Creating Java source code automatically from a diagram is known as forward engineering. This can be done very simply with Apollo for Eclipse, simply click the 'Create Class' button in the diagram editor palette, then click in the diagram to create the class.
The class source code is automatically generated, which can be verified by the presence of the .java file in the package explorer. You may edit the name, add attributes and operations, set types and default values, and more. These changes are then reflected in the Java source code.
Please refer to the section called “Editing Elements” for more information about editing in Apollo for Eclipse, including multi-valued attribute mapping.