Java套接字聊天-解决消息问题

Question

我正在研究Java项目，其主要目的是允许用户彼此聊天。当前版本就像一个聊天室，每个人都可以编写并查看其他人发送的内容。我需要更改此设置，因此用户必须选择他正在写的对象，这意味着仅向其收件人显示消息。我的问题在这里：

public class Server {

    private int port;
    public static List<PrintStream> klienci;
    private ServerSocket server;

    public static void main(String[] args) throws IOException {
        new Server(4444).run();
    }

    public Server(int port) {
        this.port = port;
        this.klienci = new ArrayList<PrintStream>();
    }

    public void run() throws IOException {
        server = new ServerSocket(port) {
            protected void finalize() throws IOException {
                this.close();
            }
        };
        System.out.println("Port 4444 works.");

        while (true) {
            Socket client = server.accept();
            System.out.println("Connected " + client.getInetAddress().getHostAddress());

            this.klienci.add(new PrintStream(client.getOutputStream()));
            new Thread(new KLIENT(this, client.getInputStream())).start();
        }
    }

    void MESSIN(String msg) {
        for (PrintStream client : this.klienci) {
            client.println(msg);
        }
    }
}

class KLIENT implements Runnable {

    private Server server;
    private InputStream client;

    public KLIENT(Server server, InputStream client) {
        this.server = server;
        this.client = client;
    }

    public void run() {
        String msg;
        Scanner sc = new Scanner(this.client);
        while (sc.hasNextLine()) {
            msg = sc.nextLine();
            server.MESSIN(msg);
        }
        sc.close();
    }
}

我正在尝试在MESSIN中添加IF条件，但是我不知道如何通过其昵称来区分客户端（昵称是第二类，即Client，我不能使用它们）。