How to compare Two date in android?

Question

I have a date from server in the format yyyy-mm-dd HH:mm:ss .I have the current time in millis by System.currentTimeMillis(). I want to compare both dates and see if date received from server is future date and if it is so schedule an alarm for the future date. so my question how to compare this 2 different format dates and how to get remaining time (future- current) so as to schedule the alarm.

Edit: I already tried parsing the date in millis but it doesnt return value as same format as System.currentTimeMillis(). so it fails

Answer 1

You can parse the yyyy-mm-dd HH:mm:ss date thanks to DateFormat class (e.g. the SimpleDateFormat class). This operation will return a Date object.

You can also create a Date object from current time in milliseconds.

When you have both objects you can compare them thanks to compareTo method.

For instance the following code:

try {
    SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
    Date parsed = sdf.parse("2016-03-10 22:05:20");

    Date now = new Date(System.currentTimeMillis()); // 2016-03-10 22:06:10

    System.out.println(parsed.compareTo(now));
} catch (Exception e) {
    e.printStackTrace();
}

will print -1, which means that parsed is before now.

EDIT:

Here it is the code of a simple but useless application that makes use of AlarmManager.

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

    Log.d("TAG", "From intent: "+getIntent().getStringExtra("MyEXTRA"));
}

@Override
protected void onResume() {
    super.onResume();

    try {
        SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
        long parsedMillis = sdf.parse("2016-03-10 22:54:30").getTime();
        long now = System.currentTimeMillis(); // 22:54:15

        if (parsedMillis > now) {
            Log.d("TAG", "In the future!");

            AlarmManager am = (AlarmManager) getSystemService(ALARM_SERVICE);
            Intent intent = new Intent(this, MyActivity.class);
            intent.putExtra("MyEXTRA", "From alarm");
            PendingIntent broadcast = PendingIntent.getActivity(this, 0, intent, 0);
            am.setExact(AlarmManager.RTC_WAKEUP, parsedMillis, broadcast);

        } else {
            Log.d("TAG", "In the past...");
        }


    } catch (Exception e) {
        e.printStackTrace();
    }
}

In the LogCat you will see:

03-10 22:54:20.925    3946-3946/com.example.myapp D/TAG﹕ From intent: null
03-10 22:54:21.227    3946-3946/com.example.myapp D/TAG﹕ In the future!
03-10 22:54:30.513    3946-3946/com.example.myapp D/TAG﹕ From intent: From alarm
03-10 22:54:30.577    3946-3946/com.example.myapp D/TAG﹕ In the past...

The last line is printed because the alarm causes the activity to be relaunched, hence the onResume will be called another time and System.currentTimeMillis() will be greater than the hardcoded time. As you can see, however, the alarm is correctly fired at the scheduled time.

Moreover you should consider edge cases: if parsedMillis is greater than now by only a few milliseconds, you could set an alarm that will never be fired because it is set in the past. This happens because the OS needs some time to execute your code, so you should check that parsedMillis > now + DELTA, where DELTA is a reasonable amout of time that depends on the code you execute between System.currentTimeMillis() and am.setExact(...).