concept all class in category java

This is an excerpt from Manning's book Java SE 11 Programmer I Certification Guide MEAP V03. Login to get full access to this book.

Packaged classes are part of a named package—a namespace—and they’re defined as being part of a package by including a package statement in a class. All classes and interfaces are packaged. If you don’t include an explicit package statement in a class or an interface, it’s part of a default package.

Figure 10.5 Adding a new property, facebookId, to all classes, with and without the base class Employee