You can use JDeveloper to work directly with database objects through a database connection. Alternatively, you can work with offline database definitions which you can subsequently generate to a database schema.
You can create new tables and views and generate the information to a database, or you can import database objects from a database schema, make the changes you want, and then generate the changes back to the same database schema, to a new database schema, or to a file that you can run against a database at a later date. You can also use JDeveloper's modeling tools to visualize your offline database objects on a diagram.
You will use in-place editing in the database diagram to visually define and edit tables, columns and key relationships.