Java

My Java expertise includes writing a Java to C compiler, with a sophisticated runtime and code optimization system. I know the ins and outs of Java, and many of the weird corner cases it offers for amusement. More importantly, I also know how to make Java run quickly and well, and how to develop nimble, powerful systems in Java without being lost in an XML metadata maze. 

Most of Java's value lies in the libraries and frameworks written for it.  A few that I have experience with are listed below, to pique your interest:

Servlets

The backbone technology for any Java web server, this API defines how a Java process will spit out web pages on demand for a given URL and query.  Some things to think about when writing servlets directly are concurrency, throughput, and memory usage. 

 JDBC

This is the API that allows Java to connect to databases.  Sporting both a very simple SQL wrapper type layer, and a more high level tables based layer, this API is built on by Hibernate, EJB's and others to store data in a scalable, safe, and potentially distributed manner.

Swing

This API is rich, complicated and powerful.  Want to create an application with a rich tabular structure?  No problem!  With Java Web Start and Java applets, these rich applications can be delivered through the web, creating interfaces far beyond a typical browser session.  Just sample a typical stock trading platform to see what I mean.

Ant

Ant is used as a build system for many Java projects.  Extensible and filled with functionality usually needed, one still needs to be careful to keep the build process simple, repeatable, and able to change as needed.   

Jython

Jython is a large subset Python in Java.   This allows applications to be easily extensible, as there's no need to build a new interpreter to support your use case -- just use Jython.  Other easy to incorporate interpreters include Groovy and Beanshell.