I recognize that as being our book. Note that on the page 6, on the line before you quoted it says "To keep things simple for now, we'll follow a subset of the rules". Only one class is a subset at least one .
In other words, we are making a simpler world so you can focus on main methods() rather than how the complexities of classes.
Hi Bear and Jeanne, yes, the book is “OCA Oracle Certified Associate Java SE 8 Programmer” by J. Boyarsky and S. Sekikoff. Thank you Jeanne for you reply, however I don’t understand what you mean by "Only one class is a subset at least one".
I guess the question is what do you mean by "Each file can contain only one class" In page 6?