URL decoder and encoder

http://meyerweb.com/eric/tools/dencoder/

Excellent Java tutorial

http://www.deitel.com/Tutorials/Freetutorialsandarticles/tabid/1575/Default.aspx

The concepts explained here are related to multi thread
like
ExecutorService
ThreadPool
Deadlock
ThreadLocal
Thread and Processes
Synchronisation
http://www.vogella.com/articles/JavaConcurrency/article.html