Question

When is a "has-a" relationship better to use than an "is-a" relationship?

Answer 1

A "has-a" relationship (composition) is better than an "is-a" relationship (inheritance) when the relationship between objects represents ownership or when the behavior of one object should not directly depend on another. "Has-a" is more flexible because it allows for modular, reusable components without tightly coupling classes. For example, if a Car class has a Engine class as a field, it reflects a "has-a" relationship since a car owns an engine, but an engine is not inherently a type of car. Composition also avoids the pitfalls of inheritance, such as inheriting unnecessary behaviors or violating the principle of single responsibility. By using "has-a," you can change or replace the associated object (Engine) without altering the parent object (Car), which promotes loose coupling and enhances code maintainability.

Answer 2

Use a "has-a" relationship (composition) when:

1. You want to reuse components across unrelated objects.
2. You need modularity and encapsulation.
3. Behavior or features may change or vary dynamically.
4. The relationship is a part-whole rather than a strict subtype. (Bird has wings)

Use an "is-a" relationship when:

1. Clear Hierarchy: The subclass is a specific type of the superclass (e.g., Dog is an Animal).
2. Shared Behavior: The subclass shares common behavior with the superclass.
3. Specialization: The subclass extends or modifies the superclass behavior.
4. Static Relationship: The parent-child relationship is fixed.
5. Code Reusability: Inherit common code to avoid duplication.