Some Useful Programs

You now know everything there is to know about the class. The tools in this class alone let you write some genuinely useful programs. Here we’ll look at two: one that queries your domain name server interactively and another that can improve the performance of your web server by processing log files offline.


nslookup is a Unix utility that converts hostnames to IP addresses and IP addresses to hostnames. It has two modes: interactive and command line. If you enter a hostname on the command line, nslookup prints the IP address of that host. If you enter an IP address on the command line, nslookup prints the hostname. If no hostname or IP address is entered on the command line, nslookup enters interactive mode, in which it reads hostnames and IP addresses from standard input and echoes back the corresponding IP addresses and hostnames until you type “exit”. Example 6.9 is a simple character mode application called HostLookup, which emulates nslookup. It doesn’t implement any of nslookup’s more complex features, but it does enough to be useful.

Example 6-9. An nslookup Clone

import*; import*; public class HostLookup { public static void main (String[] args) { if (args.length > 0) { // use command line for (int i = 0; i < args.length; i++) { System.out.println(lookup(args[i])); } } else { BufferedReader in = new BufferedReader( new InputStreamReader(; System.out.println( "Enter names and IP addresses. ...

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