Try to use one comupter as both server and client

Question

Hello, everyone, I am just trying to use my own computer as both server and client to test some codes about networking. For example, use the sample code in java tutorial which is used to test Echo server(code is listed below). Is there anything I have to do to set my computer, such as set my hostname or something like that?
I am a pure newbie. And the purpose of this question is to test some code including socket on one PC without connect to internet.
I have tried to change the name "taranis" in the following code to the computer name of my own PC, but it doesn't work, and said: Couldn't get I/O for the connection to: (my computer name).
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();

Answer 1

Did you write the EchoServer and start it on your
machine, listening on port 7?
You can have the client and server running on the same
machine or different machines, but they have to be
separate pieces of software.
Write a separate EchoServer class that starts up and
listens on that port. Then start the EchoClient and
make the connection.
%yeah, I didn't wrote the EchoServer class. But I thought it is automaticly included and therefore has run once I start my computer.
If I write a EchoServer class, then how should I set the host name of the EchoClient, just simply change "taranis" to my computer name (change "echoSocket = new Socket("taranis", 7);" to echoSocket = new Socket("(my comptuer name)", 7);"?