Can you explain the difference between aggregation and composition in object-oriented design?

During my final year, I worked on a group project for a college management system. In this project, we used aggregation to represent the relationship between students and courses. Each student could enroll in multiple courses, and a course could have multiple students. If a student decided to drop a course, they were no longer associated, but the course still existed. This illustrated aggregation, where the lifetime of the student and the course are independent, emphasizing that one does not depend on the other. It helped me understand how to model real-world relationships in a flexible way.

While volunteering at a community library, I helped develop a tracking system for library books and their authors. We utilized composition in our design, where each book was composed of an author. In this scenario, if the author was removed from the system, the book could not exist meaningfully without its author. This experience taught me that composition indicates a stronger relationship, as the lifecycle of the book is directly tied to the author. It was a practical way to see how these concepts apply in real-world settings, reinforcing the idea of ownership in design.

In my first job as a junior developer at an e-commerce platform, I encountered both aggregation and composition in our codebase. For example, products were represented with aggregation, as they could exist independently of categories. A product might belong to multiple categories but could still function without them. Conversely, we used composition for managing shopping carts, where each cart could only exist with its items. Understanding these concepts helped me contribute effectively to discussions about system architecture and design decisions, ensuring that our code was both efficient and easy to maintain.