W2561 UML Class Diagrams

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Prerequisites

• W1095 Flow Charts

Research

• Read Class Diagram (Wikipedia)
• Read (Niraj Bhatt)
• Watch UML Class Diagrams (Lucidchart - YouTube)
• Read Counterinsurgency Field Manual (Farnam Street)

Examples

Dependency

Alpha uses Bravo as a parameter to Alpha.f(). A transient relationship exists.

class Alpha {
    func f(bravo:Bravo) {
        bravo.g()
    }
}

Aggregation

Alpha has-a Bravo but Bravo's existence is independent of Alpha.

class Alpha {
    var bravo : Bravo?

    init(bravo:Bravo) {
        self.bravo = bravo
    }
}

Composition

Alpha has-a Bravo and owns Bravo; Bravo's existence is dependent on Alpha.

class Alpha {
    let bravo : Bravo

    init() {
        bravo = Bravo()
    }
}

Key Concepts

UML Class Relation Arrows

• General
  • has-a/part-of generally means that one class has (either through ownership or aggregation) another object
  • is-a generally means that one class is a subtype of another
  • can-do-a generally means that one class can perform some function as described by an interface
• Class Diagrams provide a means for communicating the general, static, conceptual model of an application
• Classes are represented by a box with with three compartments
  • The top compartment contains the name of the class, capitalized and centered
  • The middle compartment contains the attributes (properties) of the class, left-aligned in camel case
  • The bottom compartment contains the operations (methods) of the class, left-aligned in camel case
    • Abstract operations are indicated by using italics for the operation name
• An abstract class is indicated by using italics for the class name
• Visibility notations may be placed in front of the attributes and operations
  • + indicates public access
  • - indicates private access
  • # indicates protected access
• Scope notations may be used to specify the scope of attributes and operations
  • An underline indicates classifier (static) scope
  • The lack of an underline indicates instance scope
• A relationship describes logical connections between entities
  • Class-Level relationships
    • Inheritance indicates that one of the classes is a specialized form (subclass) of the other (superclass). This is also known as an is a relationship, e.g. "a dog is a mammal". This is indicated on a diagram by drawing a line from the subclass to the superclass with a hollow triangle on the superclass end.
    • Realization indicates that one of the classes (the client) realizes (implements) the behavior specified by the other class (the supplier or interface). This is indicated on the diagram by drawing a dashed line from the client to the interface with a hollow triangle on the interface end. The interface itself has an <<interface>> tag above the "class" name.
  • Instance-Level relationships
    • A dependency represents a semantic relationship between two elements. It is represented as a dashed line with an arrow head pointing in the direction of the dependency. Note that actual interaction may be transient and limited to a class reference as an argument to a method.
    • An association represents a tighter link than a dependency when the relationship generally exists throughout the lifetime of the class as a reference to another class.
      • A unidirectional association is represented as a solid line with an arrow head pointing in the direction of the association.
      • A bidirectional association is represented as a solid line between the classes.
      • Aggregation is a form of weak association with a "part-of" relationship, where the contained classes do not have a lifecycle dependency on the container yet are "used" by it. It is represented with a hollow diamond on the containing class with a line drawn to the contained class.
      • Composition is a form of strong association where the contained class is "owned" by the container. It is represented as a filled diamond on the containing class with a line drawn to the contained class.

Exercises

1. Construct a UML diagram (either on paper or electronic tool such as www.draw.io). Be sure to include all important properties and methods and ensure that relationships between classes are represented correctly.