The big idea: The classes are already in the description — they are the things it keeps talking about.
The attributes are what it says about them. The methods are what it says they do.
The nouns-and-verbs method: Underline the nouns: candidates for classes and attributes.
Underline the verbs: candidates for methods.
Crude, and it gets a first draft right surprisingly often.
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The UML box and what each part means. Step to 'The class' first.
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The three parts
- Name at the top
- Attributes in the middle: name and type
- Methods at the bottom, with their return type
- Always in that order
Visibility
- - private — reachable only inside the class
- + public — reachable from outside
- Attributes are normally private
- Methods are normally public
Relationships
- A plain line joins classes that know about each other
- A hollow triangle points at the class being inherited from
- Numbers on the line show how many, as in an ERD
Underlined means static: A UML entry drawn underlined belongs to the class itself rather than to each object.
A count of how many objects have been created is written underlined, because there is only ever one of it.
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One class, one responsibility
Methods that belong to the data
Inherit only for a genuine "is a"
Attributes private, methods public
Why design before coding: A class diagram is quick to draw and quick to change. Rewriting a class other classes already depend on is neither.
That is the argument the guide is making when it says UML aids effective design and planning.
How this is tested — you must identify sensible classes and put each method with the data it uses. It comes up two ways:
Paper 2 — working with code
- Construct a class design or UML diagram, 4-6 marks
- Identify classes and methods from a description
- Explain a design decision
Paper 2 — the algorithmic-thinking question
- Design from a description with no code required
- Say why one arrangement is better than another
The classic trap: Using inheritance where the relationship is really has a. A Library has Books; it is not a Book. Ask "is a" out loud before drawing the triangle.
A cinema books seats for screenings. Each screening shows one film in one room at a time, and customers book seats. Construct a class design.
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