The Derby DB is a simple open source database application you can download for free at http://db.apache.org/derby. The Squirrel SQL is a simple client application that can connect nearly any database. You can download it for free athttp://www.squirrelsql.org/#installation. The following five video clips explain the first steps when using these two.
The Visitor Design Pattern
The visitor design pattern allows us to add operations to objects that already exist without modifying their classes and without extending them.