How to generate custom java class for existing entity object in Oracle Fusion Applications.

I am trying to create a method validation rule for an entity object.

For some unknown reason i am not able to generate java class where i could write my custom method which would be used for entity level validation rule.

I am trying to customize entity object based on following documentation point 8.2.3 - Implementing Validation and Business Rules Programmatically

So far i am stuck at step 2 because the edit button is disabled in Jdeveloper.

Does someone have any idea what could be the problem.