FSD +002

Classes & Objects

Class

• A template that defines the attributes and methods of an object, which can be used to create many objects.
• Name of the class is noun.
• Functions are defined in the class, containing the function data.
• Many objects can be created from the same class.
• A class can be inherited by many sub-classes
• A child class can have one or more parent classes. (super-classes)
• Encapsulated data
• Behaviors of a class are exposed while the implementation is hidden.

Object

• An instance of a class, created the class template during runtime.
• Object-oriented software is composed of many objects
• a composite data type identified by its attributes (fields) and behaviors (functions), which are defined in the class.
• object-oriented design analysis: to identify the classes and relationships between the classes to model how the system would work.

Procedure & Function

Aspect	Procedure	Function
Returns value?	❌ Usually does not return a value	✅ Usually returns a value
Purpose	Perform an action	Compute and return a result

Object-Oriented Paradigm Principles

Abstraction

• hides the internal implementation of functions and expose to other class the behavior of those functions.
• Data abstraction: hide the variables stored the data and only expose the values.
• Function abstraction: hide the function code and only export the function behavior.

Encapsulation

• a fundamental principle of OOP, increases data and code security, enhances performance by reducing object interactions.
• minimize the number of interactions from outside.

Inheritance

• allows classes to have sub-classes.
• a parent class can have many children.
• a child class can have more than one parent.
• a child class inherits their parent class and can also override them.

Polymorphism

• more of the same type of object can be used interchangeably.
• object of different types to be treated as objects of a common parent.
• multiple object types with the same parent can implement the same function in different ways.
• multiple object types with the same parent can use the same attributes with different values.

Object-Oriented Design

• The process of designing a software using oop.
• Identify the classes -> Identify the classes' attributes and behaviors -> Identify the how classes interact with each other and create a visual class model

Identify classes

• Classes are derived from use cases and actors described in the requirement analysis process.
• Classes should be identified exactly as needed.
• Reduce coupling and enable extensibility.
• Identifying a class always requires to identify the fields and methods.
• A class can use the methods of another class. -> one class (the client) uses or depends on another class (the supplier).

UML Class diagram

• UML visual representation of the object-oriented software system.
• Describes the attributes and operations of classes, relationships, and any constraints.
• Used as a plan to construct executable code and define the way in which objects may interact.

Associations

• a relationship between classes indicating a meaningful connection.
• Labelled by
  • Association name
  • Role name
  • Multiplicity
  • Navigability

Multiplicity

• how many instances of type A can be associated with on instance of type B.

Recursive or reflexive association

• a class can have an association with itself.

Attributes

• m:m association can be divided into two 1:m associations.

Association Classes

• An attribute is related to an association.
• Instance of the association class have a life-time dependency on the association.

• an associative type should exist in the background somewhere.

Generalization & Specialization

• Generalization: the activity of identifying commonalities among concepts and defining superclass (general concept) and subclass (specialized concepts) relationships.
• "is-a" relationship
• a subclass inherits from a superclass.

Benefits of generalization

• Code Reusability
• Cleaner/Simplified design
• Scalability
• Extensibility
• Easier Maintenance
• Testability
• Modularity

Class inheritance

• same as generalization and specialization.
• the subclasses inherit the attributes and methods of the superclasses.

• When a hierarchy is created, statements about the superclass apply to subclasses.
• A conceptual subclass should be a member of the set of the superclass.
• The conceptual subclass is a kind of superclass.
• <subclass> is a <superclass>
• Every instance of the <subclass> can be viewed as an instance of the <superclass>.

Subclass

• has additional attributes of interest.
• has additional associations of interest.
• is handled differently than the superclass or other subclasses.
• represents an object that behaves differently than the superclass or other subclasses.

Superclass

• Create a conceptual superclass when:
  • The potential conceptual subclasses represent variations of a similar concept.
  • The subclass fully conforms to the attributes and associations of its superclass. (100% rule)
• All subclasses have the same attributes and operations which can be factored out and expressed in the superclass.
• All subclasses have the same associations which can be factored out and related to the superclass.

Abstract Class

• a parent class that cannot be instantiated.
• at least one of its operations is abstract.
• an abstract operation has its signature defined in the abstract parent class, but the implementation is defined in the child class.
• create a high-level modelling vocabulary.

Interface & Realization

• a class with no attributes.
• can't be instantiated.
• simply declares a contract that may be realized by zero or more classes.
• to separate the specification from its implementation.
• only defines a specification for what the class should do and it never implies how it should do it.
• the class implementing an interface has a realization relationship with the interface.

Aggregation

• a type of whole-part relationship in which the aggregate is made up of many parts.
• Signified with a hollow diamond.
• implies the part may be in many composite instances.

Composition

• a stronger form of aggregation.
• the multiplicity at the composite end may be at most one
• Signified with a filled diamond.
• There is a create-delete dependency. Their lifetime is bound within the lifetime of the composite.

Dependency

• a relationship between two or more model elements whereby a change to one element
• The most common dependency stereotype is <<use>>, which simply states that the client makes use of the supplier in some way.

Visibility

• + public
• - private
• # protected
• ~ package

Example

UML Tools

• Draw.io
• Lucidchart
• Visual Paradigm
• StarUML
• Creately
• PlantUML