Java Random Color String

Question

I have written this java method but sometimes the color String is only 5 characters long. Does anyone know why?

@Test
public void getRandomColorTest() {
    for (int i = 0; i < 20; i++) {
        final String s = getRandomColor();
        System.out.println("-> " + s);
    }
}

 public String getRandomColor() {
    final Random random = new Random();
    final String[] letters = "0123456789ABCDEF".split("");
    String color = "#";
    for (int i = 0; i < 6; i++) {
        color += letters[Math.round(random.nextFloat() * 15)];
    }
    return color;
}

Answer 1

Working with floats and using round is not a safe way of creating such random colors.

Actually a color code is an integer in hexadecimal formatting. You can easily create such numbers like this:

import java.util.Random;

public class R {

    public static void main(String[] args) {

        // create random object - reuse this as often as possible
        Random random = new Random();

        // create a big random number - maximum is ffffff (hex) = 16777215 (dez)
        int nextInt = random.nextInt(0xffffff + 1);

        // format it as hexadecimal string (with hashtag and leading zeros)
        String colorCode = String.format("#%06x", nextInt);

        // print it
        System.out.println(colorCode);
    }

}

Answer 2

Your split will generate an array of length 17 with an empty string at the beginning. Your generator occasionally draws that zeroth element which will not contribute to the length of the final string. (As a side effect, F will never be drawn.)

1. Accept that split has that odd behaviour and work with it: Ditch that nasty formula that uses round. Use 1 + random.nextInt(16) instead as your index.

2. Don't recreate the generator on each call of getRandomColor: that ruins the generator's statistical properties. Pass random as a parameter to getRandomColor.