What is the Relationship between Class Diagrams and ERD?

Entity-Relationship(ER) Diagrams are used to design the database perspective of  the project unlike the UML class diagram which is used to design application domain model of the project. However, the objects, which are produced according to the templates (Classes), are stored within a database for persistence purpose. So there is a semantic relation between them that's it.

You can actually generate a Class diagram into ERD or verse versa by using a modeling tool (i.e. Visual Paradigm). If you want to control the database using pure java you can actually perform object relational mapping (ORM) for relating the class diagram with the ERD and subsequently the database.

Useful UML Resources

• OMG UML Specification
• Full-featured UML modeling tool
• UML tutorials