Index, added simple code for connection.

.gitignore

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+target/
+!.mvn/wrapper/maven-wrapper.jar
+!**/src/main/**/target/
+!**/src/test/**/target/
+.kotlin
+
+### IntelliJ IDEA ###
+.idea/modules.xml
+.idea/jarRepositories.xml
+.idea/compiler.xml
+.idea/libraries/
+*.iws
+*.iml
+*.ipr
+
+### Eclipse ###
+.apt_generated
+.classpath
+.factorypath
+.project
+.settings
+.springBeans
+.sts4-cache
+
+### NetBeans ###
+/nbproject/private/
+/nbbuild/
+/dist/
+/nbdist/
+/.nb-gradle/
+build/
+!**/src/main/**/build/
+!**/src/test/**/build/
+
+### VS Code ###
+.vscode/
+
+### Mac OS ###
+.DS_Store

.idea/.gitignore

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+# Default ignored files
+/shelf/
+/workspace.xml
+# Ignored default folder with query files
+/queries/
+# Datasource local storage ignored files
+/dataSources/
+/dataSources.local.xml
+# Editor-based HTTP Client requests
+/httpRequests/

.idea/encodings.xml

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+<?xml version="1.0" encoding="UTF-8"?>
+<project version="4">
+  <component name="Encoding">
+    <file url="file://$PROJECT_DIR$/src/main/java" charset="UTF-8" />
+    <file url="file://$PROJECT_DIR$/src/main/resources" charset="UTF-8" />
+  </component>
+</project>

.idea/misc.xml

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+<?xml version="1.0" encoding="UTF-8"?>
+<project version="4">
+  <component name="ExternalStorageConfigurationManager" enabled="true" />
+  <component name="MavenProjectsManager">
+    <option name="originalFiles">
+      <list>
+        <option value="$PROJECT_DIR$/pom.xml" />
+      </list>
+    </option>
+  </component>
+  <component name="ProjectRootManager" version="2" languageLevel="JDK_21" default="true" project-jdk-name="ms-21" project-jdk-type="JavaSDK">
+    <output url="file://$PROJECT_DIR$/out" />
+  </component>
+</project>

.idea/vcs.xml

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+<?xml version="1.0" encoding="UTF-8"?>
+<project version="4">
+  <component name="VcsDirectoryMappings">
+    <mapping directory="$PROJECT_DIR$" vcs="Git" />
+  </component>
+</project>

pom.xml

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+<?xml version="1.0" encoding="UTF-8"?>
+<project xmlns="http://maven.apache.org/POM/4.0.0"
+         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
+    <modelVersion>4.0.0</modelVersion>
+
+    <groupId>pl.mrusek</groupId>
+    <artifactId>SimpleChat</artifactId>
+    <version>1.0-SNAPSHOT</version>
+
+    <properties>
+        <maven.compiler.source>21</maven.compiler.source>
+        <maven.compiler.target>21</maven.compiler.target>
+        <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
+    </properties>
+
+</project>

src/main/java/ChatServer.java

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+import java.io.BufferedReader;
+import java.io.IOException;
+import java.io.InputStreamReader;
+import java.io.PrintWriter;
+import java.net.ServerSocket;
+import java.net.Socket;
+import java.util.ArrayList;
+
+
+public class ChatServer implements Runnable {
+    private int port = 9000;
+    private ArrayList<Socket> clientSockets = new ArrayList<Socket>();
+
+    public ChatServer() {
+    }
+
+    public ChatServer(int port) {
+        this.port = port;
+    }
+
+    private User getOrCreateUser(String username) {
+        return new User(username);
+    }
+
+    private void SendMessage(User user, String msg) {
+        try {
+            for (Socket client : clientSockets) {
+                PrintWriter out = new PrintWriter(client.getOutputStream(), true);
+
+                out.println(user.getUsername() + " " + msg);
+            }
+        } catch (IOException e) {
+            e.printStackTrace();
+        }
+    }
+
+    private void handleClient(Socket socket) {
+        try {
+            clientSockets.add(socket);
+
+            String msg;
+            User user = null;
+
+            PrintWriter out = new PrintWriter(socket.getOutputStream(), true);
+            BufferedReader in = new BufferedReader(new InputStreamReader(socket.getInputStream()));
+
+            out.println("Login: /login username");
+            while ((msg = in.readLine()) != null) {
+                if (user != null) {
+                    SendMessage(user, msg);
+                }
+
+                if (user == null) {
+                    if (msg.toLowerCase().startsWith("/login")) {
+                        String username = msg.substring(6).strip();
+                        user = getOrCreateUser(username);
+                    }
+                }
+            }
+
+        } catch (IOException e) {
+            clientSockets.remove(socket);
+            e.printStackTrace();
+        }
+    }
+
+
+    @Override
+    public void run() {
+        try {
+            ServerSocket serverSocket = new ServerSocket(port);
+
+            while (true) {
+                Socket clientSocket = serverSocket.accept();
+
+                Thread thread = new Thread(() -> handleClient(clientSocket));
+                thread.start();
+            }
+
+        } catch (IOException e) {
+            throw new RuntimeException(e);
+        }
+    }
+}

src/main/java/Client.java

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+import java.io.BufferedReader;
+import java.io.IOException;
+import java.io.InputStreamReader;
+import java.io.PrintWriter;
+import java.net.Socket;
+import java.util.Scanner;
+
+public class Client {
+    private Socket clientSocket;
+    private PrintWriter out;
+    private BufferedReader in;
+
+    public void startConnection(String ip, int port) throws IOException {
+        clientSocket = new Socket(ip, port);
+        out = new PrintWriter(clientSocket.getOutputStream(), true);
+        in = new BufferedReader(new InputStreamReader(clientSocket.getInputStream()));
+    }
+
+    public void sendMessage(String message) throws IOException {
+        out.println(message);
+    }
+
+    public String loadMessage() throws IOException {
+        String resp = in.readLine();
+        return resp;
+    }
+
+    public void stopConnection() throws IOException {
+        in.close();
+        out.close();
+        clientSocket.close();
+    }
+
+    public static void main(String[] args) throws IOException {
+        Client client = new Client();
+        client.startConnection("127.0.0.1", 9000);
+
+        String msg;
+        Scanner scanner = new Scanner(System.in);
+
+        Thread thread = new Thread(() -> {
+            try {
+                String resp;
+
+                while (true) {
+                    resp = client.loadMessage();
+                    System.out.println(resp);
+                }
+            } catch (IOException e) {
+                throw new RuntimeException(e);
+            }
+        });
+        thread.start();
+
+        while (true) {
+            msg = scanner.nextLine();
+            client.sendMessage(msg);
+        }
+
+//        client.stopConnection();
+    }
+}

src/main/java/Main.java

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+public class Main {
+    public static void main(String[] args) {
+        ChatServer chatServer = new ChatServer(9000);
+
+
+        Thread serverThread = new Thread(chatServer);
+        serverThread.start();
+
+    }
+}

src/main/java/User.java

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+public class User {
+
+    private String username;
+
+    public User(String username) {
+        this.username = username;
+    }
+
+    public String getUsername() {
+        return username;
+    }
+
+    public void setUsername(String username) {
+        this.username = username;
+    }
+}