The Single Responsibility Principle is the most misread rule in SOLID: it was never about doing one thing, but about having one clear reason to change.

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

Sometimes deciding which object to create is a job in itself. The Factory pattern gives that decision one home so the rest of your code stays simple.

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.