Generating Unique Random Numbers in Java

Question

I'm trying to get random numbers between 0 and 100. But I want them to be unique, not repeated in a sequence. For example if I got 5 numbers, they should be 82,12,53,64,32 and not 82,12,53,12,32 I used this, but it generates same numbers in a sequence.

Random rand = new Random();
selected = rand.nextInt(100);

Answer 1

• 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(new Integer(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.

NOTE that the public Integer​(int value) constructor is deprecated since Java 9.

The first for loop can simply be changed to:

for (int i = 1; i < 11; i++) {
  list.add(i);
}