Mois : février 2017

Object Oriented Programming fundamentals – notes

Object != Class
A class provides the definition of the object type. It defines properties and actions.
An object is an instance of a class
The object variable is the variable that references the object result of the instantiation. It holds the state of the object. It retains the values of its properties.
Objects only exits at runtime.

A class – a cookie cutter
An object = a cookie made by the cookie cutter

Business object, entity

A business object is a class specifically defined for solving a business problem
An entity is anything from the real world that is significant enough to be represented as a class in the application

OOP
Object Oriented Programming is  an approach to build applications by focusing on objects that interact cleanly with one another in a way that is flexible, natural, well-crafted and testable.

In order to build to solve a real problem

Identify classes
Separate responsibilities
Establish relationships
Leverage reuse

The 4 pilons of OOP

Abstraction is a concept, a way to think: define appropriate classes by focusing on what is important for a purpose. Appropriate abstractions depends on the requirements of the application.

Encapsulation – is a technique to hide or encapsulate the data and implementation details within a class. It hides complexity.
Polymorphism – is the concept where a method can behave differently depending on the type of object that calls it. inheritance-based polymorphism – method String
interface-based polymorphism
Inheritance -Inheritance allows building a class that takes or inherits the members of another class. Inheritance – « is a » relationship
Separation of concerns
An application should be decomposed into parts with minimal overlap. Each part should be responsable for a separate concern.
Separation of concerns minimise coupling, maximise cohesion, simplifies maintenance and improves testability.
Coupling
Coupling represents the degree to which classes are dependent on each other.
The fewer dependencies the easiest is to write, test, maintain and update.
Low coupling – a reduced probability that changes to one class will adversely affect another class. Easier to test.
Cohesion
Cohesion represents the measure of how related everything in a class is to the purpose of the class.
High cohesion makes the class well defined and easy to understand and test
Low coupling + high cohesion = love
Collaboration – « uses a » – one class uses another class. Example, orderRepository uses the Order class to display the order or to serialise

Composition – « has a » – an object has other objects. Aggregation – an object can be composed of another object. Agregation is a special type of composition where the component parts does not exist except as part of the composition. An order has an orderItem

Reuse – share common classes, Inheritance a base class, components – build an encapsulated component and reuse it in any application
Design pattern
A design pattern is a common practice for defining an appropriate set of classes and their associated relationships.