Issue
HomeController.java
package com.project.controller;
import java.io.IOException;
import java.util.ArrayList;
import java.util.List;
import com.project.entity.User;
import com.project.model.ListUsers;
import javax.annotation.Resource;
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.sql.DataSource;
@WebServlet("/home")
public class HomeController extends HttpServlet {
private static final long serialVersionUID = 1L;
@Resource(name="jdbc/MyDB")
private DataSource dataa;
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
String page = request.getParameter("page");
page.toLowerCase();
switch (page) {
case "home": {
List<User> listUserss = new ArrayList<>() ;
listUserss= new ListUsers().listUsers(dataa);
request.setAttribute("listUserss", listUserss);
request.getRequestDispatcher("index.jsp").forward(request, response);
}
case "listUsers":{
request.getRequestDispatcher("listUsers.jsp").forward(request, response);
}
default:
request.getRequestDispatcher("error.jsp").forward(request, response);
}
}
}
ListUsers.java
package com.project.model;
import java.sql.Connection;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.util.ArrayList;
import java.util.List;
import javax.sql.DataSource;
import com.project.entity.User;
public class ListUsers {
public List<User> listUsers(DataSource dataa){
List<User> listUsers = new ArrayList<>();
//==== build SQL database ====
ResultSet re = null;
Connection connect = null;
java.sql.Statement st= null;
try {
// =====is used to establish a connection with the database.
connect =dataa.getConnection();
// Create a SQL statement string
String query = "Select * FROM newpro.users;";
st = connect.createStatement();
// Execute SQL query
re = st.executeQuery(query);
// print the result
while(re.next()) {
listUsers.add( new User(re.getString("email"),re.getString("username"),re.getInt("users_id")));
}
} catch (SQLException e) {
e.printStackTrace();
}
return listUsers;
}
}
User.java
package com.project.entity;
public class User {
private String email;
private String username;
private int users_id;
public User(String email,String username,int users_id) {
this.email = email;
this.username =username;
this.users_id= users_id;
}
public String getEmail() {
return email;
}
public void setEmail(String email) {
this.email = email;
}
public String getUsername() {
return username;
}
public void setUsername(String username) {
this.username = username;
}
public int getUsers_id() {
return users_id;
}
public void setUsers_id(int users_id) {
this.users_id = users_id;
}
}
listUsers.jsp
<%@page import="java.util.ArrayList"%>
<%@ page language="java" contentType="text/html; charset=ISO-8859-1"
pageEncoding="ISO-8859-1"%>
<%@ page import="java.util.List" %>
<%@ page import="com.project.entity.User" %>
<!DOCTYPE html>
<html>
<head>
<meta charset="ISO-8859-1">
<title>List of Users</title>
<link href="https://cdn.jsdelivr.net/npm/[email protected]/dist/css/bootstrap.min.css" rel="stylesheet" >
</head>
<body>
<header class="p-3 bg-dark text-white">
<div class="container">
<div class="d-flex flex-wrap align-items-center justify-content-center justify-content-lg-start">
<a href="<%=request.getContextPath()%>/home?page=home" class="d-flex align-items-center mb-2 mb-lg-0 text-white text-decoration-none">
<svg class="bi me-2" width="40" height="32" role="img" aria-label="Bootstrap"></svg>
</a>
<ul class="nav col-12 col-lg-auto me-lg-auto mb-2 justify-content-center mb-md-0">
<li class="nav-item"><a href="<%=request.getContextPath()%>/home?page=home " class="nav-link px-2 text-muted">Home</a></li>
<li class="nav-item"><a href="<%=request.getContextPath()%>/home?page=listUsers" class="nav-link px-2 text-muted">About</a></li>
</ul>
<form class="col-12 col-lg-auto mb-3 mb-lg-0 me-lg-3">
<input type="search" class="form-control form-control-dark" placeholder="Search..." aria-label="Search">
</form>
<div class="text-end">
<button type="button" class="btn btn-outline-light me-2">Login</button>
<button type="button" class="btn btn-warning">Sign-up</button>
</div>
</div>
</div>
</header>
<h1>List of Users</h1>
<hr/>
<table border="1">
<thead>
<th>username</th>
<th>email</th>
<th>user_id</th>
</thead>
<%
/*################################## This is the Error >>>>>>>>> ###################*/
List<User> listu = request.getAttribute("listUserss");
%>
</table>
<footer class="d-flex flex-wrap justify-content-between align-items-center py-3 my-4 border-top">
<p class="col-md-4 mb-0 text-muted">© 2022 Company, Inc</p>
<a href="/" class="col-md-4 d-flex align-items-center justify-content-center mb-3 mb-md-0 me-md-auto link-dark text-decoration-none">
<svg class="bi me-2" width="40" height="32"><use xlink:href="#bootstrap"></use></svg>
</a>
<ul class="nav col-md-4 justify-content-end">
<li class="nav-item"><a href="<%=request.getContextPath()%>/home?page=home " class="nav-link px-2 text-muted">Home</a></li>
<li class="nav-item"><a href="<%=request.getContextPath()%>/home?page=listUsers" class="nav-link px-2 text-muted">About</a></li>
</ul>
</footer>
<script src="https://cdn.jsdelivr.net/npm/[email protected]/dist/js/bootstrap.bundle.min.js" integrity="sha384-u1OknCvxWvY5kfmNBILK2hRnQC3Pr17a+RTT6rIHI7NnikvbZlHgTPOOmMi466C8" crossorigin="anonymous"></script>
</body>
</html>
the error in listUsers.jsp says: Type mismatch: cannot convert from Object to List<User>
Solution
Please cast Object to List<User>. Request's getAttribute method returns the object and it will not know in which class to convert. So you have to explicitly cast the object. Make sure request.getAttribute("listUserss") is not null else it will throw NullPointerException.
<%
List<User> listu = (List<User>) request.getAttribute("listUserss");
%>
Answered By - Dhaval Gajjar
Answer Checked By - Robin (JavaFixing Admin)
