OO Design Principles (PPT 15)

Question 1

What are the three design principles to be aware of?

Answer 1

• Open-closed principle
• Dependency-Inversion principle
• Liskov-substitution principle

Question 2

What is the Open-Closed principle?

Answer 2

Classes should be open for extension but closed to modification. Meaning you should be able to extend the behaviour of a class without modifying the already working code

Question 3

How do you implement the open closed principle?

Answer 3

Implement Inheritance

Question 4

What is the Dependency-Inversion Principle?

Answer 4

High level modules should not depend on low level modules, both should depend on an abstraction.

Question 5

How to you solve the Dependency-Inversion Principle?

Answer 5

You define an abstraction of the behaviour. Each device must derive itself from this behaviour

Question 6

What is the Liskov Substitution Principle?

Answer 6

Child classes should be substitutable for their parents

Question 7

How do we prevent the Liskov Substitution Principle?

Answer 7

• Design by contract (define pre and post conditions on operations)
• Post conditions should be at least as strong as the parent class
• Also use pre conditions