Is there any API in Java to access wikipedia data

Mediawiki, the wiki platform that wikipedia uses does have an HTTP based API. See MediaWiki API.

For example, to get pages with the title stackoverflow, you call

http://en.wikipedia.org/w/api.php?action=query&titles=Stackoverflow

There are some (incomplete) Java wrappers around the API - see the Client Code - Java section of the API page for more detail.

I had the same question and the closest I came to an out-of-the-box solution is bliki, hosted at http://code.google.com/p/gwtwiki/. I also wrote an article at Integrating Stuff to help you get started with it: http://www.integratingstuff.com/2012/04/06/hook-into-wikipedia-using-java-and-the-mediawiki-api/