Concurrent calls to new Random() not random enough

[Martin Buchholz]

Hi Sherman,

This is a bug report (sorry, no fix this time)

Synopsis: Concurrent calls to new Random() not random enough
Description:
new Random() promises this:
    /**
     * Creates a new random number generator. This constructor sets
     * the seed of the random number generator to a value very likely
     * to be distinct from any other invocation of this constructor.
     */

but if there are concurrent calls to new Random(), it does not
do very well at fulfilling its contract.

The following program should print out a number much closer to 0.

import java.util.*;

public class RandomSeed {
    public static void main(String[] args) throws Throwable {
        class RandomCollector implements Runnable {
            int[] randoms = new int[1<<21];
            public void run() {
                for (int i = 0; i < randoms.length; i++)
                    randoms[i] = new Random().nextInt();
            }};
        final int threadCount = 2;
        List<RandomCollector> collectors = new ArrayList<RandomCollector>();
        List<Thread> threads = new ArrayList<Thread>();
        for (int i = 0; i < threadCount; i++) {
            RandomCollector r = new RandomCollector();
            collectors.add(r);
            threads.add(new Thread(r));
        }
        for (Thread thread : threads)
            thread.start();
        for (Thread thread : threads)
            thread.join();
        int collisions = 0;
        HashSet<Integer> s = new HashSet<Integer>();
        for (RandomCollector r : collectors) {
            for (int x : r.randoms) {
                if (s.contains(x))
                    collisions++;
                s.add(x);
            }
        }
        System.out.println(collisions);
    }
}
---
==> javac -source 1.6 -Xlint:all RandomSeed.java
==> java -esa -ea RandomSeed
876