Question

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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.

Answer 1

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.