TCPServer

package com.company;

import java.io.DataInputStream;
import java.io.DataOutputStream;
import java.io.IOException;
import java.net.ServerSocket;
import java.net.Socket;
import java.util.HashMap;

public class TCPServer {

    private ServerSocket server;
    private HashMap<Integer, Socket> activeConnections;

    // todo: Get the required connection
    public Socket getConnection(int id) {
        return activeConnections.getOrDefault(id,null);
    }

    // todo: Add connected client to the hash map
    void addConnection(int id, Socket connection) {
        activeConnections.putIfAbsent(id,connection);
    }

    synchronized void endConnection(int id){
        activeConnections.remove(id);
    }

    //todo: Initialize server
    TCPServer(int port) throws IOException {
        server=new ServerSocket(port);
        activeConnections=new HashMap<>();
    }

    // todo: Handle server listening
    // todo: For each connection, start a separate
    // todo: thread (ServerWorkerThread) to handle the communication
    void listen() throws Exception {
        while(true)
        {
            Socket client=server.accept();
            new ServerWorkerThread(client,this).start();
        }
    }

    public static void main(String[] args) throws Exception {
        TCPServer server=new TCPServer(9876);
        server.listen();
    }
}
class ServerWorkerThread extends Thread {
    private TCPServer server;
    private Socket socket;
    private int id;
    private DataInputStream dis;
    private DataOutputStream dos;

    public ServerWorkerThread(Socket socket, TCPServer server) throws Exception {
        this.server = server;
        this.socket = socket;
        dis = new DataInputStream(socket.getInputStream());
        dos = new DataOutputStream(socket.getOutputStream());
        id = dis.readInt();
        server.addConnection(id,socket);
    }
    @Override
    public void run()
    {
        while (true)
        {
            try {
                 String line= dis.readUTF();
                 if(line.equals("END"))
                 {
                     server.endConnection(id);
                     dos=new DataOutputStream(socket.getOutputStream());
                     dos.writeUTF("END");
                     dos.flush();
                     break;
                 }
                 String lines[]=line.split(":");
                 Socket connection=server.getConnection(Integer.parseInt(lines[1]));
                 dos=new DataOutputStream(connection.getOutputStream());
                 dos.writeUTF(lines[0]);
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
    }
}