Sealed classes and interfaces: permits, final, sealed and non-sealed subclasses

A sealed class or interface restricts which other classes or interfaces may directly extend or implement it. Ordinary inheritance is open: anyone can subclass a non-final class. final closes it completely. Sealing sits in between: you name the exact set of allowed direct subtypes. This lets you model a closed set of alternatives, such as the shapes a drawing tool supports, and it lets the compiler check that a switch covers every case.