Answer by Jamie for How to generate a random alpha-numeric string?

Lots of use of StringBuilder above. I guess it's easy, but requires a function call per char, growing an array, etc... If using the stringbuilder, a suggestion is to specify the required capacity of the string ie.,

new StringBuilder(int capacity);

Here's a version that doesn't use a StringBuilder or String appending, and no dictionary.

public static String randomString(int length)
{
    SecureRandom random = new SecureRandom();
    char[] chars = new char[length];
    for(int i=0;i<chars.length;i++)
    {
        int v = random.nextInt(10 + 26 + 26);
        char c;
        if (v < 10)
        {
            c = (char)('0' + v);
        }
        else if (v < 36)
        {
            c = (char)('a' - 10 + v);
        }
        else
        {
            c = (char)('A' - 36 + v);
        }
        chars[i] = c;
    }
    return new String(chars);
}