You can start the default Java™ editor from the diagram editor to make
modifications to the source code of Java classes and interfaces.
Note: Changes that you make to the source code are automatically
reflected in the corresponding diagram.
To start the default Java editor from a diagram, in the diagram
editor, right-click a Java class or interface; then click.
Note: To open the default Java editor, you can also double-click a Java classifier
or relationship in the diagram editor.