As per the comments on your previous question:
even after im specifying the protocol...only google and aol are not working, same is working with yahoo, bing and ask.... my project is to implement a metasearch engine....i am able to extract links from yahoo, bing and ask...but same does not work with google and aol...what may be the reason..??
They've blocked your request because you're acting as a robot/leecher which may violate their terms of service. Their websites are very frequently requested and they don't want to unnecessarily waste their bandwidth to robots/leechers which actually only need a small part of the response.
Use their public web service APIs instead of parsing the HTML of the entire website. For Google, that's for example "Google Custom Search API". Other search engine providers offer similar web services. Note that those web services doesn't return bloated HTML, but compact JSON or XML data which is much easier to parse/extract using JSON/XML parsers.