Has-a relationships¶

Learning objectives

Understand the two types of 'has-a' relationships
Practice to use composition

For teachers

Prerequisites are:

Learners have .

Teaching goals are:

Learners .

Teaching form used:

.

Lesson plan:

5 mins: prior knowledge
Imagine a university, departments, professors, the department of biology, the department of physics. Which relationships are there?
A university has departments. Departments have professors. Is this the same type of relationship? Hint: what happens if the bigger one ends existing?
5 mins: presentation
10 mins: demonstration is_zero
25 mins: challenge
10 mins: feedback

`struct` relations¶

Good () data structures:

Increase expressiveness
Bundles data that belongs together
Ensures correct state of the program

Class relations¶

In this course, we use only these types of relations:

composition, a 'has-a' relationship
inheritance, a 'is-a-type-of' relationship
association, a 'uses' relationship

classDiagram
  Person
  Name
  Person *-- Name

An example of composition

In English this would be read as 'a person has a name'.

classDiagram
  Animal
  Duck
  Animal <|-- Duck

An example of inheritance

In English this would be read as 'a duck is a type of animal'.

classDiagram
  Simulation
  Results
  Simulation --> "produces" Results

An example of association

In English this would be read as 'a simulation creates results'.

A 'has-a' relationship¶

When one data type has a 'has-a' relationship with another, this is called composition.

classDiagram
  class Coordinat{
    -x
    -y
  }

A Coordinat has an x and y. This is close to English, as it should be.

However, 'has a' is not always a literal translation. Below is an incorrect class diagram:

classDiagram
  note "Incorrect class diagram!"
  class Simulation {
    - results
  }
  Results

  Simulation *-- Results

Although we could say 'a simulation has results', a simulation does not own these results. It does not manage the state of the results. Instead, a simulation has an association with the results it produces.

Composition versus aggregation¶

A university has departments, where a department has professors. These are two different 'has-a' relationships.

Has-a relationship	Composition	Aggregation
Example	A university has departments	A department has professors
Ownership of its elements	Yes, a university owns its departments	No, a department allows professors to group
What happens to elements when destroyed	Destroyed: when a university closes, all departments close	Left intact: when a department closes, all professors live afterwards

In this course, we never use aggregation

Composition of structs¶

classDiagram
  class Particle{
    -position
    -velocity
  }
  class Velocity{
    -delta_x
    -delta_y
  }
  class Position{
    -x
    -y
  }
  Particle *-- Velocity
  Particle *-- Position

C.1. Organize related data into structures (structs or classes)

Exercise 1: design a struct (15 mins)¶

Goal:

to convince design is trickier than one thinks
to convince design has implications
to grow appreciation of classes

What are their elements? Which do you guess are structures? Were they?

A coordinate in 3 dimensions
A velocity in two dimensions
A circle
A square

Exercise 2: a coordinate in 3 dimensions¶

consists out of an x, y and z coordinate
struct: all combinations valid

But:

Spherical coordinate: radial distance, polar angle, azimuthal angle
distance must be positive, hence class

Exercise 3: a velocity in two dimensions¶

consists out of a delta_x, delta_y: struct
consists out of a angle, speed (:monocle_face:): maybe class

Exercise 4: a circle¶

A circle has:

a center coordinat and ...
- a diameter (:monocle_face:): maybe struct
- a radius (:monocle_face:): maybe struct
three coordinats: class, must avoid colinearity

Exercise 5: a square¶

A square has

a center coordinat and size (:monocle_face:) and ...
- maybe an angle (:monocle_face:)
a top-left coordinat and size (:monocle_face:) and ...
- maybe an angle (:monocle_face:)
a two opposing coordinats: struct?