Design Principles

Question 1

Princple of Least Knowledge (Law of Demeter)

Answer 1

A method M in an object O can call on:

• methods in O
• methods of parameters to M
• method of object instantiated
• method of object that is part of O

Question 2

Liskov Substitution Principle

Answer 2

Subtypes should be substitutable for their base types

Question 3

Encapsulate what varies

Answer 3

• separate varying aspects of code so changing doesn’t affect rest of code
• encapsulate behaviour
• encapsulate object creation
• encapsulate method invocation (command)

Question 4

Favour composition over inheritance

Answer 4

• enables to change behaviour at run time

- delegate the implementation of a behaviour to composed class

Question 5

loosely-coupled design

Answer 5

• loosely coupled designs minimise interdependency between objects

Question 6

Hoolywood principle: don’t call us, we’ll call you

Answer 6

• enable low-level components to hook themselves into a system, but the high-level components decide “when and how” they are needed
• i.e. template method pattern

Question 7

OCP

Answer 7

• new changes must be implemented by new code not altering existing
• i.e. strategy, state, decorator and template

Question 8

SRP

Answer 8

classes should only have one reason to change = high cohesion

Question 9

Program to an interface (super-type) and not to an implementation

Answer 9

• the declared type of the variable should be a super-type (abstract class or interface)
• helps use polymorphism

Question 10

Depend upon abstractions. Do not depend on create classes

Answer 10

• A high level module should not depend on “low-level” modules, but both should depend on abstractions