How can I modify the crawler?

[suxiaoke79]

 
  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.