Sistema pulsación doble para salir App con DrawableMenu de Android

doubleBackPressed_drawableMenu.java

@Override
  public void onBackPressed() {
      DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
      if (drawer.isDrawerOpen(GravityCompat.START)) {
          drawer.closeDrawer(GravityCompat.START);
      } else {

          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();

      }
  }