Question

I’ve just started with this section of the tutorial. I only have a basic understanding of what ports are, etc.

I tried to run this code:

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

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

        Socket echoSocket = null;
        PrintWriter out = null;
        BufferedReader in = null;

        try {
            echoSocket = new Socket("taranis", 7);
            out = new PrintWriter(echoSocket.getOutputStream(), true);
            in = new BufferedReader(new InputStreamReader(
                                        echoSocket.getInputStream()));
        } catch (UnknownHostException e) {
            System.err.println("Don't know about host: taranis.");
            System.exit(1);
        } catch (IOException e) {
            System.err.println("Couldn't get I/O for "
                               + "the connection to: taranis.");
            System.exit(1);
        }

        BufferedReader stdIn = new BufferedReader(
                               new InputStreamReader(System.in));
        String userInput;

    while ((userInput = stdIn.readLine()) != null) {
        out.println(userInput);
        System.out.println("echo: " + in.readLine());
    }

    out.close();
    in.close();
    stdIn.close();
    echoSocket.close();
    }
}

---

“Don’t know about host: taranis.
Java Result: 1”

Is the error catch I get. From my limited understanding; is the echo-server something which exists on my machine? If that’s the case, what do I need to do to get this running? Or am I way off?
Also why have they chosen “taranis” as a parameter?

Ive also replaced “taranis” with “localhost” to see what happened.
It ended up catching an IOException this time.

EDIT: So I’ve found that the echo server is disabled by default in win7 and have activated it. However I cant even connect to it on telnet. I think I may just be in over my head. I’ve also tried the sockets you have recommended with no success.

Answer 1

From the same tutorial:

… The Socket constructor used here requires the name of the machine and the port number to which you want to connect. The example program uses the host name taranis. This is the name of a hypothetical machine on our local network. When you type in and run this program on your machine, change the host name to the name of a machine on your network. Make sure that the name you use is the fully qualified IP name of the machine to which you want to connect. The second argument is the port number. Port number 7 is the port on which the Echo server listens.`

In any case, you will probably want to change taranis to "localhost" and make sure an echo service is running on your machine. If it’s not, you could use something like the following code to simulate an echo server.

import java.net.Socket;
import java.util.Formatter;
import java.util.Scanner;
import java.io.IOException;
import java.net.ServerSocket;
import java.util.ArrayList;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

public class EchoServer {

    public static void main(String[] args) {
        try {
            new EchoServer(INSERTPORT).execute();
        } catch (IOException e) {
            e.printStackTrace();
            System.exit(1);
        }
    }

    private ServerSocket serverSocket;
    private int port;

    private ArrayList<Client> clientList;
    private ExecutorService clientRunner;

    public EchoServer(int port) throws IOException {
        this.port = port;
        serverSocket = new ServerSocket(port);
        clientRunner = Executors.newCachedThreadPool();
        clientList = new ArrayList<>();
    }

    public void sendMessageToAll(String message) {
        for (Client c : clientList) {
            c.displayMessage(message);
        }
    }

    public void execute() throws IOException {
        while (true) {
            clientList.add(new Client(serverSocket.accept(), this));
            clientRunner.execute(clientList.get(clientList.size()-1));
        }
    }
    private class Client implements Runnable {

        private Socket clientSocket;
        private Scanner input;
        private Formatter output;

        public Client(Socket s) throws IOException {
            clientSocket = s;

            input = new Scanner(clientSocket.getInputStream());
            output = new Formatter(clientSocket.getOutputStream());
        }

        public void displayMessage(String s) {
            output.format(s + "\n");
            output.flush();
        }
        @Override
        public void run() {
            while(clientSocket.isConnected()) {
                if(input.hasNextLine()) {
                    sendMessageToAll(input.nextLine());
                }
            }
        }
    }
}

Edit: Just for completeness, as you mentioned some problems running the code, you run the server (this code) and leave it running in the background, then run the client (the code you posted). I tested it, works fine.