Server Client Communication with multiple client support in Java using sockets

Client.java

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

public class Client {
    //declearing socket and input output streams
    private Socket socket = null;
    ObjectOutputStream dout;
    ObjectInputStream din;
    // constructor to put ip address and port
    public Client(String address, int port) throws UnknownHostException, IOException, ClassNotFoundException {
        // establish a connection
        socket = new Socket(address, port);
        System.out.println("Connected");
    }
    // initializing the input output streams
    public void initializeStreams() throws IOException{
        dout = new ObjectOutputStream(socket.getOutputStream());
        din = new ObjectInputStream(socket.getInputStream());
    }
    public void closeStreams() throws IOException {
        dout.close();
        din.close();
    }
    // method to communicate with server by sending the message and getting back the response (List of words) 
    private Object communicate(String userInput) throws IOException, ClassNotFoundException {
        Object serverResponse;
        dout.writeUTF(userInput);
        dout.flush();
        serverResponse = din.readObject();
        return serverResponse;
    }
    
    public static void main(String args[]) throws UnknownHostException, IOException, ClassNotFoundException {
        if(args.length != 1){
            System.err.println("Usage: java Client <port number>");
            System.exit(1);
        }
        int portNumber = Integer.parseInt(args[0]);
        Client client = new Client("127.0.0.1", portNumber);
        BufferedReader input = new BufferedReader(new InputStreamReader(System.in, "UTF-8"));
        String userInput = "";
        // Read until "End" is encountered
        client.initializeStreams();
        while (!userInput.equals("End")) {
            userInput = input.readLine();
            Object serverResponse = client.communicate(userInput);
            System.out.println("Server says: " + serverResponse);
        }
        //closing the input/output streams and the socket
        client.closeStreams();
        client.socket.close();
    }
}

MultiServer.java

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

public class MultiServer {
    public static void main(String[] args) throws IOException {
        if (args.length != 1) {
            System.err.println("Usage: java MultiServer <port number>");
            System.exit(1);
        }
        int portNumber = Integer.parseInt(args[0]);
        boolean listening = true;
        int clientNumber = 0;
        System.out.println("Server started");
        try (ServerSocket serverSocket = new ServerSocket(portNumber)) {
            while (listening) {
                System.out.println("Waiting for next client ...");
                Socket socket = serverSocket.accept();
                clientNumber++;
                System.out.println("Connected to client number " + clientNumber);
                new Thread(new ServerThread(socket,clientNumber)).start();
            }
        } catch (IOException exception) {
            System.err.println("Couldn't listen on port " + portNumber);
            System.exit(1);
        }
    }

}

class ServerThread implements Runnable {
    private Socket socket;
    private int clientNumber;
    public ServerThread(Socket socket, int clientNumber) {
        this.socket = socket;
        this.clientNumber = clientNumber;
    }
    //method to parse clients message and sort the words present in it by their length 
    private List<String> sortStrings(String input) {
        String[] stringList = input.split(" ");
        Arrays.sort(stringList, (a, b) -> {
            return a.length() - b.length();
        });
        return Arrays.asList(stringList);
    }

    @Override
    public void run() {
        // takes input from the client socket
        try (ObjectInputStream din = new ObjectInputStream(socket.getInputStream());
                ObjectOutputStream dout = new ObjectOutputStream(socket.getOutputStream());) {
            String clientMessage = "";

            // reads message from client until "End" is sent
            while ((!clientMessage.equals("End"))) {
                clientMessage = din.readUTF();
                System.out.println("Client " + clientNumber + " says: " + clientMessage);
                // replying to client with list of sorted words contained in the clients message
                dout.writeObject(sortStrings(clientMessage));
                dout.flush();
            }
            socket.close();
        } catch (IOException ex) {
            ex.printStackTrace();
        }

    }
}

Compile the java classes using the command javac Multiserver.java javac Client.java
Run the Compiled java class files using the following commands in different terminals

java MultiServer <port number>

java Client <port number>

Note that the must be same while running client and server
(Run the Server Class first to avoid the Client program from crashing)
send a string which consists of space seperated words
The server will respond with a List Containing the words sorted according to their lengths
To end the communication with server send "End" from the Client
Note that the server supports multiple clients just like any normal server