public static String randomString(final int len) {
        final StringBuilder b = new StringBuilder();
        for (int i = 0; i < len; i++)
            b.append(LETTERS_AND_DIGITS.charAt(SEEDED_RANDOM.nextInt(LETTERS_AND_DIGITS.length())));
        return b.toString();
    }
Generate a random string of letters and digits of the given length

@param len The length of the string
@return The random string
clients/src/test/java/org/apache/kafka/test/TestUtils.java