A question about a piece of scan port program

Question

The program as the following:
import java.net.*;
import java.io.*;
public class Scan
  public static void main(String[] args)
    String host;
    Socket theTCPsocket;
    InetAddress hostAddress;
    if (args.length < 1)
      host = "localhost";
    else
      host = args[0];
    try
      hostAddress=InetAddress.getByName(host);
   for (int i = 0; i < 65535; i++)
        try
          theTCPsocket = new Socket(hostAddress, i);
          System.out.print("TCP port " + i + " of " + host + " is open");
          switch(i)
            case 21:
            System.out.println("(maybe there is a FTP server is running)");
            break;
            case 23:
            System.out.println("(maybe there is a TELNET server is running)");
            break;
            case 25:
            System.out.println("(maybe there is a SMTP server is running)");
            break;
            case 80:
            System.out.println("(maybe there is a HTTP server is running)");
            break;
            case 110:
            System.out.println("(maybe there is a POP server is running)");
            break;
            case 139:
            System.out.println("(This server's netBIOS is reachable)");
            break;
          theTCPsocket.close();
        catch (IOException e)
     catch (UnknownHostException e)
      System.err.println("the hostname:"+host+"is unknown or can not be analysed!");
}  The question is: I don't know why the System.out.print("TCP port " + i + " of " + host + " is open"); in for (int i = 0; i < 65535; i++)
try
theTCPsocket = new Socket(hostAddress, i);
System.out.print("TCP port " + i + " of " + host + " is open"); is not run everytime when i equal from 0 to65535,but it just run when i=21 and etc?
Please help me!
Thanks!
yours
spark
Message was edited by:
spark077

Answer 1

Don't run this program except on your own computer(s), but the answer is that if the port isn't being listened to, the Socket constructor throws an IOException which you are handling with the following code:
    {}and then continuing with the loop. So if the port isn't being listened to, nothing will be printed.