petehouston/Calculator.java

## Calculator.java
package com.java;

import java.io.IOException;
import java.io.PrintWriter;

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;

public class Calculator extends HttpServlet {
    private static final long serialVersionUID = 1L;

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        String num1 = request.getParameter("num1");
        String num2 = request.getParameter("num2");
        String op = request.getParameter("op");
        String result = "";

        int a, b;
        try {
            a = Integer.parseInt(num1);
            b = Integer.parseInt(num2);
            if (op.equals("+")) {
                result = String.valueOf(a + b);
            } else if (op.equals("-")) {
                result = String.valueOf(a - b);
            } else if (op.equals("*")) {
                result = String.valueOf(a * b);
            } else if (op.equals("/")) {
                if (b == 0) {
                    result = "";
                } else {
                    result = String.valueOf(a / b);
                }
            } else {
                op = "";
                result = "";
            }
        } catch (NumberFormatException ex) {
            op = "";
            num1 = "";
            num2 = "";
            result = "";
        }

        PrintWriter out = response.getWriter();
        out.write(
        "<html>" +
            "<body>" +
            "<h1>Simple Calculator</h1>" +
            "<form action='/calculator' method='get'>" +
                "<input type='text' name='num1' value='" + num1 + "' />" +
                "<input type='text' name='op' value='" + op + "' />" +
                "<input type='text' name='num2' value='" + num2 + "' />" +
                "=" +
                "<input type='text' name='result' value='" + result + "' />" +
                "<input type='submit' value='Submit' />" +
            "</form>" +
            "</body>" +
        "</html>"
        );
    }

}

## web.xml
<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns="http://xmlns.jcp.org/xml/ns/javaee"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee http://xmlns.jcp.org/xml/ns/javaee/web-app_4_0.xsd"
         version="4.0">
    <display-name>Calculator</display-name>
    <welcome-file-list>
        <welcome-file>index.html</welcome-file>
    </welcome-file-list>
    <servlet>
        <servlet-name>calculator</servlet-name>
        <servlet-class>com.java.Calculator</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>calculator</servlet-name>
        <url-pattern>/calculator</url-pattern>
    </servlet-mapping>
</web-app>
	package com.java;

	import java.io.IOException;
	import java.io.PrintWriter;

	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;

	public class Calculator extends HttpServlet {
	private static final long serialVersionUID = 1L;

	protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
	String num1 = request.getParameter("num1");
	String num2 = request.getParameter("num2");
	String op = request.getParameter("op");
	String result = "";

	int a, b;
	try {
	a = Integer.parseInt(num1);
	b = Integer.parseInt(num2);
	if (op.equals("+")) {
	result = String.valueOf(a + b);
	} else if (op.equals("-")) {
	result = String.valueOf(a - b);
	} else if (op.equals("*")) {
	result = String.valueOf(a * b);
	} else if (op.equals("/")) {
	if (b == 0) {
	result = "";
	} else {
	result = String.valueOf(a / b);
	}
	} else {
	op = "";
	result = "";
	}
	} catch (NumberFormatException ex) {
	op = "";
	num1 = "";
	num2 = "";
	result = "";
	}

	PrintWriter out = response.getWriter();
	out.write(
	"<html>" +
	"<body>" +
	"<h1>Simple Calculator</h1>" +
	"<form action='/calculator' method='get'>" +
	"<input type='text' name='num1' value='" + num1 + "' />" +
	"<input type='text' name='op' value='" + op + "' />" +
	"<input type='text' name='num2' value='" + num2 + "' />" +
	"=" +
	"<input type='text' name='result' value='" + result + "' />" +
	"<input type='submit' value='Submit' />" +
	"</form>" +
	"</body>" +
	"</html>"
	);
	}

	}
	<?xml version="1.0" encoding="UTF-8"?>
	<web-app xmlns="http://xmlns.jcp.org/xml/ns/javaee"
	xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
	xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee http://xmlns.jcp.org/xml/ns/javaee/web-app_4_0.xsd"
	version="4.0">
	<display-name>Calculator</display-name>
	<welcome-file-list>
	<welcome-file>index.html</welcome-file>
	</welcome-file-list>
	<servlet>
	<servlet-name>calculator</servlet-name>
	<servlet-class>com.java.Calculator</servlet-class>
	</servlet>
	<servlet-mapping>
	<servlet-name>calculator</servlet-name>
	<url-pattern>/calculator</url-pattern>
	</servlet-mapping>
	</web-app>