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8532d5 lisa 2026-06-08 11:36:12
C05: add Object Descriptions & Class Diagrams page (5th design tool) Class diagram (Animal example) + notation key, properties/methods/events incl. GUI events, links to data dictionary (attributes) and pseudocode (methods, Level 9). Added to home index under C5-1 and cross-linked from pseudocode page. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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> **DRAFT** — under teacher review.
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# Object Descriptions and Class Diagrams
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The Hamilton and Alexandra College · Year 12 · 2026
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If your solution uses **objects and classes**, the fifth design tool is the **object description**. VCAA defines it as a tool that *"describes all of the relevant properties, methods and events in an object or class."* A **class diagram** is the standard way to draw a whole set of object descriptions at once and show how the classes relate.
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This is a **Level 7+** tool in C5-1, and at **Level 9** you are also expected to write pseudocode for the methods inside your classes.
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> [!NOTE]
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> **Only if you use OOP.** If your solution genuinely does not use classes and objects, you do not need object descriptions. But most SAT projects of any real size have at least a few classes — and a class diagram is the clearest way to show them.
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---
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## Reading a class diagram
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Each box is a **class**. The top section lists its **attributes** (properties), the bottom section lists its **methods** (actions), and the arrows show **relationships** between classes.
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```mermaid
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---
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title: Animal example
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---
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classDiagram
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note "From Duck till Zebra"
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Animal <|-- Duck
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note for Duck "can fly<br>can swim<br>can dive<br>can help in debugging"
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Animal <|-- Fish
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Animal <|-- Zebra
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Animal : +int age
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Animal : +String gender
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Animal: +isMammal()
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Animal: +mate()
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class Duck{
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+String beakColor
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+swim()
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+quack()
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}
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class Fish{
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-int sizeInFeet
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-canEat()
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}
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class Zebra{
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+bool is_wild
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+run()
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}
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```
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Read it like this: `Animal` is the **base class**. `Duck`, `Fish` and `Zebra` are all *a kind of* Animal, so each one **inherits** Animal's attributes (`age`, `gender`) and methods (`isMammal()`, `mate()`) and then adds its own. `Duck` adds `beakColor`, `swim()` and `quack()`; it doesn't have to redefine `age` because it already has it from `Animal`.
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---
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## The notation you need
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| Symbol | Meaning |
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|---|---|
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| `+` | **Public** — usable from outside the class |
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| `-` | **Private** — usable only inside the class |
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| `age : int` | An **attribute** and its **data type** |
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| `isMammal()` | A **method** (an action the object can perform) |
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| `isUpcoming() : bool` | A method and its **return type** |
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| `Animal <|-- Duck` | **Inheritance** — "Duck *is a* Animal" |
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> [!TIP]
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> Every attribute should have a **data type** and every method should show its **parameters and return type**. "`age`" alone is weak; "`+ age : int`" is design-ready. This is exactly the precision that lifts C5-1 from the middle bands into 7–9.
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---
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## Properties, methods — *and events*
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A class diagram shows properties and methods well. But VCAA object descriptions also include **events** — especially for **GUI objects** (buttons, text fields, menus). An event is something the object *responds to*, like a click.
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For a login button you might describe the object like this:
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| Aspect | Example for `btnLogin` (a button) |
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|---|---|
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| **Properties** | `caption` = "Log in", `enabled` = true |
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| **Methods** | `setEnabled(state)` |
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| **Events** | `onClick` → runs the *Validate User* process |
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So: the class diagram carries your properties and methods; for interface objects, add a short note or table documenting the **events** as well.
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---
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## How it connects to your other design tools
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The object description is not an island — it ties the whole detailed design together:
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- **Properties ↔ data dictionary.** Every attribute is a field. `studentID : int` becomes a data-dictionary row with a type, size and format. See [Data Dictionary — Format is not Type](/sd/C05/Data%20Dictionary%20-%20Format%20is%20not%20Type).
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- **Methods ↔ pseudocode.** At Level 9 you write pseudocode for each method. The `is_passing()` example on the pseudocode page is exactly a method from a class diagram, written out. See [VCAA Pseudocode — Not Python](/sd/C05/VCAA%20Pseudocode%20Not%20Python).
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- **Classes ↔ mock-ups and IPO charts.** The objects are what sit *behind* your screens and processes.
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---
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## Common mistakes
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### Mistake 1: An object description with no methods
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> ~~A box listing only attributes~~
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That is just a data dictionary in disguise. An object description must include the **methods** (and, for GUI objects, the **events**) — that is what makes it an *object*.
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### Mistake 2: No visibility or data types
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> ~~`beakColor`, `swim`~~
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Show whether each member is public (`+`) or private (`-`), give every attribute a **type**, and give every method its **parameters and return type**.
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### Mistake 3: Classes drawn with no relationships
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If `Teacher` is a kind of `Student`, draw the inheritance arrow. A diagram of disconnected boxes hides the design thinking the arrows are meant to show.
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### Mistake 4 (Level 9): A class diagram but no method pseudocode
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At Level 9 the methods in your diagram must be backed by pseudocode. A diagram on its own caps you below the top band.
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---
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## Check Your Understanding
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1. In the Animal diagram, name two things `Duck` inherits from `Animal`, and one thing `Duck` adds of its own.
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>| **Inherits** (any two): `age`, `gender`, `isMammal()`, `mate()`. **Adds** (any one): `beakColor`, `swim()`, `quack()`.
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2. What do `+` and `-` mean in front of an attribute or method?
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>| `+` means **public** (accessible from outside the class); `-` means **private** (accessible only inside the class).
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3. Which design tool turns the *methods* in your class diagram into step-by-step logic — and at which level is it required?
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>| **Pseudocode**, required at **Level 9** (pseudocode for the functions and methods within your classes). See [VCAA Pseudocode — Not Python](/sd/C05/VCAA%20Pseudocode%20Not%20Python).
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---
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## Credits
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- Class-diagram example adapted from the [Mermaid](https://mermaid.js.org/) documentation's *Animal* sample.
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---
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## See also
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- [VCAA Pseudocode — Not Python](/sd/C05/VCAA%20Pseudocode%20Not%20Python) — write the pseudocode for each method (Level 9)
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- [Data Dictionary — Format is not Type](/sd/C05/Data%20Dictionary%20-%20Format%20is%20not%20Type) — each attribute is a data-dictionary field
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- [Sketch vs Mock-up](/sd/C05/Sketch%20vs%20Mock-up)
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- [C05 Resources](/sd/Resources/C05-Resources)
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