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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** |
31bafd lisa 2026-06-08 11:43:19
C05: rebuild Data Dictionary page; fix class-diagram notation table - Rename 'Data Dictionary - Format is not Type' → 'Data Dictionary' (now a full page): when-to-use (data sources), 7 headings, worked student example, three videos (Kalodikis), Wikipedia data/metadata image (CC0), student-example activity, ScotRail extension; keeps the format-vs-type section. Updated 4 inbound links. - Class-diagram page: fix broken 'notation' table — the `<|--` cell contained a pipe that corrupted the markdown table; moved inheritance into prose. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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**Relationships** are shown with arrows between classes. The inheritance arrow `<|--` means *"is a kind of"* — writing `Animal <|-- Duck` says **Duck is a kind of Animal**, so Duck inherits all of Animal's attributes and methods and then adds its own. (Two others you may meet: *composition* `*--`, "is made of", and *association* `-->`, "uses".)
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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> [!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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1abde1 lisa 2026-06-08 11:55:33
C05 Object Descriptions: add written-form section with video + worked tables Adds an 'object descriptions — the written form' section: embeds the Godot abstract-classes video (Queble), with worked object-description tables for Animal/Dog and the Skill / SkillEffect / ApplyDamage hierarchy ('object descriptions about skills'), and an inheritance-pattern TIP. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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## Object descriptions — the written form
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A class diagram is the *picture*. An **object description** is the *written* version of the same information: a small table, one per class, listing each property and method (and, for interface objects, each event). VCAA accepts either form — most folios use both, the diagram for the overview and object descriptions for the detail.
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The video below builds two worked examples in a game engine. Watch how each class is captured by its properties and methods — and notice the idea of an **abstract class**: a base class that exists only as a *template* for others to inherit from, and is never created on its own (exactly like `Animal` in the diagram above — you never make a plain "Animal", only a Duck or a Dog).
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**🎯 Watch for:** two class families — `Animal → Dog`, and a `Skill` made of `SkillEffect`s — and how the base class declares a method that each child class then fills in.
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{{Video|src=https://www.youtube.com/watch?v=h21Lmg4o1Ks}}
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> [!NOTE]
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> This is a Godot (game-engine) tutorial, so the code is GDScript — but the *object-description skill* is identical to what VCAA asks for. Read it for the structure, not the language.
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### Example 1 — Animal and Dog
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**`Animal`** — an abstract base class (a template; never created on its own)
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| Member | Kind | Returns / type | Description |
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|---|---|---|---|
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| `make_sound()` | Method (abstract) | `void` | The sound the animal makes. Declared here but **not** implemented — every subclass must define its own. |
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**`Dog`** — extends `Animal`
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| Member | Kind | Returns / type | Description |
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|---|---|---|---|
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| `make_sound()` | Method (override) | `void` | Implements the abstract method — outputs "woof". |
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### Example 2 — a Skill made of effects ("object descriptions about skills")
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A `Skill` (think turn-based RPG) holds a list of effects. When you activate it, it loops through those effects and applies each one. The effects share an abstract base, `SkillEffect`, so new effects can be added without ever changing the `Skill` class.
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**`Skill`**
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| Member | Kind | Returns / type | Description |
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|---|---|---|---|
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| `effects` | Property | array of `SkillEffect` | The effects this skill applies. |
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| `activate(target)` | Method | `void` | Loops through `effects` and applies each one to the target. |
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**`SkillEffect`** — abstract base class (template only)
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| Member | Kind | Returns / type | Description |
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|---|---|---|---|
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| `apply(target)` | Method (abstract) | `void` | Applies this effect to the target. Each concrete effect defines its own. |
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**`ApplyDamage`** — extends `SkillEffect`
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| Member | Kind | Returns / type | Description |
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|---|---|---|---|
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| `damage_amount` | Property | `int` | How much damage to deal. |
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| `apply(target)` | Method (override) | `void` | Subtracts `damage_amount` from the target's health. |
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> [!TIP]
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> The same pattern runs through both examples: a **base class declares a method** (`make_sound`, `apply`) and each **child class fills it in**. Showing that in your object descriptions — the shared method on the parent, the specific version on each child — is exactly the inheritance evidence that scores at the top of C5-1.
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---
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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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## 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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31bafd lisa 2026-06-08 11:43:19
C05: rebuild Data Dictionary page; fix class-diagram notation table - Rename 'Data Dictionary - Format is not Type' → 'Data Dictionary' (now a full page): when-to-use (data sources), 7 headings, worked student example, three videos (Kalodikis), Wikipedia data/metadata image (CC0), student-example activity, ScotRail extension; keeps the format-vs-type section. Updated 4 inbound links. - Class-diagram page: fix broken 'notation' table — the `<|--` cell contained a pipe that corrupted the markdown table; moved inheritance into prose. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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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](/sd/C05/Data%20Dictionary).
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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- **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)
31bafd lisa 2026-06-08 11:43:19
C05: rebuild Data Dictionary page; fix class-diagram notation table - Rename 'Data Dictionary - Format is not Type' → 'Data Dictionary' (now a full page): when-to-use (data sources), 7 headings, worked student example, three videos (Kalodikis), Wikipedia data/metadata image (CC0), student-example activity, ScotRail extension; keeps the format-vs-type section. Updated 4 inbound links. - Class-diagram page: fix broken 'notation' table — the `<|--` cell contained a pipe that corrupted the markdown table; moved inheritance into prose. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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- [Data Dictionary](/sd/C05/Data%20Dictionary) — each attribute is a data-dictionary field
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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- [Sketch vs Mock-up](/sd/C05/Sketch%20vs%20Mock-up)
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- [C05 Resources](/sd/Resources/C05-Resources)
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