Dependency Inversion principle
The last of the SOLID principles is based on two statements, that Wikipedia states in this form:
High-level modules should not depend on low-level modules. Both should depend on abstractions.
Abstractions should not depend upon details. Details should depend upon abstractions.
As for the first statement, we should clarify what we understand by high-level and low-level modules. The terminology is related to the importance of the actions performed by the module.
Let's put it simply: if a module holds the business logic of a Customers class, and another includes the format that a list of the Customers class uses in a report, the first one would be high-class and the second would be low-class.
The second statement speaks by itself. If an abstraction depends on details, the usage as a definition contract is compromised.
In the case of our sample, we still have some code that will not grow appropriately: the SportsCar creation method depends much on what the user writes...
