Countdown Timer Notification

I'm begginer in Android programming, I have to do a CountdownTimer that starts from a number selected by the user using a two number pickers (one for Hours and other for minutes).

The characteristics must be:

-The CountdownTimer must work in background and notify the user when it arrives to 00:00. I Have this code:

package com.android.application;
import android.app.Activity;
import android.app.Notification;
import android.app.NotificationManager;
import android.app.PendingIntent;
import android.content.Intent;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.NumberPicker;

public class MainActivity extends Activity {
NotificationManager nm;
private static final int ID_NOTIFICACION_PERSONAL =1;

@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.main);

NumberPicker number1 = (NumberPicker)findViewById(R.id.horas);
NumberPicker number2 = (NumberPicker)findViewById(R.id.minutos);
Button btn_lanzar=(Button)findViewById(R.id.btn_notificacion);
number1.setMaxValue(10);
number1.setMinValue(0);
number2.setMinValue(0);
number2.setMaxValue(59);

int number3=number1.getValue();
int number4=number2.getValue();

nm = (NotificationManager)getSystemService(NOTIFICATION_SERVICE);
btn_lanzar.setOnClickListener(new View.OnClickListener(){
    @Override 
    public void onClick(View v) {
    Notification notification = new Notification(R.drawable.estacionamiento,"Estacionamiento Inteligente",System.currentTimeMillis());    
    PendingIntent intencionpendiente = PendingIntent.getActivity(getApplicationContext(),0, new Intent (getApplicationContext(),Segunda_ventana.class),0);
    notification.setLatestEventInfo(getApplicationContext(), "Estacionamiento Inteligente", "Su Tiempo se ha Terminado", intencionpendiente);
    nm.notify(ID_NOTIFICACION_PERSONAL, notification);
    }

});
}

} 

I don't know how to make that the CountdownTimer begins from the number selected by the user with the number pickers and the notification when it is done.

Please someone Help Me. :(

Use Android's Timer and TimerTask classes. Below I assume that you know how to display notifications, as it appears you do in your question.

Along with your number pickers, have a Button defined in your static layout like so:

<Button
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:text="Start CountDown"
    android:onClick="startCountdown"
 />

Above I assume that you also have your number pickers in that same layout.

Java activity code might be as follows:

NumberPicker hourPicker;
NumberPicker minutePicker;

@Override
protected void onCreate(Bundle savedInstanceState)
{
    hourPicker = (NumberPicker)findViewById(R.id.horas);
    minutePicker = (NumberPicker)findViewById(R.id.minutos);
    hourPicker.setMaxValue(10);
    hourPicker.setMinValue(0);
    minutePicker.setMinValue(0);
    minutePicker.setMaxValue(59);
}

public void startCountdown(View v)
{
    //this fires when button was tapped
    showTimerStartedNotification();

    int hours = hourPicker.getValue();
    int mins = minutePicker.getValue();

    int millis = ((hours*60)+mins)*60000; // Need milliseconds to use Timer


    new Timer().schedule(new TimerTask()
    {
        @Override
        public void run()
        {
            //code that runs when timer is done
            showTimerFinishedNotification();
        }
    }, millis);
}