1. Object- oriented Design Principles

2. The pyramid of OO
Abstract/
Philosophical
Practical
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3. Object-orientation is good!
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4. Object-orientation is good?
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5. Object-orientation is good (?)
Why are we learning about object-orientation in the first place?
Other ”paradigms” for programming exist, for instance functional programming and logic programming
However, OO is currently the dominating paradigm for programming – more or less an industry standard
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6. Object-orientation is good!
Object-orientation has proven to be a friutful way of connecting real life and software development
However, there are certain ”rules” to obey when playing the OO-game…
At the top, we rely on a few Object-Oriented concepts
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7. Object-oriented concepts
Abstraction
Encapsulation
Polymorphism
Inheritance
(Composition)
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8. Abstraction The idea that we focus only on the essence of a concept
Inessential details are ”abstracted away”
Abstraction works at multiple levels
Can be a challenge to find the appropriate level of abstraction
Abstractions are manifested through interfaces, which define behaviors
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9. Encapsulation
The idea not to expose details about how behaviors are achieved
Concepts are considered black boxes
This enables us to change how behaviors are achieved, without affecting the user of a particular concept
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10. Polymorphism
The idea that concrete instances of a behavior can take many forms
All animals can eat – but do so in very different ways…
Eating is polymorphic behavior – it can take many forms, but has the same essence
Allows us to focus on the essence of the behavoir, neglecting actual behaviors
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11. Inheritance
The idea that concepts can inherit behaviors from other, more general concepts
Allows us to reuse behaviors that have previously been defined
We can easily define hierarchies of related concepts, only needing to add genuinely new behaviors
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12. Composition
The idea that new concepts can be com-posed by combining existing concepts
A supplement/alternative to inheritance
A different approach to reuse
Composition helps us realise has-a relations between concepts, inheritance is used for is-a relations
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13. Fine, but… The OO-concepts are nice, but not particularly operational
We need something more concrete
The OO-principles provide more specific guidelines
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14. The Object-oriented principles
1. Encapsulate the aspects of your application that vary
2. Favor composition over inheritance
3. Program to an interface, not an implementation
4. Strive for loose couplings between objects that interact
5. Make classes open for extension, closed for modification
6. Depend on abstractions, not concrete classes
7. Only talk to your closest friends
8. Don’t call us, we’ll call you
9. A class should only have one reason to change
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15. OO principles Where do these principles come from…?
Not from ”proofs” or theory…
Condensed best practices from real life
Principles are to some extent overlapping
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16. OO principles ”Encapsulate the aspects of your applica-tion that vary”
Promotes reusability
When designing classes for a system of concepts, put the varying elements into separate classes
Can be realised both through inheritance and composition, often using interfaces
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17. OO principles ”Program to an interface, not an imple-mentation”
An essential idea; if we know the interface, we do not need to know any details about a specific implementation (weak coupling)
Allows us the change the implementation without affecting the user
This is more or less encapsulation
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18. OO principles
”Make classes open for extension, closed for modification”
Very important principle!
Once we have a well-documented, well-tested class, we should not change it!
If we need more functionality, achieve it through extension, using inheritance and/or composition
Keep classes as ”pure” as possible
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19. OO principles ”Only talk to your closest friends”
Hmm, what does that mean…?
A typical application architecture divides classes into a number of layers
Classes in one layer should only know classes in the next layer (loose couplings)
Avoid reliance on the specific composition of a class
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20. OO principles What is best, and why…?
double temp = station.GetThermometer().GetTemperature();
double temp = station.GetStationTemperature();
We should not rely on specifics about how a Station is implemented
Only one ”.” in a line of code…
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21. OO principles ”Don’t call us, we’ll call you”
The Hollywood Agent principle…
A more advanced principle; instead of imple-menting an algorithm using a lot of calls to generic classes, implement a generic algo-rithm with ”plugins” for specialised behavior
Also known as Inversion of Control
”Encapsulate what varies…”
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22. Beyond principles
We can use these OO principles directly when developing software
Next step is Design Patterns, which describe designs for solving common problems, relying on the OO principles
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