desarrolladores-android-pregunta-login

SharedPreferencesRepository.java

public class SharedPreferencesRepository {
  
  private final Context context;
  
  public SharedPreferencesRepository(Context context) {
    this.context = context;
  }
  
  public boolean saveString(String key, String source) {
    SharedPreferences.Editor editor = getSharedPreferences().edit();
    editor.putString(key, source);
    return editor.commit();
  }

  public String getStringValue(String key) {
    SharedPreferences sharedPreferences = getSharedPreferences();
    return sharedPreferences.getString(key, "");
  }
  
  private SharedPreferences getSharedPreferences() {
    return context.getSharedPreferences("tuapp_preferences_o_como_quieras", Context.MODE_PRIVATE);
  }
  
}

TuLoginActivity.java

public class TuLoginActivity extends Activity {
  
  private SharedPreferencesRepository sharedPreferences;
  
  @Override
  protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    sharedPreferences = new SharedPreferencesRepository(this);
    String token = sharedPreferences.getStringValue("user_token");
    if (token == null || token.isEmpty()) {
      // muestras la UI de login
      setContentView(R.layout.login_activity);
    } else {
      // aqui metes todo lo relacionado ó redireccionas a la actividad/fragmento/whatever que estes loggeado
      startActivity(new Intent(this, LoggedInActivity.class);
    }
  }
  
}