Random gen method in java doesn't seem random

Question

Possible Duplicate:
True random generation in Java
Java random always returns the same number when I set the seed?

I run this piece of code in one of my programs.

public String[] gen_phase_zero() {
        Random generator = new Random();
        int r = generator.nextInt(2);
        if (r == 1) {
            String big = "A";
            String small = "a";
            return new String[] { big, small };
        } else {
            String big = "B";
            String small = "b";
            return new String[] { big, small };
        }
    }

If I run this a few times my output is like this.

Aa Aa Aa Aa Bb Aa Aa Aa Bb

It's not alwasy in that order. But it's almost never anything close to 50/50

Update:

I'm not expeccting fifty fifty, but it seems that if "Aa" is selected first, then it'll be next around 3 more times, but if Bb is selected first, it'll be selected the next three times as well.

Of course you cannot rely on a `Random` class to generate random numbers with `50-50` probability. — Rohit Jain, Nov 11 '12 at 17:54
Refer http://stackoverflow.com/questions/381037/true-random-generation-in-java — Ankur, Nov 11 '12 at 17:54
See [this](http://stackoverflow.com/q/8044570/1037210) question. — Lion, Nov 11 '12 at 18:00

score 5 · Accepted Answer · answered Nov 11 '12 at 17:56

5

Well it doesn't look too bad to me. Let's create a more statistically significant test:

import java.util.Random;

public class Test {
    public static void main(String[] args) throws Exception {

        Random rng = new Random();
        int total = 0;
        for (int i = 0; i < 1000000; i++) {
            total += rng.nextInt(2);
        }
        System.out.println("Total: " + total);
    }
}

Sample output from 5 runs:

Total: 501184
Total: 499740
Total: 500116
Total: 500374
Total: 500413

Doesn't look hugely biased to me...

I got the same results when calling new Random() inside the loop instead of just once, too - although it's not a good idea to do that.

answered Nov 11 '12 at 17:56

Jon Skeet

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What's the significance of `total += rng.nextInt(2);` I'm really asking about the "2" at the end there. Still consider myself fairly new to java. Thanks. – EGHDK Nov 11 '12 at 18:05
@EGHDK: Well it matches your original code - it's an exclusive upper bound. Read the documentation for `Random.nextInt`. Basically that call will either return 0 or 1. I'm adding up the results, so you'd expect it to be about half of the number of iterations. – Jon Skeet Nov 11 '12 at 18:09
Interesting. Alright, thanks! – EGHDK Nov 11 '12 at 18:10

score 1 · Answer 2 · answered Nov 11 '12 at 17:56

1

Do that 10000 times and it will approach 50:50.

Then toss a coin 9 times - the result will probably be similar to what random.nextInt() gives you.

The key here is to statistically significant amount of data.

And btw, use random.nextBoolean()

answered Nov 11 '12 at 17:56

Bozho

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score -1 · Answer 3 · answered Nov 11 '12 at 17:58

-1

You need to add a seed to your random class. Typically it's a timestamp. So try something like

Random generator = new Random(System.currentTimeMillis);

answered Nov 11 '12 at 17:58

donebizkit

458
3
9

You have to be careful with that, as you might be seeding all your "random" generators with the same seed. @EGHDK's best bet would actually to be to create as few Random instances as possible. – Neil Nov 11 '12 at 18:39
This will not help - it is actually worse than the default constructor (which uses a timestamp **and** a static counter.) – finnw Nov 12 '12 at 19:24

Random gen method in java doesn't seem random

Update:

3 Answers3