How could this PendingIntent alarm cancelled?

Question

I've managed to understand this code. This code is used in an reminder app. What this code do is to set and cancel alarm. However, i don't know how the newly created reference could cancel existing PendingIntent. Please explain me this code (the execute method).

Here's the code:

public class AlarmService extends IntentService {

    private static final String TAG = "AlarmService";

    public static final String POPULATE = "POPULATE";
    public static final String CREATE = "CREATE";
    public static final String CANCEL = "CANCEL";

    private IntentFilter matcher;

    public AlarmService() {
        super(TAG);
        matcher = new IntentFilter();
        matcher.addAction(POPULATE);
        matcher.addAction(CREATE);
        matcher.addAction(CANCEL);
    }

    @Override
    protected void onHandleIntent(Intent intent) {
        String action = intent.getAction();
        String alarmId = intent.getStringExtra(AlarmMsg.COL_ALARMID);
        String alarmMsgId = intent.getStringExtra(AlarmMsg.COL_ID);
        String startTime = intent.getStringExtra(Alarm.COL_FROMDATE);
        String endTime = intent.getStringExtra(Alarm.COL_TODATE);

        if (matcher.matchAction(action)) {
            if (POPULATE.equals(action)) {
                RemindMe.dbHelper.populate(Long.parseLong(alarmId), RemindMe.db);
                execute(CREATE, alarmId);
            }

            if (CREATE.equals(action)) {
                execute(CREATE, alarmId, alarmMsgId, startTime, endTime);
            }

            if (CANCEL.equals(action)) {
                execute(CANCEL, alarmId, alarmMsgId, startTime, endTime);
                RemindMe.dbHelper.shred(RemindMe.db);
            }           
        }
    }

    /**
     * @param action
     * @param args {alarmId, alarmMsgId, startTime, endTime}
     */ 
    private void execute(String action, String... args) {       
        Intent i;
        PendingIntent pi;               
        AlarmManager am = (AlarmManager) getSystemService(Context.ALARM_SERVICE);
        Cursor c;

        String alarmId = (args!=null && args.length>0) ? args[0] : null;
        String alarmMsgId = (args!=null && args.length>1) ? args[1] : null;
        String startTime = (args!=null && args.length>2) ? args[2] : null;
        String endTime = (args!=null && args.length>3) ? args[3] : null;

        String status = CANCEL.equals(action) ? AlarmMsg.CANCELLED : AlarmMsg.ACTIVE;

        if (alarmMsgId != null) {
            c = RemindMe.db.query(AlarmMsg.TABLE_NAME, null, AlarmMsg.COL_ID+" = ?", new String[]{alarmMsgId}, null, null, null);

        } else {
            c = AlarmMsg.list(RemindMe.db, alarmId, startTime, endTime, status);
        }

        if (c.moveToFirst()) {
            long now = System.currentTimeMillis();
            long time, diff;
//          long count = c.getCount();
//          double MAX = alarmId!=null && count>30 ? Util.MONTH : Util.YEAR; 
            do {
                i = new Intent(this, AlarmReceiver.class);
                i.putExtra(AlarmMsg.COL_ID, c.getLong(c.getColumnIndex(AlarmMsg.COL_ID)));
                i.putExtra(AlarmMsg.COL_ALARMID, c.getLong(c.getColumnIndex(AlarmMsg.COL_ALARMID)));

                pi = PendingIntent.getBroadcast(this, 0, i, PendingIntent.FLAG_UPDATE_CURRENT);
//              pi = PendingIntent.getService(context, requestCode, intent, flags);

                time = c.getLong(c.getColumnIndex(AlarmMsg.COL_DATETIME));
                diff = time-now + (long)Util.MIN;
                if (CREATE.equals(action)) {
                    if (diff > 0 && diff < Util.YEAR)
                        am.set(AlarmManager.RTC_WAKEUP, time, pi);
                    //am.setRepeating(AlarmManager.RTC_WAKEUP, triggerAtTime, interval, operation);

                } else if (CANCEL.equals(action)) {
                    am.cancel(pi);
                }                       
            } while(c.moveToNext());
        }
        c.close();      
    }

}

Answer 1

This code cancels the alarm like this:

            } else if (CANCEL.equals(action)) {
                am.cancel(pi);

This doesn't cancel the PendingIntent, it just cancels the alarm.

This code will never set more than one alarm, because attempts to set a second alarm will automatically cancel the first one. This is because the same PendingIntent is used every time.