Discussion

A good business idea, they say, can be explained in one sentence. Similarly, each program entity should have one clear purpose.

An entity with several disparate purposes is generally disproportionately harder to use, because it carries more than the sum of the intellectual overhead, complexity, and bugs of its parts. Such an entity is larger (often without good reason) and harder to use and reuse. Also, such an entity often offers crippled interfaces for any of its specific purposes because the partial overlap among various areas of functionality blurs the vision needed for crisply implementing each.

Entities with disparate responsibilities are typically hard to design and implement. "Multiple responsibilities" frequently implies "multiple personalities"a combinatorial number of possible behaviors and states. Prefer brief single-purpose functions (see also Item 39), small single-purpose classes, and cohesive modules with clean boundaries.

Prefer to build higher-level abstractions from smaller lower-level abstractions. Avoid collecting several low-level abstractions into a larger low-level conglomerate. Implementing a complex behavior out of several simple ones is easier than the reverse.