Dependency Injection

Dependency injection means you get HANDED your tools instead of making them yourself. Picture a chef who gets today's ingredients from the kitchen, instead of growing their own vegetables. The chef can cook with whatever they're handed, so tomorrow's ingredients can be totally different, and the chef doesn't need to change at all. In code, an object receives the other objects it needs from the outside, instead of creating them itself.

1. 1Instead of a class building the things it depends on inside itself, it accepts them as inputs — usually in its constructor (the setup method that runs when you create the object).
2. 2Whoever creates the object decides which exact version to hand it: a real one, a fake one for testing, or a different brand entirely.
3. 3The class just uses whatever it was handed, without caring which specific version it got.

Use this when you want to swap parts easily — a real database vs. a fake one for tests, different payment providers, or different engines — without rewriting the class that uses them.

• ⚠Don't let the class create its own dependency internally 'for convenience'. That locks you back into one fixed version.
• ⚠Don't hand in so many small pieces that the constructor becomes a long, confusing list. Sometimes a few really do belong bundled together.