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IB DP Computer Science Study Notes

D.1.7 Constructing Related Objects

In the realm of object-oriented programming (OOP), the construction of related objects is a central concept that facilitates the modelling of complex systems through simple, interacting components. This section provides an in-depth look at defining and interlinking a limited set of objects to encapsulate a problem's requirements effectively.

Object Definition and Classes

  • Class Overview: A class serves as a template for objects, defining the attributes and behaviours that the instantiated objects will possess.
    • Attributes and Their Significance: Attributes, also known as properties or fields, hold the data pertinent to objects. For instance, a `Book` class may include attributes such as `title`, `author`, and `ISBN`.
    • Methods and Operations: Methods, the operations that can be performed on an object's data, are defined within the class. In a `Book` class, methods could include `borrow()`, `returnBook()`, and `reserve()`.

Object Instantiation

  • Instantiation Process: The instantiation is the act of creating an actual object from a class, equipping it with its attributes and methods defined by the class.
    • Instance Characteristics: Each instance is a unique entity. For example, two instances of a `Book` class represent two different books, each with its own title and author.

Relationships Among Objects

  • Types of Relationships: Understanding how objects relate and interact is crucial in OOP, typically encapsulated as dependency, aggregation, and inheritance relationships.
    • Dependency Relationship: A dependency exists when an object's method invokes another object. For instance, a `Library` object might depend on a `Book` object to perform the `lendOut()` method.
    • Aggregation Relationship: This 'has-a' relationship suggests that one object is a part of another larger object. A `Library` may aggregate `Book` objects.
    • Inheritance Relationship: This 'is-a' relationship allows a class to inherit attributes and methods from another class, known as the superclass. An `EBook` class could inherit from `Book`, indicating that an `EBook` is a type of `Book`.

Case Study: Library Management System

  • Defining the Objects: For a library management system, we might define three primary objects: `Library`, `Book`, and `Patron`.
    • Library Object:
      • Attributes: `name`, `location`, `catalogue`.
      • Methods: `addBook()`, `registerPatron()`, `lendOut()`.
    • Book Object:
      • Attributes: `title`, `author`, `isCheckedOut`.
      • Methods: `borrow()`, `returnBook()`, `reserve()`.
    • Patron Object:
      • Attributes: `patronId`, `name`, `borrowedBooks`.
      • Methods: `borrowBook()`, `returnBook()`, `searchCatalogue()`.

Constructing the Objects

  • Library Object Construction:
    • Attributes: The `name` might be a string, while `catalogue` could be a collection of `Book` objects.
    • Methods: `addBook()` adds a new book to the catalogue, while `registerPatron()` adds a new library member.
  • Book Object Construction:
    • Attributes: `title` and `author` are strings, whereas `isCheckedOut` is a Boolean indicating availability.
    • Methods: `borrow()` changes the `isCheckedOut` status and records the borrower.
  • Patron Object Construction:
    • Attributes: `patronId` could be an integer or string, uniquely identifying a library member.
    • Methods: `searchCatalogue()` allows the patron to search for books within the library's catalogue.

Inter-Object Communication

  • Library-Book Interaction: A `Patron` object interacts with the `Library` to borrow a `Book`, which in turn updates the `Book` object's `isCheckedOut` attribute.
    • Implementation: When a `Patron` calls the` borrowBook()` method on the `Library`, the `Library` invokes the `borrow()` method on the `Book`.
  • Patron-Library Interaction: The `Patron` uses the `Library` object's `searchCatalogue()` method to find books, demonstrating a dependency relationship.

Reducing Dependencies

  • Encapsulation: By keeping an object's data internal and exposing only the necessary methods, dependencies between objects can be reduced. For instance, the `Book` doesn't need to know about the `Patron`'s details, just that it can be borrowed.
  • Interfaces and Contracts: Interfaces can be employed to define a set of methods that must be implemented by any class that agrees to the contract, without enforcing how the methods are to be executed.

Data Types and Objects

  • Choosing Appropriate Data Types: Selecting the correct data types for an object's attributes is essential for ensuring that operations on the data are logical and efficient.
    • Primitive Data Types: These data types include integer, real, string, and Boolean. For example, a `Patron`'s `patronId` might be an integer for easy management.

Passing Data Between Objects

  • Parameter Passing: Objects can communicate by passing data to and from methods as parameters. This can be seen when a `Patron` passes a `Book` object to the `borrowBook()` method of the `Library`.
    • Return Types: Methods can return data items to the calling object, which allows for continuous interaction and data exchange between objects.

This detailed exploration of constructing related objects within the OOP paradigm underscores the importance of defining clear relationships and interactions between objects. The case study illustrates practical application, showing how objects representing real-world entities can be created, linked, and utilised to solve complex problems. Understanding these concepts paves the way for students to develop efficient, effective, and scalable object-oriented applications.


Polymorphism allows objects to be treated as instances of their parent class rather than their actual class. This affects the construction and interaction of related objects by enabling a single method to operate on objects of different classes. For example, if a `PaymentProcessor` class has a method `processPayment` that takes a `Payment` object, polymorphism allows for instances of any subclass of `Payment`, like `CreditCardPayment` or `PaypalPayment`, to be passed to this method. This enables the construction of flexible systems where related objects can be interchanged and managed under a common interface, greatly simplifying the management of object relationships and interactions.

Encapsulation strengthens relationships between objects by controlling the access to and from an object's data and behaviour. This concept promotes the idea of 'information hiding' which means that the internal representation of an object is hidden from the outside. In doing so, objects do not need to reveal their implementation details to other objects they interact with, only their interfaces. This allows objects to be changed internally without affecting other objects that use them, making the relationships more flexible and maintainable. For instance, a `BankAccount` object can change the way it calculates interest internally without the `Customer` object, which holds a reference to the `BankAccount`, needing to know any of the implementation details.

Access modifiers are keywords in object-oriented languages that set the accessibility of classes, methods, and other members. They are essential for regulating the relationships between objects as they determine how one object can access another's data and methods. For instance, the `public` access modifier allows members to be accessed from any other class, which can be useful for methods that are intended to be called across different objects. The `private` modifier restricts access to the class itself, which is important for encapsulating the internal workings of the object and only exposing what is necessary for interaction. The `protected` modifier allows access within the same class and its subclasses, facilitating inheritance relationships. Access modifiers are thus key to building robust object-oriented designs that properly control how objects are constructed and interact.

Method overloading is the process of defining multiple methods with the same name but different parameter lists within the same class. Its significance in constructing related objects is that it provides flexibility in how objects interact with each other. For example, in a class `Database`, one could overload the `addRecord` method to accept different types of objects, such as `Employee`, `Department`, or `Project`. This allows for a variety of objects to be related to the database in different ways, depending on the information they carry. It also contributes to cleaner and more intuitive code, as the method's functionality can be tailored to the type of object it is dealing with, making the relationships between objects clearer and more robust.

Constructors are special methods in a class that are called when a new object is created. They play a crucial role in constructing related objects as they initialise the new object's state with default or provided values. For instance, in a class `Person`, a constructor might initialise the person's `name` and `age` attributes. When creating related objects, constructors can also set up relationships. For example, a `Person` object could be passed to a `Job` object's constructor to link the person with a specific role, establishing a relationship between these two objects. This is essential in constructing an ecosystem of interrelated objects where each object is correctly initialised to interact with others from the moment it is created.

Practice Questions

Describe the process of constructing a set of related objects in an object-oriented program and provide an example that includes at least two different types of relationships between the objects.

The process begins by defining classes which serve as templates for objects, specifying their properties and behaviours. Instantiation is then used to create objects from these classes. In a school management system, for instance, a `Student` object might have a dependency relationship with a `Classroom` object as it uses methods from the `Classroom` to attend classes. There might also be an aggregation relationship between a `School` object and `Classroom` objects, since a school comprises multiple classrooms. This exemplifies how related objects interact to form a cohesive program.

Given the following classes: Car, Engine, and Wheel, identify the most likely type of relationship between them and justify your answer.

A `Car` class would most likely have an aggregation relationship with `Engine` and `Wheel` classes, because a car is composed of these parts but they can also exist independently. This 'has-a' relationship indicates that while the `Engine` and `Wheel` are integral to the functionality of the `Car`, they are distinct entities that the `Car` object aggregates. The `Engine` and `Wheel` do not inherit from `Car` as they are not types of cars, nor are they mere dependencies; they are essential components that make up the `Car` object's structure.

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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