TCP Server and Client in Java

Question

I tried building a TCP server and client using Java. They can connect, they work well, but I have a single error.

This is the server side:

package com.company;
import java.io.*;
import java.net.*;

public class Main {

    public static void main (String[] args) throws IOException {

        System.out.println("The server is ready");
        ServerSocket serverSocket = new ServerSocket (1234);
        Socket clientSocket = serverSocket.accept ();

        BufferedReader in = new BufferedReader (new InputStreamReader (clientSocket.getInputStream ()));
        PrintWriter out = new PrintWriter(clientSocket.getOutputStream(), true);
        String message, modifiedMessage;
        message = in.readLine ();
        System.out.print("The received message from client: " + message);
        modifiedMessage = message.toUpperCase();
        out.print(modifiedMessage);
        System.out.println ("\nModified message which is sent to client: " + modifiedMessage);
    }

}

The server will have to receive a message from a client, then transforming it in an upper case string.

The client side is:

package com.company;
import java.io.*;
import java.net.*;

public class Main {

    public static void main(String[] args) throws IOException {

        Socket socket = new Socket("127.0.0.1", 1234);
        PrintWriter out = new PrintWriter(socket.getOutputStream(), true);
        System.out.println("Enter a lowercase sentence: ");
        BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));
       BufferedReader in = new BufferedReader((new InputStreamReader(socket.getInputStream())));

        String messageSent = reader.readLine();
        System.out.println("The message sent is: " + messageSent);
        out.println(messageSent);


        String messageReceived = in.readLine();
        System.out.println("The modified message is: " + messageReceived);
    }
}

I want the client to be able to print both the lower case sentence and the received (modified) upper case sentence. The problem is that, when I enter a simple word, say hello, my client will only print the original string, not the modified one.

The output of the server is:

The received message from client: hello 
The modified message sent to the client is: HELLO

But the output of the client is:

The message sent is: hello 
The modified message is: null

I know that the server is able to convert the string to the upper-case version and to connect to my client. Why doesn't my client print the received message? Doesn't it actually receive it?

Don Ha Don Ha · Accepted Answer · 2020-11-06T18:39:41

You need to flush the message. PrintWriter calls flush in println, print doesn't.

on the server side you need to change to: out.println(modifiedMessage); instead of out.print(modifiedMessage);

TCP Server and Client in Java

1 Answers