How to generate a secure random alphanumeric string in Java efficiently?

Question

How do you generate a secure random (or pseudo-random) alphanumeric string in Java efficiently?

Answer 1

Initialize an array containing all the accepted chars (CHARS_ARRAY), then instantiate a SecureRandom instance, and call nextInt(CHARS_ARRAY.length) repeatedly to get a random index in your char array. Append each char to a StringBuilder until you get the expected number of chars.