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IB DP Computer Science Study Notes

D.1.5 Relationships Between Objects

In Object-Oriented Programming (OOP), understanding how objects interact and connect with one another is fundamental. These interactions are defined through relationships that dictate how objects communicate and coexist within a system. This exploration into object relationships is vital for IB Computer Science students to grasp the complexities of software design.

The Essence of Object Relationships

Objects in OOP are not isolated; they exist in a state of co-dependency and hierarchy. This section delves into the three primary relationships defined in the IB Computer Science syllabus: dependency, aggregation, and inheritance.

Dependency: The "Uses" Relationship

Dependency occurs when one object requires another to perform its function. It’s a relationship that signifies a one-way interaction where one object 'uses' the functionalities or data of another.

  • Characteristics of Dependency:
    • Temporal Nature: The user object only needs the supplier object for a certain period.
    • Signal of Change: If the supplier object changes, the user object might also need to adapt.
    • Illustration: An 'Order' object may depend on a 'PaymentGateway' object to process payments.

Aggregation: The "Has a" Relationship

Aggregation describes a whole-part relationship, where one object is composed of several others, yet they exist independently.

  • Characteristics of Aggregation:
    • Part-whole Relationship: It represents an association where the part can exist independently of the whole.
    • Transitive Ownership: The whole object might be responsible for the lifecycle of its parts, but not necessarily.
    • Illustration: A 'Classroom' object contains multiple 'Desk' objects, but desks can exist outside of the classroom.

Inheritance: The "Is a" Relationship

Inheritance establishes a hierarchy where one class inherits properties and behaviors from another, forming an 'is-a' relationship.

  • Characteristics of Inheritance:
    • Hierarchical Classification: Objects are classified based on a generalization-specialization relationship.
    • Behavioural Inheritance: The inheriting object adopts the methods and attributes of its parent.
    • Illustration: A 'Bird' class can inherit from an 'Animal' class, as all birds are animals.

Delving Deeper: The Nature of Object Relationships

These relationships are not merely academic concepts; they are applied practices that significantly impact how software is structured and maintained.

Dependency in Software Design

  • Handling Dependencies:
    • Loose Coupling: Aim for minimal dependencies to reduce the ripple effect of changes.
    • Interface Usage: Define an interface for dependencies to promote flexibility and interchangeability.

Aggregation and Composition

Aggregation and composition both describe whole-part relationships, but with different intensities of ownership.

  • Differentiating Aggregation from Composition:
    • Aggregation: Implies a weaker association where parts can exist independently of the whole.
    • Composition: Suggests a stronger relationship where the parts do not typically exist without the whole.

Inheritance for Code Reusability

Inheritance is a powerful tool for code reuse but must be used judiciously to avoid creating unnecessary complexities.

  • Strategic Inheritance:
    • Avoid Deep Hierarchies: Deep inheritance trees can make code difficult to read and maintain.
    • Favor Composition Over Inheritance: When possible, use composition to achieve code reuse without the constraints of inheritance.

Utilising UML for Object Relationships

Unified Modelling Language (UML) diagrams serve as a blueprint for object relationships, providing a visual representation of a system's architecture.

Dependencies in UML

  • UML for Dependencies:
    • Notation: A dashed arrowhead line indicates a dependency relationship in UML diagrams, pointing from the dependent to the supplier.

Aggregations in UML

  • UML for Aggregations:
    • Notation: Aggregation is shown with a hollow diamond at the aggregate (whole) end of the line, connecting to the part.

Inheritance in UML

  • UML for Inheritance:
    • Notation: Inheritance is depicted with a plain arrowhead line, pointing from the subclass to the superclass.

Practical Scenarios: Applying Object Relationships

Understanding the theory behind object relationships is the first step. Applying them to solve real-world problems is where they truly shine.

Case Study: An Online Shopping Platform

  • Dependency in Action:
    • A 'Customer' object uses a 'ShoppingCart' object to make purchases. The customer object is dependent on the shopping cart for the duration of the shopping session.
  • Aggregation Illustrated:
    • The 'ShoppingCart' aggregates 'Product' objects, meaning the cart contains products but does not define them.
  • Inheritance Implemented:
    • A 'DigitalProduct' class inherits from a 'Product' class, extending its functionalities to cater to digital goods specifically.

Object Relationships: The Bigger Picture

Mastering the understanding of object relationships is more than an academic exercise; it equips students with the foundational skills necessary to architect robust and efficient software systems.

The Role of Relationships in Software Architecture

  • Building Flexible Systems:
    • Design Patterns: Many design patterns, like Observer or Strategy, depend on object relationships to decouple system components.
    • System Evolution: A well-thought-out object relationship structure allows the system to evolve with less friction and fewer errors.

Relationships and System Maintenance

  • Easing Maintenance:
    • Refactoring Ease: Properly defined relationships make refactoring a system much easier, as the connections between objects are clear.
    • Testing Simplicity: Object relationships, when well designed, can simplify the creation of unit tests and improve code coverage.


Through this detailed exploration of object relationships, IB Computer Science students are better equipped to analyse, design, and implement software that is both modular and scalable. These relationships form the bedrock of object-oriented design, enabling the construction of systems that are both robust and flexible, prepared to adapt to new challenges and requirements.


Yes, objects can have more than one type of relationship with other objects in a system. For instance, in an e-commerce system, a 'Customer' object may have a dependency relationship with a 'ShoppingCart' object, as the customer uses the shopping cart to add products. Simultaneously, the same 'Customer' object could be part of an aggregation relationship with a 'CustomerAccounts' object, where the latter aggregates all 'Customer' objects. These diverse relationships reflect the multifaceted interactions objects can have within a system, and recognising them is key to accurately modelling the system's behaviour.

An object relationship is characterised as aggregation rather than composition when the contained objects can exist independently of the containing object. Aggregation implies a binary relationship where the lifetime of the 'part' is not strictly managed by the 'whole'. For example, a 'Class' object aggregating 'Student' objects would be aggregation because students can exist without the class. In contrast, composition implies a stronger relationship where the 'part' cannot exist without the 'whole'. It's important to distinguish between the two to accurately model and implement the system's relationships, reflecting the real-world constraints and dependencies of the domain.

Minimising dependencies between objects is important to reduce the coupling within a system. High coupling leads to systems where objects are tightly interconnected, making changes difficult and error-prone, as alterations in one object could have unforeseen consequences on dependent objects. To minimise dependencies, one can employ design patterns such as the Dependency Inversion Principle, which suggests depending on abstractions rather than concrete implementations. Using interfaces or abstract classes to define contracts that objects must adhere to can also decouple the system. Moreover, segregating responsibilities clearly through the Single Responsibility Principle ensures that objects do not have unnecessary dependencies.

One common pitfall when using inheritance is creating unnecessarily deep inheritance hierarchies, which can lead to a complex and rigid class structure that is difficult to understand and maintain. This often occurs when inheritance is used for code reuse rather than to model a natural hierarchical relationship. Another issue is the misuse of inheritance when composition would be more appropriate, known as the "inheritance versus composition" problem. Students should recognise that just because two classes share some functionality, it does not mean one should inherit from the other; sometimes, it's better to have shared functionality in a separate class that is used by both classes.

Identifying object relationships early in the design process significantly benefits the development lifecycle by establishing a clear structure and communication protocol amongst objects. This foresight helps in creating a more logical and cohesive architecture, which eases implementation, testing, and maintenance. For instance, understanding the 'has-a' and 'is-a' relationships can avoid unnecessary redundancy in code, as developers can leverage inheritance for shared attributes and behaviours, and use composition or aggregation to express ownership and containment without duplication. Early identification of dependencies can also highlight potential areas for decoupling, allowing for more modular and reusable code, ultimately leading to a more robust and adaptable system.

Practice Questions

Explain how the 'uses' relationship (dependency) between objects in a software system can affect system maintenance.

A 'uses' relationship, or dependency, means that an object requires another to function. Excessive dependencies can complicate system maintenance since a change in one object might necessitate changes across all dependent objects, leading to a higher risk of bugs. For instance, if a 'PaymentProcessor' object is heavily used by various objects for transaction services, any modification in its methods requires a careful update of all dependent objects. This interconnectivity demands rigorous impact analysis and testing, making maintenance more challenging and time-consuming, highlighting the importance of minimizing and managing dependencies.

Describe a scenario in a software system where an aggregation relationship might change to a composition relationship. Justify the reason for this change.

In a university management system, 'Department' objects initially aggregate 'Lecturer' objects, implying lecturers can belong to multiple departments. However, the system's policy changes to assign lecturers exclusively to one department. The relationship shifts to composition, where each 'Department' object now composes 'Lecturer' objects. This change reflects the new policy, enforcing that a lecturer's lifecycle is tightly coupled with their department, ensuring that if the department ceases to exist, so does the lecturer's association with the university. This enforces a stricter and more representative relationship within the system.

Alfie avatar
Written by: Alfie
Cambridge University - BA Maths

A Cambridge alumnus, Alfie is a qualified teacher, and specialises creating educational materials for Computer Science for high school students.

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