Servlet simple calculator 2.0, servlet calculator 2.0

Servlet simple calculator 2.0, servlet calculator 2.0

Jsp input interface:

1 <% @ page language = "java" contentType = "text/html; charset = UTF-8" 2 pageEncoding = "UTF-8" %> 3 <! DOCTYPE html PUBLIC "-// W3C // dtd html 4.01 Transitional // EN" http://www.w3.org/TR/html4/loose.dtd "> 4 
Servlet background processing:
 1 package com.sp.web; 2  3 import java.io.IOException; 4 import javax.servlet.ServletException; 5 import javax.servlet.http.HttpServlet; 6 import javax.servlet.http.HttpServletRequest; 7 import javax.servlet.http.HttpServletResponse; 8 import javax.servlet.http.HttpSession; 9 10 public class Test_JSQ extends HttpServlet {11     private static final long serialVersionUID = 1L;12 13     public Test_JSQ() {14         super();15 16     }17 18     protected void doGet(HttpServletRequest request, HttpServletResponse response)19             throws ServletException, IOException {20 21         response.setCharacterEncoding("UTF-8");22         response.setContentType("text/html");23         String st = request.getParameter("text").trim();24 25         int a = 0;26         if (st.contains("+")) {27             a = st.indexOf("+");28         } else if (st.contains("-")) {29             a = st.indexOf("-");30         } else if (st.contains("x")) {31             a = st.indexOf("x");32         } else if (st.contains("/")) {33             a = st.indexOf("/");34         } else if (st.contains("%")) {35             a = st.indexOf("%");36         }37         Double n1 = Double.parseDouble(st.substring(0, a));38         Double n2 = Double.parseDouble(st.substring(a + 1, st.length() - 1));39         String s = st.charAt(a) + "";40 41         String rs = "";42         if (s.equals("+")) {43             rs = n1 + n2 + "";44             45         } else if (s.equals("-")) {46             rs = n1 - n2 + "";47             48         } else if (s.equals("x")) {49             rs = n1 * n2 + "";50             51         } else if (s.equals("/")) {52             if (n2 != 0.0) {53                 rs = n1 / n2 +"";54             55             } else {56                 rs = "error by 0";57             }58         } else if (s.equals("%")) {59             rs = n1 % n2 + "";60             61         }62         request.getSession().setAttribute("jg", rs);63         response.sendRedirect("jisuanqi.jsp");64         ;65     }66 67     protected void doPost(HttpServletRequest request, HttpServletResponse response)68             throws ServletException, IOException {69 70         doGet(request, response);71     }72 73 }
Interface display: When the divisor is 0:

The key issue is that when you click each button, the content you want to enter is displayed in the text field. When you click "Submit", the entered content is automatically moved forward.
I still don't know why. There is no problem when I calculate the addition, subtraction, and Multiplication Modulo. However, when I perform Division operations, I always report a data format error, so after changing "handler" to "/", it will be inexplicably solved.