15. Cohesion, coupling and the Law of Demeter

• High cohesion: a class does one focused job; its members belong together.
• Low coupling: classes depend on few others, through abstractions.
• Law of Demeter ("talk to friends, not strangers"): avoid order.Customer.Address.City.Name chains; ask the object to do the work: order.ShippingCity().
• Tell, don't ask: tell the object what to do rather than pulling data out and deciding for it.

These lead straight into SOLID, covered in the next course.