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

Efficent and short.

/**
 * Utility class for generating random Strings.
 */
public interface RandomUtil {

    int    DEF_COUNT = 20;
    Random RANDOM    = new SecureRandom();

    /**
     * Generate a password.
     *
     * @return the generated password
     */
    static String generatePassword() {
        return generate(true, true);
    }

    /**
     * Generate an activation key.
     *
     * @return the generated activation key
     */
    static String generateActivationKey() {
        return generate(false, true);
    }

    /**
     * Generate a reset key.
     *
     * @return the generated reset key
     */
    static String generateResetKey() {
        return generate(false, true);
    }

    static String generate(boolean letters, boolean numbers) {
        int
            start = ' ',
            end   = 'z' + 1,
            count = DEF_COUNT,
            gap   = end - start;
        StringBuilder builder = new StringBuilder(count);
        while (count-- != 0) {
            int codePoint = RANDOM.nextInt(gap) + start;
            switch (getType(codePoint)) {
                case UNASSIGNED:
                case PRIVATE_USE:
                case SURROGATE:
                    count++;
                    continue;
            }
            int numberOfChars = charCount(codePoint);
            if (count == 0 && numberOfChars > 1) { count++; continue; }
            if (letters && isLetter(codePoint)
                || numbers && isDigit(codePoint)
                || !letters && !numbers) {
                builder.appendCodePoint(codePoint);
                if (numberOfChars == 2) count--;
            } else count++;
        }
        return builder.toString();
    }

}