Generating Unique Random Numbers in Java

by
  • Add each number in the range sequentially in a list structure.
  • Shuffle it.
  • Take the first ‘n’.

Here is a simple implementation. This will print 3 unique random numbers from the range 1-10.

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<3; i++) {
            System.out.println(list.get(i));
        }
    }
}

The first part of the fix with the original approach, as Mark Byers pointed out in an answer now deleted, is to use only a single Random instance.

That is what is causing the numbers to be identical. A Random instance is seeded by the current time in milliseconds. For a particular seed value, the ‘random’ instance will return the exact same sequence of pseudo random numbers.

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