Multi Client Simple Chat(non-GUI) Server in Java using threads

Question

I am unable to figure out how to stop the message from appearing twice on both the client's screen.

The Actual output should be something like this:

Steps for Running the code:
1. Run Server on one terminal
2. Run two clients on two different terminals

When I run the Server - main method creates a Server object:

public static void main(String[] args) throws IOException {
            Server server = new Server();
    }

Server Constructor:

Server() throws IOException {
            Date dNow = new Date();
            System.out.println("MultiThreadServer started at " + String.format("%tc", dNow));
            System.out.println();

            ServerSocket server = new ServerSocket(8000);
            ClientSockets = new Vector<Socket>();


            while (true) {
                    Socket client = server.accept();
                    AcceptClient acceptClient = new AcceptClient(client);
                    System.out.println("Connection from Socket " + "[addr = " + client.getLocalAddress() + ",port = "
                                    + client.getPort() + ",localport = " + client.getLocalPort() + "] at "
                                    + String.format("%tc", dNow));
                    System.out.println();
                    //System.out.println(clientCount);

            }
            //server.close();
    }

I am using Socket to connect to the server. Here is my Server code.

Server.java

import java.io.IOException;
import java.net.*;
import java.util.Vector;
import java.io.*;
import java.util.*;
import java.io.DataInputStream;
import java.net.ServerSocket;
import java.net.Socket;
import java.time.LocalDateTime;
import java.util.logging.Level;
import java.util.logging.Logger;
import java.text.*;
import java.util.Scanner;



public class Server {
    static Vector<Socket> ClientSockets;
    int clientCount = 0;
    //int i = 0;


    Server() throws IOException {
            Date dNow = new Date();
            System.out.println("MultiThreadServer started at " + String.format("%tc", dNow));
            System.out.println();

            ServerSocket server = new ServerSocket(8000);
            ClientSockets = new Vector<Socket>();


            while (true) {
                    Socket client = server.accept();
                    AcceptClient acceptClient = new AcceptClient(client);
                    System.out.println("Connection from Socket " + "[addr = " + client.getLocalAddress() + ",port = "
                                    + client.getPort() + ",localport = " + client.getLocalPort() + "] at "
                                    + String.format("%tc", dNow));
                    System.out.println();
                    //System.out.println(clientCount);

            }
            //server.close();
    }

    public static void main(String[] args) throws IOException {
            Server server = new Server();
    }

    class AcceptClient extends Thread {
            Socket ClientSocket;
            DataInputStream din;
            DataOutputStream dout;

            AcceptClient(Socket client) throws IOException {
                    ClientSocket = client;
                    din = new DataInputStream(ClientSocket.getInputStream());
                    dout = new DataOutputStream(ClientSocket.getOutputStream());

                    //String LoginName = din.readUTF();
                    //i = clientCount;
                    clientCount++;
                    ClientSockets.add(ClientSocket);
                    //System.out.println(ClientSockets.elementAt(i));
                    //System.out.println(ClientSockets.elementAt(1));

                    start();
            }

            public void run() {

                            try {
                                    while (true) {
                                    String msgFromClient = din.readUTF();
                                    System.out.println(msgFromClient);
                                    for (int i = 0; i < ClientSockets.size(); i++) {
                                            Socket pSocket = (Socket) ClientSockets.elementAt(i);
                                            DataOutputStream pOut = new DataOutputStream(pSocket.getOutputStream());
                                            pOut.writeUTF(msgFromClient);
                                            pOut.flush();
                                    }
                            }

                            } catch (IOException e) {
                                    e.printStackTrace();
                            }

            }
    }

}

Client.java

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


public class Client implements Runnable{

    Socket socketConnection;
    DataOutputStream outToServer; 
    DataInputStream din;


    Client() throws UnknownHostException, IOException {

            socketConnection = new Socket("127.0.0.1", 8000);
            outToServer = new DataOutputStream(socketConnection.getOutputStream());
            din = new DataInputStream(socketConnection.getInputStream());


            Thread thread;
            thread = new Thread(this);
            thread.start();

            BufferedReader br = null;
            String ClientName = null;
            Scanner input = new Scanner(System.in);
            String SQL = "";
            try {
                    System.out.print("Enter you name: ");
                    ClientName = input.next();
                    ClientName += ": ";
                    //QUERY PASSING

                    br = new BufferedReader(new InputStreamReader(System.in));
                    while (!SQL.equalsIgnoreCase("exit")) {
                            System.out.println();
                            System.out.print(ClientName);

                            SQL = br.readLine();
                            //SQL = input.next();
                            outToServer.writeUTF(ClientName + SQL);
                            //outToServer.flush();
                            //System.out.println(din.readUTF());

                    }

            } catch (Exception e) {
                    System.out.println(e);
            }
    }

    public static void main(String[] arg) throws UnknownHostException, IOException {
            Client client = new Client();
    }


    public void run() {
            while (true) {
                    try {
                            System.out.println("\n" + din.readUTF());

                    } catch (IOException e) {
                            e.printStackTrace();
                    }

            }
    }

}

Answer 1

The reason you have this is because you're sending the server whatever the client has written in the console, and the server sends it back to all of the clients (including the sender).

So you're writing a message in the console (and you see it) and then you're receiving it back as one of the clients (and you see it again).

A simple fix would be not to send the just received message back to the client (he already sees it in the console). Add this to the Server.AcceptClient#run method:

for (int i = 0; i < ClientSockets.size(); i++) {
    Socket pSocket = (Socket) ClientSockets.elementAt(i);

    if(ClientSocket.equals(pSocket)){
        continue;
    }
    ...