CAIE 9618 · Paper 4 · Topic 4.2.1

UML &
Class Design

Class Diagrams · Attributes · Methods · Relationships · Multiplicity · Association vs Inheritance

CSZone Cambridge International AS & A Level Computer Science 9618

UML Class Diagram Structure

Three-Section Box Format

ClassName

- attributeName : DataType

- anotherAttr : DataType

+ MethodName(param:Type) : ReturnType

+ AnotherMethod() : VOID

- = private, + = public

BankAccount

- AccountNumber : STRING

- Balance : REAL

+ NEW(AccNo:STRING, Bal:REAL)

+ GetBalance() : REAL

+ Deposit(Amount:REAL) : VOID

+ Withdraw(Amount:REAL) : BOOLEAN

THREE SECTIONS

Section 1 (top) — class name, bold or underlined

Section 2 (middle) — attributes with visibility (+/-), name, and data type

Section 3 (bottom) — methods/operations with visibility, parameters, and return type

VISIBILITY SYMBOLS

Symbol	Meaning
+	Public
-	Private
#	Protected (accessible by subclasses)

Relationships in UML

How Classes Relate to Each Other

Relationship	UML Notation	Meaning	Example
Inheritance	Hollow triangle arrowhead (→ superclass)	IS-A: subclass extends superclass	Car IS-A Vehicle
Association	Solid line with optional arrowhead	HAS-A relationship; one class uses another	Student takes Exam
Aggregation	Hollow diamond at the whole end	Whole-part; part can exist without whole	Library HAS Books (book exists without library)
Composition	Filled diamond at the whole end	Stronger whole-part; part cannot exist without whole	House HAS Rooms (room cannot exist without house)

MULTIPLICITY (cardinality)

1
Exactly one

0..1
Zero or one

1..*
One or more

0..*
Zero or more

Example: Student (0..*) ——— (1) Tutor means each student has exactly 1 tutor; a tutor has 0 or more students.

Exam Practice

Cambridge-style questions

Question 1

A school system has classes Teacher and Department. Each department is managed by exactly one teacher, and each teacher can manage zero or one departments. Draw a UML class diagram showing the Teacher class (with attributes: Name:STRING, StaffID:STRING and method GetName():STRING) and the relationship between Teacher and Department with multiplicity labels. [4]

1

Teacher class box with three sections: class name, attributes (- Name : STRING, - StaffID : STRING), methods (+ GetName() : STRING).

1

Department class box shown (can be empty or with at least class name).

1

Association line (solid) connecting Teacher and Department.

1

Correct multiplicity: Teacher side: 0..1 (a department has zero or one teacher managing it); Department side: 1 (each teacher manages exactly one department) — OR the reverse depending on how the relationship direction is interpreted. Mark awarded for correct 1:1 or 0..1 labels with justification.

Common Mistakes

Don't lose easy marks

1

Leaving the visibility symbols off attributes and methods — CAIE mark schemes always look for + (public) and - (private). Forgetting these loses marks even if the attribute names and types are correct.

2

Drawing inheritance arrows the wrong way — the arrow always points TO the superclass (parent), not to the subclass. "Dog inherits from Animal" = arrow from Dog pointing to Animal.

3

Confusing aggregation and composition — both are "has-a" relationships but they differ in lifetime dependency. Composition = parts CANNOT exist without the whole. Aggregation = parts CAN exist without the whole. The exam often asks you to distinguish them with an example.

Topic Summary — 4.2.1

What You Need to Know

CLASS BOX FORMAT

3 sections: class name | attributes (- name : Type) | methods (+ Name(params) : ReturnType). + = public, - = private, # = protected.

RELATIONSHIPS

Inheritance (IS-A, hollow triangle → superclass). Association (HAS-A, solid line). Aggregation (hollow diamond, parts survive whole). Composition (filled diamond, parts die with whole).

MULTIPLICITY

1 (exactly one), 0..1 (zero or one), 1..* (one or more), 0..* (zero or more). Always label both ends of an association line.

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Next Video

4.2.2

Design Patterns

Observer · Strategy · Factory · Singleton

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