换行问题

​package com.servlet;

import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;
import java.io.PrintWriter;

public class SampleServlet extends HttpServlet {
    @Override
    protected void service(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        req.setCharacterEncoding("utf-8");
        resp.setCharacterEncoding("utf-8");
        resp.setContentType("text/html;charset=utf-8");

        PrintWriter writer=resp.getWriter();
        writer.println("Addition Calculator：");
        writer.println();
        String num1 = req.getParameter("num1");
        String num2 = req.getParameter("num2");
        int number1=Integer.valueOf(num1);
        int number2=Integer.valueOf(num2);
        int sum=number1+number2;
        writer.println("Calculation results:"+sum);



    }
}

为什么打了println 但是不换行