Blue Pelican java lesson 15 project, What’s that diameter?

[Solved]
Henry Wong wrote: And inner class instances require an instance of the outer class to exist. The main method is in the outer class class, but it is a static method — and static content does not have an outer class instance (via the this reference) … hence, the error
https://www.widecoder.com/blue-pelican-java-lesson-15-project-whats-that-diameter-solved/