Inheritance vs Aggregation

TODO: Rework to https://www.codeproject.com/Articles/330447/Understanding-Association-Aggregation-and-Composit

Abstraction is a good thing, but in all except the most trivial applications, we may find many more different abstractions than we can comprehend at one time. Encapsulation helps manage this complexity by hiding the inside view of our abstractions. Modularity helps also, by giving us a way to cluster logically related abstractions. Still, this is not enough. A set of abstractions often forms a hierarchy, and by identifying these hierarchies in our design, we greatly simplify our understanding of the problem.

Hierarchy is a ranking or ordering of abstractions.

Examples of hierarchy:

• Single Inheritance

• Multiple Inheritance

• Aggregation

Single Inheritance

Inheritance defines a relationship among classes, wherein one class shares the structure or behavior defined in one or more classes. Inheritance thus represents a hierarchy of abstractions, in which a subclass inherits from one or more superclasses.

Typically, a subclass augments or redefines the existing structure and behavior of its superclasses.

Semantically, inheritance denotes an “is a” relationship.

Multiple Inheritance

A better way to express our abstractions and thereby avoid redundancy is to use multiple inheritance. It's conceptually straightforward, but it does introduce some practical complexities for programming languages.

Languages must address two issues:

• Clashes among names from different superclasses. Clashes will occur when two or more superclasses provide a field or operation with the same name or signature as a peer superclass.

• Repeated inheritance. Repeated inheritance occurs when two or more peer superclasses share a common superclass.

Aggregation

Whereas these “is a” hierarchies denote generalization/specialization relationships, “part of” hierarchies describe aggregation relationships.

For example, consider the abstraction of a garden. We can contend that a garden consists of a collection of plants together with a growing plan. In other words, plants are “part of” the garden, and the growing plan is “part of” the garden. This “part of” relationship is known as aggregation.