Techniques for good Design

Question 1

What are basic symptoms of Degenerative design?

Answer 1

Fragility - Changes lead to unforeseen errors
Rigidity - changes often bring with them lots of additional changes
Low reusablity - parts are not or cannot be reused

Question 2

What are techniques for good design? (SOLID…)

Answer 2

• Single Responsiblity Principle
• Open-Closed Principle
• Liskov’s substitution principle
• Interface seggregation principle
• Dependency Inversion
• Low Coupling
• High cohesion
• Resolvation of cyclic dependencies

Question 3

Describe the Single Responsibility Principle (SRP)

Answer 3

• classes should only have one reason to change
• functional unit should only be responsible for a single aspect of a system’s requierement
• an apsect of a requierement is a trait/property of requierements which can change seperately from other aspects

Question 4

Describe the open-closed principle

Answer 4

Sofware Modules should be
- open for extension (through polymorphism/inheritance)
- closed for modification (given logic remains reliable and unchanged)

Question 5

Describe Liskov’s Substitution principle

Answer 5

A basic class should always be replaceable by its derived classes (subclasses)

Subclasses should behave in a expected and similiar way as the parent class.

Question 6

Describe the Interface Seggregation Principle (ISP)

Answer 6

Extensive and complex interfaces should be subdevided, based on
- semantic context
- area of responsibility
- user groups

Question 7

Describe the Dependency Inversion Principle

Answer 7

• Dependencies should always be to abstract descriptions instead of concrete implementations
• making the implementations interchangeable without necessary changes to the using class
• no direct dependencies, instead only dependencies to abstractions

Question 8

Describe the loose coupling principle

Answer 8

• coupling refers to the relationship between building blocks and how strong the resulting dependency is
• loose coupling aims to reduce the complexity of structures
• the looser the coupling the easier an individual building block is to understand
• and the easier it is to modify

Question 9

Name a few types of coupling

Answer 9

• Call: when a class uses directly another class my calling a method of that class
• Generation: when a building block generates another building block
• Data: when classes communicate via global data structures or method parameters
• Execution location: hardware-based coupling, when building blocks have to run on same VM
• Time: when chronological sequence of building block call impacts end results
• Inheritance: subclass is coupled to parent class, level of coupling dependen on amount of inherited attributes

Question 10

Describe the high cohesion principle

Answer 10

• cohesion describes how parts of a building block deal with the same theme
• a cohesive class solves a single problem
• influences how easily building blocks can be modified
• low cohesion leads to one change needing additional changes in different building blocks or classes
• classes should not be grouped by packages but instead by systems and subsystems