static RestMethods buildHTTPClient() {

        //TODO Replace with your URL [Must have backslash '/' in end]
        Retrofit retrofit = new Retrofit.Builder()
                .baseUrl("https://my-json-server.typicode.com/umair13adil/demo/")
                .client(getClient())
                .addConverterFactory(GsonConverterFactory.create())
                .addCallAdapterFactory(RxJava2CallAdapterFactory.create())
                .build();
        return retrofit.create(RestMethods.class);
    }

    //Create OKHttp Client used by retrofit
    private static OkHttpClient getClient() {
        return new OkHttpClient.Builder()
                .addInterceptor(provideHttpLoggingInterceptor())
                .build();
    }

    //Attach logging intercept to print Logs in LogCat
    private static HttpLoggingInterceptor provideHttpLoggingInterceptor() {
        HttpLoggingInterceptor httpLoggingInterceptor =
                new HttpLoggingInterceptor(new HttpLoggingInterceptor.Logger() {
                    @Override
                    public void log(@NonNull String message) {
                        Log.d("HTTP", message);
                    }
                });
        httpLoggingInterceptor.setLevel(BODY);
        return httpLoggingInterceptor;
    }
}

RestMethods.java

import retrofit2.Call;
import retrofit2.http.POST;
import retrofit2.http.Query;

/**
 * Created by Umair Adil on 10/12/2016.
 */

public interface RestMethods {

    //TODO Replace with your API's Login Method
    @POST("YOUR_API_METHOD")
    Call<LoginResponse> login(@Query("email") String email, @Query("password") String password);
}

How would you change this to handle login to Github, to get a token from it?