Question

The following code should create two Random objects with identical seeds:

System.out.println("System time before: " + System.currentTimeMillis());
Random r1 = new Random();
Random r2 = new Random(System.currentTimeMillis());
System.out.println("System time after: " + System.currentTimeMillis());

System.out.println("r1: " + r1.nextInt());
System.out.println("r2: " + r2.nextInt());

The seeds should be identical since System.currentTimeMillis() did not change before and after creating the two objects as shown in the output:

System time before: 1331889186449
System time after: 1331889186449
r1: -1836225474
r2: 2070673752

From the docs, the constructor without any arguments is simply:

public Random() { this(System.currentTimeMillis()); }

So what gives? Can anyone explain why the two generators return different outputs when they should have the same seed?

Answer 1

If you are using java.util.Random, this is the default no-args constructor I see – now it might depend on the version of JDK you are using (this code seems to be used for sun JDK 6 & 7 at least):

public Random() {
    this(seedUniquifier() ^ System.nanoTime());
}

private static long seedUniquifier() {
    // L'Ecuyer, "Tables of Linear Congruential Generators of
    // Different Sizes and Good Lattice Structure", 1999
    for (;;) {
        long current = seedUniquifier.get();
        long next = current * 181783497276652981L;
        if (seedUniquifier.compareAndSet(current, next))
            return next;
    }
}

And just to confirm it, here is a code to check if the seeds are identical:

public static void main(String args[]) throws NoSuchFieldException, IllegalArgumentException, IllegalAccessException {
    System.out.println("System time before: " + System.currentTimeMillis());
    Random r1 = new Random();
    Random r2 = new Random(System.currentTimeMillis());
    System.out.println("System time after: " + System.currentTimeMillis());

    Field seed = Random.class.getDeclaredField("seed");
    seed.setAccessible(true);
    AtomicLong seed1 = (AtomicLong) seed.get(r1);
    AtomicLong seed2 = (AtomicLong) seed.get(r2);

    System.out.println("seed1 = " + seed1);
    System.out.println("seed2 = " + seed2);
}