Math Captcha for Java Portlets

Generate a MathCaptcha

private static final String[] NUMBERS = {"zero","one","two","three","four","five","six","seven","eight","nine","ten"};
private static final String[] OPERATIONS = {"+", "-", "×"};
private static final Random RAND = new Random(); // not high entropy, but not a big deal here

private Map<String,String> generateCaptcha(PortletSession session) {
    Map<String,String> captcha = new HashMap<String, String>();
    
    int first, second, third, op, pos, solution;
    
    op = RAND.nextInt(OPERATIONS.length);
    captcha.put("operation", OPERATIONS[op]);
    switch (op) {
    case 2: // multiplication; limit generated args to [1-5]
        first = RAND.nextInt(5) + 1;
        second = RAND.nextInt(5) + 1;
        third = first * second;
        break;
    case 1: // subtraction
        first = RAND.nextInt(NUMBERS.length);
        second = RAND.nextInt(NUMBERS.length);
        if (first < second) {
            // reorder to ensure positive solution
            int tmp = first;
            first = second;
            second = tmp;
        }
        third = first - second;
        break;
    default: // addition
        first = RAND.nextInt(NUMBERS.length);
        second = RAND.nextInt(NUMBERS.length);
        third = first + second;
    }
    
    String rFirst = NUMBERS[first], rSecond = NUMBERS[second], rThird;
    if (third < NUMBERS.length) {
        rThird = NUMBERS[third];
    } else {
        rThird = Integer.toString(third);
    }
    
    pos = RAND.nextInt(3);
    switch (pos) {
    case 0:
        solution = first;
        captcha.put("second", rSecond);
        captcha.put("third", rThird);
        break;
    case 1:
        captcha.put("first", rFirst);
        solution = second;
        captcha.put("third", rThird);
        break;
    default:
        captcha.put("first", rFirst);
        captcha.put("second", rSecond);
        solution = third;
        break;
    }
    session.setAttribute("captcha_solution", Integer.toString(solution));
    
    return captcha;
}