Composition and inheritance

Steven Lowe has an article on the ThoughtWorks blog, elaborating on Composition vs. Inheritance: How to Choose? The text covers examples of how to misuse inheritance.

The purpose of composition is obvious: make wholes out of parts.

Inheritance captures semantics (meaning) in a classification hierarchy (a taxonomy), arranging concepts from generalized to specialized. […] This makes the subclass more tightly coupled to its superclass than it would be if it merely used an instance of the superclass as a component instead of inheriting from it. […] Inheritance captures mechanics by encoding the representation of the data (fields) and behavior (methods) of a class and making it available for reuse and augmentation in subclasses. Mechanically, the subclass will inherit the implementation of the superclass and thus also its interface.

By Steven´s guidance, inheritance should only be used when:

• Both classes are in the same logical domain
• The subclass is a proper subtype of the superclass
• The superclass’s implementation is necessary or appropriate for the subclass
• The enhancements made by the subclass are primarily additive.