Internet Addresses - Java Network Programming - page 95

Most of the time, you can use hostnames and let DNS handle the translation to IP
addresses. As long as you can connect to a domain name server, you don't need to worry
about the details of how names and addresses are passed between your machine, the
local domain name server, and the rest of the Internet. However, you will need access
to at least one domain name server to use the examples in this chapter and most of the
rest of this topic. These programs will not work on a standalone computer. Your machine
must be connected to the Internet.
The InetAddress Class
The java.net.InetAddress class is Java's high-level representation of an IP address,
both IPv4 and IPv6. It is used by most of the other networking classes, including Socket ,
ServerSocket , URL , DatagramSocket , DatagramPacket , and more. Usually, it includes
both a hostname and an IP address.
Creating New InetAddress Objects
There are no public constructors in the InetAddress class. Instead, InetAddress has
static factory methods that connect to a DNS server to resolve a hostname. The most
common is InetAddress.getByName() . For example, this is how you look up
www.oreilly.com :
InetAddress address = InetAddress . getByName ( "www.oreilly.com" );
This method does not merely set a private String field in the InetAddress class. It
actually makes a connection to the local DNS server to look up the name and the numeric
address. (If you've looked up this host previously, the information may be cached locally,
in which case a network connection is not required.) If the DNS server can't find the
address, this method throws an UnknownHostException , a subclass of IOException .
Example 4-1 shows a complete program that creates an InetAddress object for
www.oreilly.com including all the necessary imports and exception handling.
Example 4-1. A program that prints the address of www.oreilly.com
import java.net.* ;
public class OReillyByName {
public static void main ( String [] args ) {
try {
InetAddress address = InetAddress . getByName ( "www.oreilly.com" );
System . out . println ( address );
} catch ( UnknownHostException ex ) {
System . out . println ( "Could not find www.oreilly.com" );
}