A float can hold any number, which is exactly the problem. Primitive obsession is why the same validation keeps reappearing, and value objects are the fix.

The moment a class builds its own collaborators, it quietly turns untestable and rigid. Dependency injection is the small habit that fixes both.

A growing if/else chain that switches on a type is a design pattern in disguise. Here's how the Strategy pattern turns it into small, swappable pieces.

Inheritance looks tidy until the hierarchy fights you. Here's the concrete point where 'is-a' breaks and 'has-a' scales, shown with real code.

Deeply nested if/else blocks are hard to read and easy to break. Guard clauses flatten your logic with early returns so the happy path stays obvious.