We start a Java Internet application by importing java.net.*, that contains the core networking routines. It contains the InetAddress class, that allows us a simple method of converting between numeric and DNS form addresses.

Here is an example of using the InetAddress class. The source code is below and in NetApp1.java .

 1 
 2    
 3  // An application to find out the current address and DNS name
 4  // of the local computer - Derek Molloy.
 5 
 6  import java.net.*;
 7 
 8  public class NetApp1
 9  {
10    public static void main(String args[])
11    {
12      try	// try to do the following
13      {
14      	 // get local address
15          InetAddress localaddr = InetAddress.getLocalHost(); 
16          // find the dns name
17          String hostName = localaddr.getHostName();          
18          System.out.println ("Local IP Address : " + localaddr);
19          System.out.println ("Local hostname : " + hostName);
20      }
21      catch (UnknownHostException e)	// catch the unknown host exception
22      {
23          System.err.println ("Can't detect localhost : " + e);
24      }                
25    }
26  }
27 
28 

This simple application shows us how Java treats networked applications. Again the class structure comes into play where methods associated with the Internet addresses are a part of the InetAddress class. This application is compiled by typing javac NetApp1.java and run by typing java NetApp1. The output looks like Figure 10.1, “InetAddress Class Example”.

Figure 10.1. InetAddress Class Example

In this case the local IP address is listed as portalm/136.206.37.8 - the Microsoft Networks name of my PC, and the fixed IP address as I was not connected to the Internet with, when I ran the application. The local hostname is extracted using the getHostName() method and returns the String object "portalm".

We can modify the code that we just wrote to create a simple "Who Is?" application. If can write this simply by the following code (as below and in WhoIs.java).

 1 
 2    
 3  // An application to find out the address of any named
 4  // computer - Derek Molloy.
 5 
 6  import java.net.*;
 7 
 8  public class WhoIs
 9  {
10     public static void main(String args[])
11     {
12       if(args.length != 1)
13       {
14         System.err.println("Usage: WhoIs [Name]");
15         System.exit(0);
16       }
17       try{
18         InetAddress a = InetAddress.getByName(args[0]);
19         System.out.println(a.toString());
20       }
21       catch(UnknownHostException e)
22       {
23         System.out.println("Unknown Host Exception!");
24       }
25     }
26  }
27  
28 

When this application is run it expects the name of a machine, either specified by the full Internet name, or the name of a machine on the same network segment. If you forget to provide the parameters, it prompts you for the correct parameters (See Figure 10.2, “WhoIs Example”)

Figure 10.2. WhoIs Example

In Figure 10.2, “WhoIs Example” the first time I ran java WhoIs I did not pass the hostname parameter so I was prompted to do so. The second time I ran java WhoIs www.cnn.com and this worked well, returning the IP address 64.236.16.20. This could change each time you run the command as there would be several servers for www.cnn.com. The third time I made up a server and executed java WhoIs made.up.com and since there is no such host (until someone registers it!) and UnknownHostException occurs and our code prints out the message "Unknown Host Exception!".

Remember if your computer is not connected to the Internet you can use the loopback address of 127.0.0.1 which is referred to as the hostname localhost, provided that you have installed a TCP stack to your computer. This is useful if you are working at home and don't want to be billed by your ISP while you are developing networking applications.