How can create reproducable random numbers

Question

how can create reproducable random numbers in java 8+ parallel Streams?

Example:

Random r = new Random(1337);

Arrays.stream(new int[]{1,2,3})
.parallel()
.map(i -> i + r.nextInt(10))
.forEach(i -> System.out.println(i));

Random and ThreadLocalRandom are not generating the same sequence if i execute serveral times.

My expected output is that if i execute this Code snippet serveral times i get always the same output. I dont need a random sequence of ints. I need that the same random number is generate for the same i.

Answer 1

Update:

Based on the valuable comment by Holger, given below should be the right way of doing it:

Arrays.stream(new int[]{1,2,3})
.parallel()
.map(i -> i + new Random(i).nextInt(10))
.forEach(i -> System.out.println(i));

Using i as seed, i.e. .map(i -> i + new Random(i).nextInt(10)) will still reproducibly get the same random number for a particular i, but not a predictable constant value as returned by new Random(1337).nextInt(10).

Original answer:

Do it as follows:

Arrays.stream(new int[]{1,2,3})
.parallel()
.map(i -> i + new Random(1337).nextInt(10))
.forEach(i -> System.out.println(i));

Demo:

import java.util.Arrays;
import java.util.Random;

public class Main {
    public static void main(String args[]) {
        for (int k = 1; k <= 100; k++) {
            Arrays.stream(new int[]{1,2,3})
            .parallel()
            .map(i -> i + new Random(1337).nextInt(10))
            .forEach(i -> System.out.println(i));
        }
    }
}