[Android] เขียนโค้ดนับถอยหลังแล้วเวลาหมด ให้ขึ้น pop up Android code

import android.app.Activity;
import android.app.AlertDialog;
import android.os.Bundle;
import android.widget.TextView;
import android.os.CountDownTimer;

public class MainActivity extends Activity {
 private TextView mTextField;
 /** Called when the activity is first created. */
 @Override
 public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
 
 mTextField = (TextView)findViewById(R.id.showCount);
 
  new CountDownTimer(30000, 1000) {
	  
      public void onTick(long millisUntilFinished) {
          mTextField.setText("seconds remaining: " + millisUntilFinished / 1000);
      }
      public void onFinish() {
    	  
          mTextField.setText("done!");
       
      }
   }.start();
 }
}