CREATE TABLE IF NOT EXISTS user(
    user_id VARCHAR(255),
    user_password VARCHAR(255) NOT NULL,
    user_last_name VARCHAR(255),
    user_first_name VARCHAR(255),
    user_email VARCHAR(255) UNIQUE NOT NULL,
    user_type TINYINT UNSIGNED NOT NULL, /* VALUES: 0 - Guest, 1 - Admin, 2 - User */
    PRIMARY KEY(user_id)
);

protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    String userId = request.getParameter("username");
    String userFirstName = request.getParameter("firstname");
    String userLastName = request.getParameter("lastname");
    String userEmail1 = request.getParameter("email1");
    String userEmail2 = request.getParameter("email2");
    String userPassword1 = request.getParameter("pass1");
    String userPassword2 = request.getParameter("pass2");
    String captchaAnswer = request.getParameter("answer");

    try {
        // simple captcha
        HttpSession session = request.getSession(true);
        Captcha captcha = (Captcha) session.getAttribute(Captcha.NAME);
        request.setCharacterEncoding("UTF-8");

        boolean isCaptchaCorrect = captcha.isCorrect(captchaAnswer);
        session.setAttribute("isCaptchaCorrect", isCaptchaCorrect);

        session.setAttribute("userId", userId);
        session.setAttribute("userFirstName", userFirstName);
        session.setAttribute("userLastName", userLastName);
        session.setAttribute("userEmail1", userEmail1);
        session.setAttribute("userEmail2", userEmail2);

        if(isCaptchaCorrect) {
            // put database entries into a String[]
            DatabaseManipulator dm = new DatabaseManipulator();
            String[] usernameArray = dm.dbEntriesToArray("user_id");
            String[] emailArray = dm.dbEntriesToArray("user_email");

            // validate inputs
            RegistrationModule rm = new RegistrationModule();
            boolean hasDuplicateUsername = rm.hasDuplicate(usernameArray, userId);
            boolean hasDuplicateEmail = rm.hasDuplicate(emailArray, userEmail1);
            boolean isEmailMatch = rm.isMatch(userEmail1, userEmail2);
            boolean isPasswordMatch = rm.isMatch(userPassword1, userPassword2);

            // bind objects to session
            session.setAttribute("hasDuplicateUsername", hasDuplicateUsername);
            session.setAttribute("hasDuplicateEmail", hasDuplicateEmail);
            session.setAttribute("isEmailMatch", isEmailMatch);
            session.setAttribute("isPasswordMatch", isPasswordMatch);

            // throw user-defined exceptions
            if(hasDuplicateUsername) {
                try {
                    throw new UsernameAlreadyExistsException();

                } catch(UsernameAlreadyExistsException uaee) {
                    // redirect to result page
                    response.sendRedirect("register-result.jsp");
                }

            } else if(hasDuplicateEmail) {
                try {
                    throw new EmailAlreadyExistsException();

                } catch(EmailAlreadyExistsException eaee) {
                    response.sendRedirect("register-result.jsp");
                }

            } else if(!isEmailMatch) {
                try {
                    throw new MismatchedEmailsException();

                } catch(MismatchedEmailsException mee) {
                    response.sendRedirect("register-result.jsp");
                }

            } else if(!isPasswordMatch) {
                try {
                    throw new MismatchedPasswordsException();

                } catch(MismatchedPasswordsException mpe) {
                    response.sendRedirect("register-result.jsp");
                }

            // register success
            } else {
                // assign if match
                String userPassword = userPassword1;
                String userEmail = userEmail1;

                // assemble user bean object
                User user = UserAssembler.getInstance(
                    userId,
                    userPassword,
                    userLastName,
                    userFirstName,
                    userEmail,
                    2 // 2 = User
                );

                // insert user into database
                dm.registerUser(user);

                response.sendRedirect("register-result.jsp");
            }

        // wrong captcha answer
        } else {
            response.sendRedirect("register-result.jsp");
        }

    } catch(NullPointerException npe) {
        // redirect when servlet is illegally accessed
        response.sendRedirect("index.jsp");
    }
}

try {
     throw new SomeException();
} catch (SomeException uaee) {
     response.sendRedirect("some-result.jsp");
}

response.sendRedirect("some-result.jsp");

if(hasDuplicateUsername) {
    response.sendRedirect("register-result.jsp");
} else if(hasDuplicateEmail) {
    response.sendRedirect("register-result.jsp");
} else if(!isEmailMatch) {
    response.sendRedirect("register-result.jsp");
} else if(!isPasswordMatch) {
    response.sendRedirect("register-result.jsp");
}

String userId = request.getParameter("username");
String userFirstName = request.getParameter("firstname");
String userLastName = request.getParameter("lastname");
String userEmail1 = request.getParameter("email1");
String userEmail2 = request.getParameter("email2");
String userPassword1 = request.getParameter("pass1");
String userPassword2 = request.getParameter("pass2");
String captchaAnswer = request.getParameter("answer");

if (userId == null || userFirstName == null || userLastName == null ||
  userEmail1 == null || ...) {
    response.sendRedirect("index.jsp");
    return;
}

try {
    throw new UsernameAlreadyExistsException();
} catch(UsernameAlreadyExistsException uaee) {
    // redirect to result page
    response.sendRedirect("register-result.jsp");
}

try {
    throw new SQLException();
} catch(SQLException uaee) {
    response.sendRedirect("register-result.jsp");
}

// wrong captcha answer
} else {

user_type TINYINT UNSIGNED NOT NULL, /* VALUES: 0 - Guest, 1 - Admin, 2 - User */

2 // 2 = User

enum UserType {GUEST, ADMIN, USER}

public boolean areElementsNull(String[] requiredUserDetails) {
    for(String detail : requiredUserDetails) {
        if(detail == null) {
            return true;
        }
    }

    return false;
}

String[] requiredUserDetails = {
    userId,
    userEmail1,
    userEmail2,
    userPassword1,
    userPassword2,
    captchaAnswer,
};

RegistrationModule rm = new RegistrationModule();
boolean areElementsNull = rm.areElementsNull(requiredUserDetails);

if(!areElementsNull) {
    // code here

} else {
    response.sendRedirect("index.jsp");
}