Create a new application from the application templates with a Navigation Drawer Activity.

Check the AndroidX artifacts options and click finish

Create a new class Activity called LoginActivity and add it to AndroidManifest.xml.

Create a new class Activity called LaunchActivity and add it to AndroidManifest.xml.

Change your AndroidManifest.xml so the LaunchActivity has the action MAIN and LAUNCHER CATEGORY.

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

  <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=".LaunchActivity"
        android:label="@string/app_name"
        android:theme="@style/AppTheme.NoActionBar">
      <intent-filter>
        <action android:name="android.intent.action.MAIN" />

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

    <activity android:name=".LoginActivity"/>
    <activity android:name=".MainActivity"/>


  </application>

</manifest>

Read and understand how SharedPreferences work on Android https://developer.android.com/training/data-storage/shared-preferences#java

Use the SharedPreferences to verify in the LaunchActivity if the user has been authenticated or not:

 public class LaunchActivity
     extends AppCompatActivity
 {
 
     public static final String TOKEN_KEY = "TOKEN_KEY";
 
     @Override
     protected void onCreate( @Nullable Bundle savedInstanceState )
     {
         super.onCreate( savedInstanceState );
         SharedPreferences sharedPref =
             getSharedPreferences( getString( R.string.preference_file_key ), Context.MODE_PRIVATE );
 
         if(sharedPref.contains(TOKEN_KEY)){
             //TODO go to MainActivity
         }else{
             //TODO go to LoginActivity
         }
     }
 }

Implement the LoginActivity Layout:

Implement a function that validates the input fields when the Login Button is clicked (use the setError method to show the error messages to the user)

Read and understand the Retrofit library documentation: https://square.github.io/retrofit/

Add the Internet permission to your app on your AndroidManifest.xml

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

 <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">
 ...

  </application>

</manifest>

Add the Retrofit library dependency to your build.gradle (app).

Add the Gson converter dependency to your build.gradle (app).

  com.squareup.retrofit2:converter-gson:<retrofit-version>

Create a Token class that matches the Token response from the API when the authentication is successful.

Create a POJO called LoginWrapper that represents the Login object send to the API (String email and String password).

Create a AuthService interface with @POST declaration to call the auth endpoint of the provided API.

Instantiate the Retrofit class and service as shown on the documentation:

 Retrofit retrofit = new Retrofit.Builder()
     .baseUrl("http:/10.0.2.2:8080") //localhost for emulator
     .addConverterFactory(GsonConverterFactory.create())
     .build();
 
 AuthService authService = retrofit.create(AuthService.class);

Connect the logic of the LoginActivity so when the Login form is fill and is correct then call the authService.login method.

If your App crashed no worries this happens all the time. Read the console on the Logcat tab and find out why the error happened.

hint: https://developer.android.com/guide/components/processes-and-threads

Create an instance of the ExecutorService class. This will allow you to switch to the background thread to be able to make the API Request.

Store the access token on the SharedPreferences and start the MainActivity.

Close your App and open it again. Verify that LaunchActivity takes you directly to the MainActivity.

Implement a logout function that clears the Token from the SharedPreferences and redirects to the LoginActivity (Do not forget to call finish() on the MainActivity to remove it from the activities stack).