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next() in Java from The art of computer programming

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xkcd: Random NumberI have always used Random’s nextInt() or nextLong(). So was curious on how “random” is it actually? Then I found this protected method next().

protected int next(int bits)


Implementation of via Google code:

    protected int next(int bits) {
        long oldseed, nextseed;
        AtomicLong seed = this.seed;
        do {
            oldseed = seed.get();
            nextseed = (oldseed * multiplier + addend) & mask;
        } while (!seed.compareAndSet(oldseed, nextseed));
        return (int)(nextseed >>> (48 - bits));

The general contract of next is that it returns an int value and if the argument bits is between 1 and 32 (inclusive), then that many low-order bits of the returned value will be (approximately) independently chosen bit values, each of which is (approximately) equally likely to be 0 or 1. The method next is implemented by class Random by atomically updating the seed to

(seed * 0x5DEECE66DL + 0xBL) & ((1L << 48) - 1)

and returning

(int)(seed >>> (48 - bits))

This is a linear congruential pseudorandom number generator, as defined by D. H. Lehmer and described by Donald E. Knuth in The Art of Computer Programming, Volume 3: Seminumerical Algorithms, section 3.2.1.


EDIT: Something even more interesting: Why this code is giving strange result? So Random?

Link to JavaDocs for Random.

Written by Utkarsh

August 1st, 2011 at 11:47 pm

Posted in Technology

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