Object Orientation and Design Principles: Encapsulation, Inheritance, and Code Smells

The value of object orientation is placing behavior near the data and rules it owns, so objects can protect their invariants.

Core Concepts

Encapsulation hides internal state and exposes meaningful operations. Inheritance expresses stable is-a relationships, but deep inheritance makes change hard. Composition is often easier to evolve.

classDiagram
    class Order {
      +Submit()
      +Cancel()
      -EnsureCanSubmit()
    }
    class Money {
      +Amount
      +Currency
    }
    Order --> Money : total

Code Smells

• God Class: one class owns everything.
• Feature Envy: a method mostly manipulates other objects.
• Long Parameter List: behavior lacks a proper object.
• Primitive Obsession: all concepts are expressed as strings or numbers.

Best Practices

• Ask who should own the rule.
• Keep invariants inside objects.
• Prefer composition and use inheritance carefully.
• Fix responsibility boundaries when smells appear.