smart way to generate unique random number

user_103089 0
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i want to generate a sequence of unique random numbers in the range of 00000001 to 99999999.

So the first one might be 00001010, the second 40002928 etc.

The easy way is to generate a random number and store it in the database, and every next time do it again and check in the database if the number already exists and if so, generate a new one, check it again, etc. But that doesn't look right, i could be regenerating a number maybe 100 times if the number of generated items gets large.

Is there a smarter way?

EDIT as allways i forgot to say WHY i wanted this, and it will probably make things clearer and maybe get an alternative, and it is: we want to generate an ordernumber for a booking, so we could just use 000001, 000002 etc. But we don't want to give the competitors a clue of how much orders are created (because it's not a high volume market, and we don't want them to know if we are on order 30 after 2 months or at order 100. So we want to have an order number which is random (yet unique)

random c#
21 Answers

You can genearate number like below if you are ok with consumption of memory.

import java.util.ArrayList;
import java.util.Collections;

public class UniqueRandomNumbers {
    
    public static void main(String[] args) {
        ArrayList<Integer> list = new ArrayList<Integer>();
        for (int i=1; i<11; i++) {
            list.add(i);
        }
        Collections.shuffle(list);
        for (int i=0; i<11; i++) {
            System.out.println(list.get(i));
        }
    }
}
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