How can I modify the crawler?

[suxiaoke04]

   I want to realize a topic-based search engine through modifing the nutch. 
For example I define a computer topic so I hope that I only find some 
information about computer. I can't find the appropriate point where I can 
insert myself sentence in Fetcher.java. Please tell me how can I modify the 
Fetcher and the parser? thanks.