Generate unique no using Calendar class/ generate unique code for using hash code

I need to pass unique no to server every time with updated user data.  For that I think, first I need to generate random no and store in a variable.  When ever I need to pass updated user data, I generate random no again and compare it with stored random no.  If It is different I pass it with user data otherwise I genarete it again and again until I fond different no. This sceneraio is not good for me.  For generate unique no I use Calender class and get the current time in millisecond. I did not need to compare just generate the no and pass with user data.

import java.util.Calendar;
import java.util.TimeZone;

Calendar.getInstance(TimeZone.getTimeZone("UTC")).getTimeInMillis();

above code return the time in milliseconds from 1970.

Using "UTC" for getting accurate value from same time zone.
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