Programming lesson
Building a Chat App and Form Validation: A Hands-On Web Development Tutorial (2026)
Learn to build a multithreaded chat application and client-side form validation using Java and JavaScript, with practical examples inspired by modern tech trends.
Introduction
Welcome to this comprehensive tutorial covering two foundational web development skills: creating a multithreaded chat application in Java and implementing client-side form validation with JavaScript. These tasks are inspired by common assignments like "COM S/SE 319" and are essential for building interactive, user-friendly web applications. By the end, you'll understand server-client architecture, thread management, and robust input validation—skills used in everything from real-time gaming apps to AI chatbots.
We'll use timely examples: imagine you're building a simple chat feature for a trending AI assistant app, or validating user input for a fantasy sports league registration form. Let's dive in.
Part 1: Building a Multithreaded Chat Application (Java)
1.1 Understanding the Server-Client Model
In a chat application, the server acts as a central hub that manages connections and broadcasts messages. Each client connects to the server via sockets. For this tutorial, we'll run everything on localhost. The server must handle multiple clients simultaneously—this is where multithreading comes in.
1.2 Server Implementation
Create a Server class that listens on a port (e.g., 1234). When a client connects, spawn a new thread to handle that client. Use a list to keep track of all connected client handlers so you can broadcast messages.
import java.io.*;
import java.net.*;
import java.util.*;
public class Server {
private static List<ClientHandler> clients = new ArrayList<>();
public static void main(String[] args) throws IOException {
ServerSocket serverSocket = new ServerSocket(1234);
System.out.println("Server started on port 1234");
while (true) {
Socket socket = serverSocket.accept();
ClientHandler clientHandler = new ClientHandler(socket);
clients.add(clientHandler);
new Thread(clientHandler).start();
}
}
public static void broadcast(String message, ClientHandler sender) {
for (ClientHandler client : clients) {
if (client != sender) {
client.sendMessage(message);
}
}
}
}The ClientHandler class implements Runnable and reads messages from the client, then calls Server.broadcast().
1.3 Client Implementation
The client should prompt for a username, then continuously read user input and send it to the server. Use a separate thread to listen for incoming messages from the server so the UI remains responsive.
import java.io.*;
import java.net.*;
public class Client {
public static void main(String[] args) throws IOException {
Socket socket = new Socket("localhost", 1234);
BufferedReader console = new BufferedReader(new InputStreamReader(System.in));
PrintWriter out = new PrintWriter(socket.getOutputStream(), true);
BufferedReader in = new BufferedReader(new InputStreamReader(socket.getInputStream()));
System.out.print("Enter your Name: ");
String name = console.readLine();
out.println(name);
// Thread to read server messages
new Thread(() -> {
try {
String serverMsg;
while ((serverMsg = in.readLine()) != null) {
System.out.println(serverMsg);
}
} catch (IOException e) { e.printStackTrace(); }
}).start();
// Main thread to send messages
String userInput;
while ((userInput = console.readLine()) != null) {
out.println(name + ": " + userInput);
}
}
}1.4 Testing and Final Touches
Run the server first, then multiple clients. Each client can send messages that appear on all other clients and the server console. This pattern is similar to how real-time collaboration tools like Google Docs or live-streaming chat work.
Part 2: Client-Side Form Validation with JavaScript
2.1 Why Validate on the Client?
Client-side validation improves user experience by providing instant feedback. For example, when signing up for a fantasy sports app or an AI image generator, you want to catch errors before submitting. We'll build two forms: a simple validation form and a contact information form.
2.2 Form 1: Validation Form
Create validation1.html with fields: First Name, Last Name, Gender (dropdown), State (dropdown). All fields are required. First and Last names allow only alphanumeric characters. Use JavaScript to check on button click.
<!DOCTYPE html>
<html>
<head>
<title>Validation Form</title>
<script src="validation1.js"></script>
</head>
<body>
<form id="form1">
First Name: <input type="text" id="fname"><br>
Last Name: <input type="text" id="lname"><br>
Gender: <select id="gender">
<option value="">Select</option>
<option value="male">Male</option>
<option value="female">Female</option>
</select><br>
State: <select id="state">
<option value="">Select</option>
<option value="California">California</option>
<option value="Florida">Florida</option>
</select><br>
<button type="button" onclick="validateForm1()">Continue</button>
</form>
<div id="results"></div>
</body>
</html>In validation1.js, write a function that checks each field and displays a green checkmark or red X. If all pass, redirect to validation2.html.
function validateForm1() {
let fname = document.getElementById('fname').value.trim();
let lname = document.getElementById('lname').value.trim();
let gender = document.getElementById('gender').value;
let state = document.getElementById('state').value;
let errors = [];
// Validate first name
if (fname === "" || !/^[a-zA-Z0-9]+$/.test(fname)) {
errors.push("First Name: ❌");
} else {
errors.push("First Name: ✅");
}
// Validate last name
if (lname === "" || !/^[a-zA-Z0-9]+$/.test(lname)) {
errors.push("Last Name: ❌");
} else {
errors.push("Last Name: ✅");
}
// Validate gender
if (gender === "") {
errors.push("Gender: ❌");
} else {
errors.push("Gender: ✅");
}
// Validate state
if (state === "") {
errors.push("State: ❌");
} else {
errors.push("State: ✅");
}
document.getElementById('results').innerHTML = errors.join('<br>');
// If all pass, go to next page
if (errors.every(e => e.includes('✅'))) {
window.location.href = 'validation2.html';
}
}2.3 Form 2: Contact Information
Create validation2.html with Email, Phone, Address fields. Use patterns: email must be [email protected], phone xxx-xxx-xxxx or xxxxxxxxxx, address must be "City,State" format.
<!DOCTYPE html>
<html>
<head>
<title>Contact information</title>
<script src="validation2.js"></script>
</head>
<body>
<form id="form2">
Email: <input type="text" id="email"><br>
Phone: <input type="text" id="phone"><br>
Address: <input type="text" id="address" placeholder="e.g., Ames,IA"><br>
<button type="button" onclick="validateForm2()">Submit</button>
</form>
<div id="results2"></div>
</body>
</html>In validation2.js, use regular expressions to validate each field.
function validateForm2() {
let email = document.getElementById('email').value.trim();
let phone = document.getElementById('phone').value.trim();
let address = document.getElementById('address').value.trim();
let errors = [];
// Email: [email protected]
if (!/^[a-zA-Z0-9]+@[a-zA-Z0-9]+\.[a-zA-Z0-9]+$/.test(email)) {
errors.push("Email: ❌");
} else {
errors.push("Email: ✅");
}
// Phone: xxx-xxx-xxxx or xxxxxxxxxx
if (!/^(\d{3}-\d{3}-\d{4}|\d{10})$/.test(phone)) {
errors.push("Phone: ❌");
} else {
errors.push("Phone: ✅");
}
// Address: city,state (e.g., Ames,IA)
if (!/^[a-zA-Z]+,[a-zA-Z]{2}$/.test(address)) {
errors.push("Address: ❌");
} else {
errors.push("Address: ✅");
}
document.getElementById('results2').innerHTML = errors.join('<br>');
}Conclusion
You've now built a multithreaded chat server-client in Java and client-side form validation in JavaScript. These skills are directly applicable to modern web development—whether you're creating a live sports chat, an AI tool signup, or a school project. Practice by extending the chat with private messaging or adding more validation rules.
Remember to test thoroughly and check your browser's developer console for errors. Happy coding!