Classes and Their Relationships: Modeling Real-World Entities and Interactions

In object-oriented programming (OOP), one of the fundamental steps is identifying classes and defining the relationships between them. Classes are used to model real-world entities, while relationships define how these entities interact. This tutorial will explain the process of identifying classes and their relationships, focusing on real-world examples and applying the principles from the previous tutorials on OOP, similar to the Car and Driver example used earlier.

1. Identifying Classes

Classes are blueprints for objects, representing entities in the real world. They encapsulate attributes (data) and methods (behaviors) associated with those entities. The first step in modeling is to identify key entities that need to be represented in your system.

Steps to Identify Classes:

• Look for nouns in the problem description. Nouns often represent entities that can become classes.
• Think about the real-world objects you are dealing with.
• Each class should represent a meaningful entity and should encapsulate all the data and behavior related to that entity.

Example (from the earlier Car-Driver analogy):

In the context of a Car example:

• Car is a class because it is a real-world object with identifiable attributes and methods.
• Driver is another class, as it represents a person who drives the car and interacts with it.

The Car class might have attributes like make, model, and year, and methods like drive() and brake(). Similarly, the Driver class might have attributes like name, licenseNumber, and age, and methods like driveCar() and parkCar().

Another Example (Library Management System):

In a library management system, some possible classes could be:

• Book: Represents a book in the library.
• Member: Represents a member who can borrow books.
• Librarian: Manages the library’s inventory and member information.
• Issue/Loan: Represents the process of a book being borrowed by a member.

2. Defining Attributes and Methods

Once the classes are identified, the next step is to define their attributes (data members) and methods (behaviors).

• Attributes are the properties of the class that describe its state. For example, the Car class may have attributes such as make, model, color, and year.
• Methods are actions the class can perform. For example, a Car can drive(), stop(), or park().

Example (Car Class):

Attributes:

• String make
• String model
• int year
• String color

Methods:

• void drive(): The car can be driven.
• void park(): The car can be parked.

Similarly, in the Library Management System, we can define:

Book Class:

• Attributes: title, author, isbn, availableCopies.
• Methods: borrow(), returnBook().

Member Class:

• Attributes: name, membershipID, address.
• Methods: borrowBook(), returnBook().

3. Identifying Relationships Between Classes

Classes in a system do not operate in isolation. They interact with one another, and these interactions are modeled through relationships. The primary types of relationships in OOP include Association, Aggregation, Composition, and Inheritance.

3.1 Association

The association represents a general relationship between two classes. It describes how objects from one class use or interact with objects from another class. It is a “uses-a” relationship.

• Example: A Driver uses a Car to drive.
• The Driver class and Car class are associated because a driver can drive a car.

In a library system:

• A Member can borrow a Book. This is a form of association between the Member class and the Book class.

3.2 Aggregation

Aggregation is a special type of association where one class contains objects of another class. This is often referred to as a “whole-part” relationship. However, the contained objects can exist independently of the container.

• Example: A Car contains Wheels, but if the car is destroyed, the wheels may still exist separately.
• The Car class has a relationship with the Wheel class, but the wheel can exist without the car.

In a library system:

• A Library contains many Books, but if the library is closed, the books can still exist elsewhere.

3.3 Composition

Composition is a stronger form of aggregation where one class is composed of other classes, but the contained objects do not exist independently. If the container object is destroyed, so are the contained objects.

• Example: A Car is composed of an Engine. If the car is destroyed, the engine (as a component of this car) also ceases to exist in its context.

In a library system:

• A Issue/Loan is composed of a Book and a Member. If the loan record is deleted, the connection between that specific book and member is also deleted.

3.4 Inheritance

Inheritance represents an “is-a” relationship where one class inherits the properties and behavior of another class. It allows the reuse of code and the creation of a more hierarchical design.

• Example: A SportsCar is a type of Car, so the SportsCar class can inherit from the Car class. This means it can use the attributes and methods of Car, but also have additional properties and methods specific to a sports car (e.g., turboBoost()).

In a library system:

• A Librarian might be a specialized form of a Person. The Librarian class can inherit from a general Person class and have extra responsibilities like managing books.

4. Modeling Real-World Interactions

After identifying the classes and their relationships, the next step is to model the interactions between these entities. This helps in understanding how different classes will work together to perform various operations.

Example:

In the library system:

• A Member interacts with the Book class by borrowing a book.
• A Loan class records the interaction between a Member and a Book, with attributes such as loanDate and returnDate.

This interaction can be modeled as:

1. A Member borrows a Book.
2. A Loan is created, linking the Member to the Book.
3. The Librarian manages this entire process, updating records accordingly.

Summary of Key Relationships:

• Association: Represents a general interaction (e.g., Member borrows Book).
• Aggregation: Represents a whole-part relationship where the part can exist independently (e.g., Library contains Books).
• Composition: A whole-part relationship where the part cannot exist independently (e.g., Loan contains Book and Member).
• Inheritance: An “is-a” relationship where one class inherits from another (e.g., SportsCar inherits from Car).

Self Assessment

1. Can you identify real-world entities that can be modeled as classes from a problem description? Example: What objects would you identify in a system for managing a library?
2. How would you differentiate between attributes and methods when defining a class? Example: In a Car class, what would be considered attributes, and what would be methods?
3. Can you explain the difference between association, aggregation, and composition relationships? Example: How would you describe the relationship between a Library and Book classes?
4. How do you decide when to use inheritance in class design? Example: When would you create a SportsCar class that inherits from a Car class?
5. Can you map real-world interactions into class relationships in a system design? Example: In a hospital management system, how would you model interactions between Doctor, Patient, and Prescription classes?

You can visit the presentation here.