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

    Log.i(TAG,  "onCreate() from MainActivity");

    session = new TempSession(this);

    // check if logged in. If not, take the user back to login activity.
    session.checkLogin(); // <---- HERE

    Toast.makeText(this, "logged in as " + session.getUsername(), Toast.LENGTH_SHORT).show();

    // Prevents screen from turning off when in this Activity.
    getWindow().addFlags(WindowManager.LayoutParams.FLAG_KEEP_SCREEN_ON);

    ....

public void checkLogin(){
    // Check login status
    if(!this.isLoggedIn() || getUsername() == null) {
        // user is not logged in redirect him to Login Activity
        Intent i = new Intent(context, LoginActivity.class);

        // Closing all the Activities
        i.addFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP | Intent.FLAG_ACTIVITY_NEW_TASK);

        // Add new Flag to start new Activity
        i.addFlags(Intent.FLAG_ACTIVITY_NO_HISTORY);

        // Staring Login Activity
        context.startActivity(i);
    }
}