Parse html with Java

In my project, I need to parse html files, for example, retrive the contents under a particular tag or search for a string in the file. I know there is some modules in Perl to accomplish those tasks, but I don't know much Perl. I want to use Java. I find there are packages like: javax.swing.text.html in JDK. So, I think there might have a solution in Java, but I don't know how to start. Could anybody give me a clue? Is there any book or on-line article about this topic? Thank you for your help.

Qingcai Guan