Association is the most basic and generic form of relationship between classes. It represents a connection between two classes where one class is aware of and can interact with another class. This relationship is often described as a “uses-a” relationship.
Aggregation is a specialised form of association that represents a “whole-part” or “has-a” relationship between classes. In aggregation, one class (the whole) contains references to objects of another class (the part), but the part can exist independently of the whole.
Composition is a stronger form of aggregation. It’s a “whole-part” relationship where the part cannot exist independently of the whole. In other words, the lifetime of the part is tied to the lifetime of the whole.
Inheritance is a fundamental concept in OOP that allows a class (subclass or derived class) to inherit properties and methods from another class (superclass or base class). It represents an “is-a” relationship between classes.
Realisation, also known as implementation, is a relationship between a class and an interface. It indicates that a class implements the behaviour specified by an interface.
Dependency is the weakest form of relationship between classes. It exists when one class uses another class, typically as a method parameter, local variable, or return type.
Understanding class relationships is crucial for effective object-oriented design and programming. We’ve explored various types of relationships including association, aggregation, composition, inheritance, realisation, and dependency. Each of these relationships serves a specific purpose and has its own strengths and weaknesses.