与设计模式的“抽象”概念混淆

Question

I was learning about design patterns and I came accross this PDF

On Slide 12, its says the following 3 rules:

1. No variable should hold a reference to a concrete class
2. No class should derive from a concrete class
3. No method should override an implemented method of its base classes

Now Suppose I have an abstract Class say "A". Then, I have 3 concrete classes that should implement this abstract class, they are classes "B","C","D". The issue is that D is a special case of C. D has only one instance variable more.

My question are,

• First, are the 3 rules above golden rules that guarantees a good design
• Second, What problem might happen if "D" extends "C" and overrides methods of "C" to add more behaviour

Answer 1

If you start adding subtypes down the graph, then the abstraction becomes weakened of course. Why? Because people have to learn more, and the footprint of what is required expands. That said, there could be valid cases, though I am straining to see them in a factory situation. One possibility, to answer your question directly for that dowstream case would be to use Decorator, because it more or less preserves the abstraction but layers in some functionality that was not provided for.