This week's book giveaway is in the General Computing forum. We're giving away four copies of Arduino in Action and have Martin Evans, Joshua Noble, and Jordan Hochenbaum on-line! See this thread for details.
JDBC uses interfaces because it only provides a contract for the classes. The actually implementation is provided by the JDBC driver. For example, if you use Oracle, you have a driver in the classes12.zip file. This file contains all of Oracle's custom classes that implement the JDBC interfaces.