IB Computer Science HL · Year 2 · Lesson 19
Aggregation vs composition
Multiple-class OOP adds relationships and flexible behaviour through inheritance, polymorphism, abstraction, composition and patterns.
Paper 250 minutesB3.2.4
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Today’s targets
What you need to be able to do
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- ExplainThe role of composition and aggregation in class relationships
B3.2.4
Paper 2 lensMatch the depth of every response to the command term. Previously learned content can move quickly, but retrieval must still be accurate.
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Retrieve
Rapid Recall Deck
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Say the answer aloud before flipping. Mark secure knowledge quickly and spend time on the gaps.
- What is aggregation in a class relationship?: A 'has-a' relationship where the component object can exist independently of the object that uses or groups it.
- What is composition in a class relationship?: A stronger 'has-a' relationship where the component is tightly owned and its lifecycle is tied to the containing object.
- What is the main difference between aggregation and composition?: Aggregated components can exist independently; composed components are considered part of the whole and do not meaningfully exist outside it.
- Why use composition or aggregation instead of inheritance?: They build larger objects from smaller components without claiming an 'is-a' relationship, often reducing unnecessary coupling.
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B3.2.4 · Learn
Core knowledge and application
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B3.2.4Explain
The role of composition and aggregation in class relationships
Multiple-class OOP adds relationships and flexible behaviour through inheritance, polymorphism, abstraction, composition and patterns.
Exam moveGive the mechanism or relationship and make the reasons/causes explicit.
Required detail 1How to design objects by leveraging smaller component objects through composition and aggregation
Required detail 2That aggregation implies that the subcomponents can function independently of the aggregating class, while in composition, the subcomponents are tightly coupled and cannot exist outside the aggregating class
Assessment boundaryPaper 2
Explain it without notes
Explain: The role of composition and aggregation in class relationships in the context of a school platform with multiple interacting object types.
- How to design objects by leveraging smaller component objects through composition and aggregation
- That aggregation implies that the subcomponents can function independently of the aggregating class, while in composition, the subcomponents are tightly coupled and cannot exist outside the aggregating class
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Trace / construct
Programming checkpoint
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class Engine:
def start(self): return "running"
class Car:
def __init__(self):
self.engine = Engine() # compositionclass Engine { String start(){ return "running"; } }
class Car {
private final Engine engine = new Engine(); // composition
}Before you run itTrace the code by hand. Identify state changes, branch/loop behaviour, and the final result.
Modify it
Change one condition, input or operation so the program solves a slightly different problem. Predict the effect before editing.
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Apply
Transfer to a new scenario
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ScenarioA school platform with multiple interacting object types needs a design or technical decision related to today’s topic. Explain what matters and why.
- How to design objects by leveraging smaller component objects through composition and aggregation
- That aggregation implies that the subcomponents can function independently of the aggregating class, while in composition, the subcomponents are tightly coupled and cannot exist outside the aggregating class
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Exam lens
Paper 2 practice
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Build the response before checking notesUse precise terminology and match the required depth.
- Explain: The role of composition and aggregation in class relationships in the context of a school platform with multiple interacting object types.
Self-checkAnswer the exact command term. For explain, include mechanism/reason; for compare, pair criteria; for discuss/evaluate/justify, build supported reasoning and a conclusion.
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Homework
Finish the learning cycle
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IA — main task
Continue Criterion D development. Keep code readable, record meaningful implementation decisions, and maintain testing/evidence notes as you work.
Syllabus — short
Complete one targeted Paper 2/Paper 1 retrieval task from today’s lesson.
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End of Lesson 19.