You don’t have to create any Book objects to call class ... belongs to the same package (discussed later in the Java 101 series) as the class containing the member that is to be accessed.

This works because the two are closely related. In Java, nested classes are categorized as either static member classes or inner classes. Inner classes are non-static member classes, local classes ...

proposed in JEP 409 and available since Java 17, let developers limit and control how deeply a component's type hierarchy can extend. With sealed classes, a developer can essentially create a ...

You need a whole new class to implement a single method. Furthermore, the logic is separated from the point in which it is needed. This will make troubleshooting and code maintenance more difficult.