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

/** Given an e-mail address of the form user@host,
 *  connect to port 25 of the host and issue an
 *  'expn' request for the user. Print the results.
 *
 *  Taken from Core Web Programming from
 *  Prentice Hall and Sun Microsystems Press,
 *  .
 *  © 2001 Marty Hall and Larry Brown;
 *  may be freely used or adapted.
 */

public class AddressVerifier extends NetworkClient {
  private String username;

  public static void main(String[] args) {
    if (args.length != 1) {
      usage();
   }
    MailAddress address = new MailAddress(args[0]);
    AddressVerifier verifier
      = new AddressVerifier(address.getUsername(),
                            address.getHostname(), 25);
    verifier.connect();
  }

  public AddressVerifier(String username, String hostname,
                         int port) {
    super(hostname, port);
    this.username = username;
  }

  /** NetworkClient, the parent class, automatically establishes
   *  the connection and then passes the Socket to
   *  handleConnection. This method does all the real work
   *  of talking to the mail server.
   */

  // You can't use readLine, because it blocks. Blocking I/O
  // by readLine is only appropriate when you know how many
  // lines to read. Note that mail servers send a varying
  // number of lines when you first connect or send no line
  // closing the connection (as HTTP servers do), yielding
  // null for readLine. Also, we'll assume that 1000 bytes
  // is more than enough to handle any server welcome
  // message and the actual EXPN response.

  protected void handleConnection(Socket client) {
    try {
      PrintWriter out = SocketUtil.getWriter(client);
      InputStream in = client.getInputStream();
      byte[] response = new byte[1000];
      // Clear out mail server's welcome message.
      in.read(response);
      out.println("EXPN " + username);
      // Read the response to the EXPN command.
      int numBytes = in.read(response);
      // The 0 means to use normal ASCII encoding.
      System.out.write(response, 0, numBytes);
      out.println("QUIT");
      client.close();
    } catch(IOException ioe) {
      System.out.println("Couldn't make connection: " + ioe);
    }
  }

  /** If the wrong arguments, thn warn user. */

  public static void usage() {
    System.out.println ("You must supply an email address " +
       "of the form 'username@hostname'.");
    System.exit(-1);
  }
}


MailAddress.java  Separates the user and host components of an email address. 

import java.util.*;

/** Takes a string of the form "user@host" and
 *  separates it into the "user" and "host" parts.
 *
 *  Taken from Core Web Programming from
 *  Prentice Hall and Sun Microsystems Press,
 *  .
 *  © 2001 Marty Hall and Larry Brown;
 *  may be freely used or adapted.
 */

public class MailAddress {
  private String username, hostname;

  public MailAddress(String emailAddress) {
    StringTokenizer tokenizer
      = new StringTokenizer(emailAddress, "@");
    this.username = getArg(tokenizer);
    this.hostname = getArg(tokenizer);
  }

  private static String getArg(StringTokenizer tok) {
    try { return(tok.nextToken()); }
    catch (NoSuchElementException nsee) {
      System.out.println("Illegal email address");
      System.exit(-1);
      return(null);
    }
  }

  public String getUsername() {
    return(username);
  }

  public String getHostname() {
    return(hostname);
  }
}


NetworkClient.java  Starting point for a network client to communicate with a remote computer.

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

/** A starting point for network clients. You'll need to
 *  override handleConnection, but in many cases connect can
 *  remain unchanged. It uses SocketUtil to simplify the
 *  creation of the PrintWriter and BufferedReader.
 *
 *  Taken from Core Web Programming from
 *  Prentice Hall and Sun Microsystems Press,
 *  .
 *  © 2001 Marty Hall and Larry Brown;
 *  may be freely used or adapted.
 */

public class NetworkClient {
  protected String host;
  protected int port;

  /** Register host and port. The connection won't
   *  actually be established until you call
   *  connect.
   */

  public NetworkClient(String host, int port) {
    this.host = host;
    this.port = port;
  }

  /** Establishes the connection, then passes the socket
   *  to handleConnection.
   */

  public void connect() {
    try {
      Socket client = new Socket(host, port);
      handleConnection(client);
    } catch(UnknownHostException uhe) {
      System.out.println("Unknown host: " + host);
      uhe.printStackTrace();
    } catch(IOException ioe) {
      System.out.println("IOException: " + ioe);
      ioe.printStackTrace();
    }
  }

  /** This is the method you will override when
   *  making a network client for your task.
   *  The default version sends a single line
   *  ("Generic Network Client") to the server,
   *  reads one line of response, prints it, then exits.
   */

  protected void handleConnection(Socket client)
    throws IOException {
    PrintWriter out = SocketUtil.getWriter(client);
    BufferedReader in = SocketUtil.getReader(client);
    out.println("Generic Network Client");
    System.out.println
      ("Generic Network Client:\n" +
       "Made connection to " + host +
       " and got '" + in.readLine() + "' in response");
    client.close();
  }

  /** The hostname of the server we're contacting. */

  public String getHost() {
    return(host);
  }

  /** The port connection will be made on. */

  public int getPort() {
    return(port);
  }
}


SocketUtil.java  Provides utilities for wrapping a BufferedReader  and PrintWriter around the Socket's input and output streams, respectively. 

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

/** A shorthand way to create BufferedReaders and
 *  PrintWriters associated with a Socket.
 *
 *  Taken from Core Web Programming from
 *  Prentice Hall and Sun Microsystems Press,
 *  .
 *  © 2001 Marty Hall and Larry Brown;
 *  may be freely used or adapted.
 */

public class SocketUtil {
  /** Make a BufferedReader to get incoming data. */

  public static BufferedReader getReader(Socket s)
      throws IOException {
    return(new BufferedReader(
       new InputStreamReader(s.getInputStream())));
  }

  /** Make a PrintWriter to send outgoing data.
   *  This PrintWriter will automatically flush stream
   *  when println is called.
   */

  public static PrintWriter getWriter(Socket s)
      throws IOException {
    // Second argument of true means autoflush.
    return(new PrintWriter(s.getOutputStream(), true));
  }
}