Smell Primitive Obsession

Using primitive types for things that deserve small objects, such as money classes that combine number and currency, ranges with upper and lower bounds, and special strings such as telephone numbers and ZIP codes.

• Using primitives for domain concepts (money, phone numbers, etc.)
• Type codes or enums that affect behavior
• Groups of primitives that should be an object
• Arrays used as objects

Martin Fowler — Refactoring: Improving the Design of Existing Code, 1st Edition (1999)