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The code in Listing 6 produces the output shown in Figure 6 .
| Figure 6 - Google host name using reverse lookup. |
|---|
Display Google name using reverse lookup.
dfw06s39-in-f17.1e100.netdfw06s39-in-f17.1e100.net |
The canonical host name
To me, the most interesting thing in Figure 6 is that both methods return the canonical host name. Neither method returns the domain name.
Although it isn't shown here, calling the getHostName method on one of the InetAddress objects contained in the addresses array in Listing 1 returns the domain name or www.google.com . Calling the getCanonicalHostName on the same InetAddress object returns thecanonical name shown in Figure 5 .
Apparently when you create an InetAddress object on the basis of the domain name, the object knows both the domain name and the canonical host name.However, when you create an InetAddress object using the IP address, the only name that it knows is the canonical host name.
The InetAddress class also has a method named getByAddress that apparently provides the same behavior when the IP address is converted toan array of bytes and passed to the method in that format. However, I haven't tested that method.
Can you access the site with the canonical host name
With respect to Google, the canonical host name, the domain name, or one of the IP addresses can be used in your browser address field to access the site.
However, for those websites in the above list that have a canonical name that ends with akamaitechnologies.com , it appears that you cannot use either the canonical name or the IP address to access the web site. (I will leave it as an exercise for the student to investigate this further.)
The end of the program
The code in Listing 7 takes care of the administrative details necessary to properly end the program.
| Listing 7 - End of the program. |
|---|
}catch(UnknownHostException e){
e.printStackTrace();}//end catch}//end main
}//end class Java4630a |
I encourage you to copy the code from Listing 8 . Compile the code and execute it while you are connected to the Internet. Experiment with the code,making changes, and observing the results of your changes. Make certain that you can explain why your changes behave as they do.
The next module will deal with the URL class and the URLEncoder class.
This section contains a variety of miscellaneous information.
Financial : Although the Connexions site makes it possible for you to download a PDF file for thismodule at no charge, and also makes it possible for you to purchase a pre-printed version of the PDF file, you should beaware that some of the HTML elements in this module may not translate well into PDF.
I also want you to know that, I receive no financial compensation from the Connexions website even if you purchase the PDF version of the module.
In the past, unknown individuals have copied my modules from cnx.org, converted them to Kindle books, and placed them for sale on Amazon.com showing me as the author. Ineither receive compensation for those sales nor do I know who does receive compensation. If you purchase such a book, please beaware that it is a copy of a module that is freely available on cnx.org and that it was made and published withoutmy prior knowledge.
Affiliation : I am a professor of Computer Information Technology at Austin Community College in Austin, TX.
Listing 8 - Complete program listing.
/*File Java4630a.java Copyright 1998, R.G.Baldwin
Revised 01/03/14This program exercises several of the methods of the
InetAddress class.**********************************************************/
import java.net.*;public class Java4630a{
public static void main(String[]args){
try{System.out.println(
"Get and display InetAddress(es) of Google URL");InetAddress[] addresses =InetAddress.getAllByName("www.google.com");
for(int cnt=0; cnt<addresses.length;cnt++){
System.out.println(addresses[cnt]);
}//end for loopSystem.out.println();//blank line
System.out.println("Get and display current " +"InetAddress of LocalHost");
InetAddress address = InetAddress.getLocalHost();System.out.println(address);
System.out.println();//blank lineSystem.out.println("Extract and display current " +
"name of LocalHost");System.out.println(address.getHostName());
System.out.println();//blank lineSystem.out.println("Extract and display current " +
"address of LocalHost");System.out.println(address.getHostAddress());System.out.println();//blank line
System.out.println("Display canonical host name for Google");
//Get InetAddress containing one of Google's// IP addresses.
address = InetAddress.getByName("www.google.com");System.out.println(address.getCanonicalHostName());
System.out.println("Display Google name using reverse lookup.");
String googleAddress = address.getHostAddress();System.out.println(InetAddress.getByName(
googleAddress).getHostName());System.out.println(InetAddress.getByName(
googleAddress).getCanonicalHostName());}catch(UnknownHostException e){
e.printStackTrace();}//end catch}//end main
}//end class Java4630a
-end-
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