Question

Name three major relationships that could exist between classes \(A\) and \(B\). Describe them in your own words. Express each in UML and in a typical Java implementation.

Short answer

Major relationships are Association, Aggregation, and Composition, differing by dependency strength and implementation in UML and Java.

Step-by-step solution

Understanding Associations

The first major relationship is 'Association'. Association is the most basic relationship between two classes. It signifies that objects of one class can be associated with objects of another through a loosely defined connection. In UML, this is represented by a simple line connecting the two class boxes. In Java, this is typically seen as one class having a member variable of another class type.

Understanding Aggregation

The second relationship is 'Aggregation'. Aggregation implies a weaker relationship where the contained object can exist independently of the container object. For instance, a department may contain professors, but a professor can exist without a department. In UML, aggregation is shown with a line and an empty diamond at the container class end. In Java, this is implemented by having a class maintain references to objects of another class typically through a list or collection.

Understanding Composition

The third relationship is 'Composition'. Composition represents a strong ownership relationship where the contained object cannot exist without the container object. For example, an engine is part of a car and does not exist independently. In UML, it is denoted by a line with a filled diamond at the container class end. In Java, this relationship is often implemented similarly to aggregation, but the lifecycle of the contained objects is tied to the lifecycle of the container object through direct instantiation or explicit destruction.

Key concepts

Association

Association is a foundational concept in object-oriented programming that relates two classes through a flexible and often informal connection. This type of relationship suggests that the two objects can "work together," or simply have some level of interaction.
Imagine a software system where a `Student` class interacts with a `Library` class. The `Library` class can keep track of which students borrow books. In this scenario, the `Student` class is associated with the `Library` class. The association just means they can communicate or interact in some way.

In UML, this relationship is visually represented by a simple line connecting two class boxes. It doesn't imply direction or ownership, just that there is some relationship.
In Java, association might be portrayed by having a class with a member variable of another class type. For example, a `Library` might contain a method that accepts a `Student` as a parameter, allowing them to interact without requiring them to own each other. Thus, this kind of relationship is loose and flexible, serving as a straightforward way to connect classes without imposing any dependency or lifecycle constraints.

Aggregation

Aggregation represents a subtle yet important form of object-oriented relationship where one class (the child) can exist independently of another class (the parent).
For example, in a university environment, a `Department` might contain instances of a `Professor`. However, each `Professor` can exist independently of the `Department`. They might move to a different department or perform tasks unrelated to any department. This autonomy is a key aspect of aggregation.

In UML diagrams, this relationship is depicted as a line with an open diamond shape at the parent end, indicating a weak "whole-part" relationship.
In Java, aggregation is usually implemented by having the parent class keep references to child objects, like a list of `Professor` objects in the `Department` class. The primary aspect of aggregation is that it is a more relaxed relationship since the lifecycle of child objects doesn't depend on the parent object. Child objects in aggregation can live independently and be shared across multiple parents, fostering reusability and flexibility.

Composition

Composition is the strongest form of the object-oriented relationship, where one class (the container or whole) completely owns the lifecycle of another class (the part).
Consider the relationship between a `Car` and its `Engine`. An `Engine` cannot function outside the context of a `Car`. If the `Car` is disposed of, so is its `Engine`. This tight coupling ensures that the existence and functionality of the part depend entirely on the whole.

In UML, composition is represented by a line with a filled diamond at the whole's end. This symbol indicates a "strong" whole-part relationship where the part does not exist independently of the whole.
In Java, composition is realized by directly instantiating the part objects within the parent class or managing their lifecycle explicitly. When a `Car` is created, it creates its own `Engine` object, ensuring the `Engine` does not exist without the `Car`. Therefore, composition provides robust encapsulation and control over the lifecycle since parts are created and destroyed alongside the whole, ensuring tight cohesion in object-oriented design.