How do you apply composition and aggregation in your design?

Composition is a design principle where one class is composed of objects from other classes, forming a strong "has-a" relationship. The contained objects are integral to the whole and cannot exist independently if the whole is destroyed. For example, a Car object may have Engine and Tire objects as its parts. If the Car is destroyed, the engine and tires are also destroyed. This approach promotes modular, reusable, and maintainable code by breaking down complex systems into smaller, interdependent parts. Composition enhances encapsulation, as the internal parts are typically hidden from outside access, only being manipulated through the container class. It is widely used when designing systems with clear part-whole relationships.

Composition is the relationship between two things, when one comprises one or more instances of the other. For instance, the relation between School and Department can be Composition, because the School comprises different departments.

Composition is a design principle where one object is made up of, or composed of, other objects. Instead of inheriting behavior from a parent class (like in inheritance), a class can include other objects as part of its structure to achieve its functionality. For example, a car can be composed of an engine, wheels, and seats. Each part has its functionality, forming a complete car. In programming, composition promotes flexibility because you can change or replace parts (objects) without affecting the whole system, making it more modular and easier to maintain.

Aggregation is a design principle where one object contains a reference to another object, but the contained object can exist independently of the parent. It's a has-a relationship. For example, a library contains books, but the books can exist on their own even if the library is gone. In programming, aggregation allows objects to be loosely connected, so changes to one object won’t directly affect the other. This makes the system more flexible and helps manage complex relationships between objects while keeping them independent.

Aggregation is a kind of connection that shows a "has-a" relationship—that is, when two classes are related to one other by reference—between them. It represents a whole-part relationship, although the coupling is not as strong as the composition.

Aggregation is a weaker form of composition where objects maintain a "has-a" relationship but can exist independently of the container object. For instance, a Classroom may contain Student objects, but students are not destroyed if the classroom is. Aggregation allows the objects to be shared across different contexts without strict lifecycle dependency. The contained objects (students, in this case) can be created and managed outside the scope of the container (classroom), making it more flexible than composition. Aggregation is suitable for scenarios where objects need to exist independently but are part of larger structures or relationships. This design principle supports better code reuse and flexibility in object management.

Aggregation: The part can exist independently of the whole. E.g., a classroom has students, but students can exist without the classroom. Composition: The part cannot exist independently of the whole. E.g., a house has rooms, and if the house is destroyed, the rooms are also destroyed.

The difference between composition and aggregation lies in object dependency. In composition, the part depends on the whole for its existence, meaning if the whole is destroyed, the part is destroyed too. For example, a Computer and its Processor have a composition relationship—without the computer, the processor is useless. In aggregation, the part can exist independently of the whole, as with a School and Teacher. If the school closes, teachers continue to exist and work elsewhere. Composition implies ownership and lifecycle dependency, while aggregation signifies association without ownership. UML diagrams differentiate them: composition uses a filled diamond, while aggregation uses a hollow one.

Think of composition like a car and its engine. If the car is destroyed, the engine is also gone, because the engine is a part of the car and can't exist without it. Now, think of aggregation like a school and students. The students go to the school, but if the school closes, the students still exist and can go to a different school. Composition means one can't live without the other, while aggregation means they can exist separately.

Composition offers tight control over objects, enabling better modularity, encapsulation, and lifecycle management, as objects are created and destroyed together. It simplifies code by ensuring that dependencies are well-contained and hidden. Aggregation, on the other hand, promotes flexibility and reusability. The contained objects can exist independently and be shared across different contexts, making the design more adaptable to changes. It’s easier to manage objects in systems that do not have strict part-whole relationships. Both approaches allow for code reuse, scalability, and modular system architecture, but composition excels in encapsulation while aggregation enhances flexibility.

Imagine a tree and its leaves. The leaves are part of the tree; if the tree dies, the leaves die too. This is like composition in programming, where one part fully depends on the other. Composition keeps everything together and controlled, making the system easy to manage as a single unit. Now think of a library and its books. The books belong to the library but can exist without it. This is like aggregation in programming, where parts are independent. Aggregation gives flexibility, allowing parts to be reused or moved without affecting the whole system.

In composition, tightly coupling objects can lead to higher complexity when changes are required, as modifying one part can impact the whole system. It can also increase memory overhead due to deep object hierarchies. Aggregation, while more flexible, introduces challenges in managing independent object lifecycles. It requires careful handling of dependencies, as the objects can outlive their containers, leading to potential memory leaks or inconsistent states. Aggregation also increases the risk of breaking encapsulation if objects are shared too freely. The trade-off between composition and aggregation depends on whether control and encapsulation are prioritized over flexibility and reusability.