Simple chat server in Java.

Chat.java

import java.io.*;
import java.net.*;
import java.util.*;

public class Chat implements Runnable {
    private static ArrayList<Chat> users = new ArrayList<>();

    private Socket socket;
    private PrintStream writer;
    private String name;

    public Chat(Socket socket) {
        this.socket = socket;
    }

    public void run() {
        try {
            users.add(this);
            BufferedReader reader = new BufferedReader(new InputStreamReader(socket.getInputStream()));
            writer = new PrintStream(socket.getOutputStream(), false, "UTF-8");
            writer.println("Welcome to chat! What's your name?");
            String line = reader.readLine();
            if (line != null && line.length() > 0) {
                name = line;
                broadcast("user " + name + " joined.");
                while ((line = reader.readLine()) != null) {
                    broadcast(name + " : " + line);
                }
            }
            socket.close();
        } catch (IOException e) {
            e.printStackTrace();
        } finally {
            users.remove(this);
        }
    }

    public static void broadcast(String message) {
        for (Chat user : users) {
            if (user.name != null) {
                user.writer.println(message);
            }
        }
    }

    public static void main(String[] args) {
        try {
            ServerSocket listener = new ServerSocket(9000);
            while (true) {
                Socket socket = listener.accept();
                Chat chat = new Chat(socket);
                new Thread(chat).start();
            }
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}