If you want to extend the Expr class, you must explicitly add the permits clause. Only a library developer can add that clause. Sealed class rules Sealed classes offer precise control over inheritance ...

Abstract classes and interfaces in Java serve fundamentally different purposes. Learn the differences between these Java language elements and how to use them in your programs.

These let you add a function to an existing class or interface. This feature is well-known in the JavaScript world, but it really stands out in the context of Java-like classes.