The best Database on this planet for Java application today is Versant.
No need to map Java to Tables and mess with crazy SQL statements.
Works at least 50 times faster than ORACLE and take 50% time less in development.
Checkout http://www.versant.com/products/vds/index.html

Do not believe me , try yourself.
Download at:
http://www.versant.com/products/download/index.cfm


Shay