IS-A vs HAS-A
IS-A represents specialization and is normally modeled through inheritance. HAS-A represents ownership, collaboration or composition, where one object contains or uses another object. For example, a Car IS-A Vehicle, while a Car HAS-A Engine. In architecture, HAS-A relationships are generally more flexible because components can be replaced independently and the owning class does not need to inherit the implementation details of its dependency.
IS-A: Represents specialization or substitutability.
HAS-A: Represents composition, ownership or collaboration.
Inheritance creates a stronger coupling between types.
Composition allows dependencies to be replaced or configured.
A good design distinguishes domain specialization from implementation reuse.
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