Sistema pulsación doble para salir App de Android

doubleBackPressed.java

  private long firstClickTime;
  ...
  @Override
  public void onBackPressed() {
    if (firstClickTime + Constants.TIME_INTERVAL > System.currentTimeMillis()) {
        super.onBackPressed();
        return;
    } else {
        Toast.makeText(this, getString(R.string.double_back_pressed), Toast.LENGTH_SHORT).show();
    }
    firstClickTime = System.currentTimeMillis();