print() method of a socket output stream doesn't work

I'm trying to write a code for a simple chat app with java sockets. This is the code of the server

public class MyServerSocket implements Runnable {

    List<Connexion> connexions = new ArrayList<>();
    final int PORT=7878;

    @Override
    public  void run() {
        ServerSocket serverSocket = null;
        try {
            serverSocket = new ServerSocket(PORT);
            System.out.println ("I'm waiting for clients to connect");
            Socket socket = null;
            while (true) {
                socket = serverSocket.accept();
                System.out.println("A client is connected, IP : " + socket.getInetAddress());
                Connexion connexion = new Connexion(socket);
                connexions.add(connexion);
                Thread process_connexion = new Thread(connexion);
                process_connexion.start();
            }
        } catch (IOException e) {
            throw new RuntimeException(e);
        }

        // serverSocket.close();
    }

    public void broadcast(String message) {
        this.connexions.forEach(connexion -> {
            if (connexion != null) {
                connexion.sendMessage(message);
            }
        });
    }

    class Connexion implements Runnable{

        private Socket socket = null;
        private String nickName;
        private BufferedReader in;
        private PrintWriter out;

        public Connexion (Socket s) {
            this.socket = s;
        }

        public void run() {
            try {
                InputStream IS = socket.getInputStream();
                InputStreamReader ISR = new InputStreamReader(IS);
                in = new BufferedReader(ISR);
                OutputStream OS = socket.getOutputStream();
                out = new PrintWriter(OS, true);



                // here is the problem
                // when i replace println with print method
                // the code won't work any more
                out.println("Hi Client");
                out.println("Enter your name : ");





                String requete = in.readLine();
                this.nickName = requete;
                out.println("You are connected, you can type what you want!");
                while ((requete = in.readLine()) != null) {
                    broadcast(this.nickName + " : " + requete);
                    System.out.println(requete);
                }
                out.println("Connection lost!");
                in.close();
                out.close();
                socket.close();
            } catch (Exception e) {
                try {
                    socket.close();
                } catch (IOException ex) {
                    throw new RuntimeException(ex);
                }
            }
        }

        public void sendMessage(String message) {
            out.println(message);
        }

    }

}

The code is working, but when i replace the mentioned println method (above in the code), or any println method related to output stream of socket, to print problems occur.

When i use println, the output of the server looks like output of the server and client output looks like client output But when i use print instead, the output of the server looks like output of the server and client output looks like client output I use the same code of client in the two exemples. My question is why the print method doesn't work like println does.

From the Javadoc:

Unlike the PrintStream class, if automatic flushing is enabled it will be done only when one of the println, printf, or format methods is invoked, rather than whenever a newline character happens to be output. These methods use the platform's own notion of line separator rather than the newline character.

Since you're not calling println anymore, there will be no automatic flushing. Call flush() manually to send the data.