if elseをうまく作れないです。 どうぞよろしくお願いします。

doPostメソッドで、新規登録で入力した値がnullでないか?
またはその値が、すでにデータベースにないか?
そのどちらでもない場合に、新規登録を実行して、データベースに登録したいです。
どのように書けば、if else がしっかり組み立てられるのでしょうか？

package servlet;

import java.io.IOException;

import java.util.List;
import javax.servlet.RequestDispatcher;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;

import model.Mutter;
import model.User;
import model.SetLogic;
import model.LoginLogic;

/**
 * Servlet implementation class Set
 */
@WebServlet("/Set")
public class Set extends HttpServlet {
    private static final long serialVersionUID = 1L;

    /**
     * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
     */
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {


        RequestDispatcher dispatcher=request.getRequestDispatcher("/WEB-INF/jsp/set.jsp");
        dispatcher.forward(request, response);

    }

    /**
     * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
     */
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {

        request.setCharacterEncoding("UTF-8");
        String name=request.getParameter("name");
        String pass=request.getParameter("pass");


        User user=new User(name,pass);

        if(name.length()==0 || pass.length()==0){
            request.setAttribute("newerrorMsg", "未入力があります。");

            RequestDispatcher dispatcher=request.getRequestDispatcher("/WEB-INF/jsp/set.jsp");
            dispatcher.forward(request, response);

        }

        //LoginLogicを使ってmutterListを取得
        LoginLogic loginLogic=new LoginLogic();

        List<Mutter>mutterList=loginLogic.execute(user);


        if(mutterList.size()!=0){
                request.setAttribute("setErrorMsg","すでに登録されています。");

                RequestDispatcher dispatcher=request.getRequestDispatcher("/WEB-INF/jsp/set.jsp");
                dispatcher.forward(request, response);
            }
            //mutter1とする
        for(Mutter mutter1:mutterList){
         if(mutter1.getUserName().length()!=0||mutter1.getUserPass().length()!=0){
                 request.setAttribute("setErrorMsg", "すでに登録されています。");

                RequestDispatcher dispatcher=request.getRequestDispatcher("/WEB-INF/jsp/set.jsp");
                dispatcher.forward(request, response);

         }else{
                HttpSession session=request.getSession();
                session.setAttribute("loginUser",user);

         User loginUser=(User)session.getAttribute("loginUser");

         Mutter mutter=new Mutter(loginUser.getName(),loginUser.getPass());
         SetLogic setLogic=new SetLogic();

            try{
                    setLogic.execute(mutter);
            }catch(ClassNotFoundException e){
                    e.printStackTrace();
                }

          RequestDispatcher dispatcher=request.getRequestDispatcher("/WEB-INF/jsp/setResult.jsp");
          dispatcher.forward(request, response);
                }
        }
    }
}