Q: Adding pin to authenticate user to android app I am working on a project where we want the user to be able to add a 4 digit pin. I want to be able to save it and have the app check upon launch that a pin is set, so that it asks the user for the pin if set or if not set just launch the app. I cannot save this pin in shared preference.
What would be the correct procedure to get this functionality? I will also be adding fingerprint scanner down the line so I need to make sure it works with that.

A: You can simply do that through Shared Preference also . Get a activity for Set password , make it Default activity . Application will be launch and ask to set password , if user set password then store it through Shared preference and forward them to Main Activity . And check saved password in SetPassword onCreate(),onStart() when it would be created next time .
On the other hand in Main activity onStart() check the key also .
Here i am sharing with some simple code -
 <activity android:name=".SetPassword">
        <intent-filter>
        <action android:name="android.intent.action.MAIN" />

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

SetPassword Class-
public class SetPassword extends AppCompatActivity {

EditText passwordview ;
Button save;
private SharedPreferences prefs;

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


     try {
    prefs = PreferenceManager.getDefaultSharedPreferences(this);
    String password = prefs.getString("password", "");
    if (!password.isEmpty()) {
        sendToMain();
      }
    } catch (Exception e) {
    e.printStackTrace();
    }

    passwordview = findViewById(R.id.password);
    save = findViewById(R.id.save);
    save.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            String password = passwordview.getText().toString();

            if (password.isEmpty()) {
                passwordview.setError("Password required");
                return;
            }

            if (password.contains(" ")) {
                passwordview.setError("Don't support space");
                return;
            }

            if (password.length() < 4 || password.length() >4) {
                passwordview.setError("Support 4 digint password");
                return;
            }

            SharedPreferences pref = PreferenceManager.getDefaultSharedPreferences(getApplicationContext());
            SharedPreferences.Editor editor = pref.edit();
            editor.putString("password", password);
            editor.commit();
            sendToMain();


        }
    });
}

    @Override
protected void onStart() {
    super.onStart();
     SharedPreferences defPref = PreferenceManager.getDefaultSharedPreferences(getApplicationContext());
     String password  = defPref.getString("password", "");
     if (!password.isEmpty()) {
        sendToMain();
    }
}

private void sendToMain() {
    Intent mainIntent = new Intent(SetPassword.this, MainActivity.class);
    startActivity(mainIntent);
    finish();
}
  }

activity_set_password.xml
    <?xml version="1.0" encoding="utf-8"?>
  <androidx.constraintlayout.widget.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"
tools:context=".SetPassword">

<EditText
    android:id="@+id/password"
    android:layout_width="0dp"
    android:layout_height="wrap_content"
    android:layout_marginTop="231dp"
    android:hint="password"
    android:inputType="numberPassword"
    app:layout_constraintEnd_toEndOf="parent"
    app:layout_constraintStart_toStartOf="parent"
    app:layout_constraintTop_toTopOf="parent" />

<Button
    android:id="@+id/save"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_marginStart="143dp"
    android:layout_marginLeft="143dp"
    android:layout_marginBottom="232dp"
    android:text="Button"
    app:layout_constraintBottom_toBottomOf="parent"
    app:layout_constraintStart_toStartOf="parent" />
  </androidx.constraintlayout.widget.ConstraintLayout>

MainActivity class-
public class MainActivity extends AppCompatActivity {
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);


    }

    @Override
    protected void onStart() {
        super.onStart();
          SharedPreferences defPref = PreferenceManager.getDefaultSharedPreferences(getApplicationContext());
          String password  = defPref.getString("password", "");
          if (password.isEmpty()) {
           startActivity(new Intent(MainActivity.this, SetPassword.class));
           finish();
          }
    }

}

If you think about encryption also then you can check AES encryption .

A: Well you can use SharedPreferences or you can save your data Using normal java InputStream/OutputStream classes
-in case of shared prefs: you can see the link below
https://developer.android.com/training/data-storage/shared-preferences
-in case of java native classes you can access the link below
https://www.tutorialspoint.com/javaexamples/file_write.htm
