How to do this?

[Yuan Ye]

I want to make an application that can serve as an IP/HostName loopup. It can return the corresponding HostName for an IP and vice vesa. I am not quite sure how to achieve this, Could anyone give me some advice? Thanks.

[Ernest Friedman-Hill]

Well, there are three parts to this:
One part is the interface this program presents to other programs. Don't know if it's a GUI, or an RMI service, or a web page, or what.
The second part is the internal data structure that stores the name/address data. Something as simple as a HashMap may be enough here.
Third, there's getting ahold of the actual information. Again, I don't know what your plans are: do you just want to get the information from DNS, or is it from a flat file or database, or where?
Finally, I should ask: why in Java? Why not simply run BIND?