Sockets programming in Java: A tutorial

import java.io.*;
import java.net.*;
public class smtpClient {
    public static void main(String[] args) {
// declaration section:
// smtpClient: our client socket
// os: output stream
// is: input stream
        Socket smtpSocket = null;  
        DataOutputStream os = null;
        DataInputStream is = null;
// Initialization section:
// Try to open a socket on port 25
// Try to open input and output streams
        try {
            smtpSocket = new Socket("hostname", 25);
            os = new DataOutputStream(smtpSocket.getOutputStream());
            is = new DataInputStream(smtpSocket.getInputStream());
        } catch (UnknownHostException e) {
            System.err.println("Don't know about host: hostname");
        } catch (IOException e) {
            System.err.println("Couldn't get I/O for the connection to: hostname");
        }
// If everything has been initialized then we want to write some data
// to the socket we have opened a connection to on port 25
    if (smtpSocket != null && os != null && is != null) {
            try {
// The capital string before each colon has a special meaning to SMTP
// you may want to read the SMTP specification, RFC1822/3
        os.writeBytes("HELO\n");    
                os.writeBytes("MAIL From: k3is@fundy.csd.unbsj.ca\n");
                os.writeBytes("RCPT To: k3is@fundy.csd.unbsj.ca\n");
                os.writeBytes("DATA\n");
                os.writeBytes("From: k3is@fundy.csd.unbsj.ca\n");
                os.writeBytes("Subject: testing\n");
                os.writeBytes("Hi there\n"); // message body
                os.writeBytes("\n.\n");
        os.writeBytes("QUIT");
// keep on reading from/to the socket till we receive the "Ok" from SMTP,
// once we received that then we want to break.
                String responseLine;
                while ((responseLine = is.readLine()) != null) {
                    System.out.println("Server: " + responseLine);
                    if (responseLine.indexOf("Ok") != -1) {
                      break;
                    }
                }
// clean up:
// close the output stream
// close the input stream
// close the socket
        os.close();
                is.close();
                smtpSocket.close();   
            } catch (UnknownHostException e) {
                System.err.println("Trying to connect to unknown host: " + e);
            } catch (IOException e) {
                System.err.println("IOException:  " + e);
            }
        }
    }           
}

import java.io.*;
import java.net.*;
public class echo3 {
    public static void main(String args[]) {
// declaration section:
// declare a server socket and a client socket for the server
// declare an input and an output stream
        ServerSocket echoServer = null;
        String line;
        DataInputStream is;
        PrintStream os;
        Socket clientSocket = null;
// Try to open a server socket on port 9999
// Note that we can't choose a port less than 1023 if we are not
// privileged users (root)
        try {
           echoServer = new ServerSocket(9999);
        }
        catch (IOException e) {
           System.out.println(e);
        }   
// Create a socket object from the ServerSocket to listen and accept 
// connections.
// Open input and output streams
    try {
           clientSocket = echoServer.accept();
           is = new DataInputStream(clientSocket.getInputStream());
           os = new PrintStream(clientSocket.getOutputStream());
// As long as we receive data, echo that data back to the client.
           while (true) {
             line = is.readLine();
             os.println(line); 
           }
        }   
    catch (IOException e) {
           System.out.println(e);
        }
    }
}