How to show a dialog to confirm that the user wishes to exit an Android Activity?

private Toast toast;
private long lastBackPressTime = 0;

@Override
public void onBackPressed() {
  if (this.lastBackPressTime < System.currentTimeMillis() - 4000) {
    toast = Toast.makeText(this, "Press back again to close this app", 4000);
    toast.show();
    this.lastBackPressTime = System.currentTimeMillis();
  } else {
    if (toast != null) {
    toast.cancel();
  }
  super.onBackPressed();
 }
}