Stage 2 · Lesson 5 of 7
Dependency inversion
The says high-level policy, such as "when an order is placed, notify the customer", should not depend on low-level details, such as an SMTP email library. Both should depend on an , such as a Notifier interface, which the business code owns.
The result is that the rules sit at the stable centre of the code, and details plug in at the edges. You can switch email for SMS or WhatsApp, or pass a fake in a , without touching OrderService. This is also the idea behind layered and clean architectures that you will meet in larger systems.
Common trapgiving a field an interface type but still creating the with new inside the , which changes nothing.
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Resources for this stage
Start with· follow one of these
- CoursePaidGrokking SOLID Design Principles
Design Gurus · Beginner · $50 one-time
One chapter per principle, each with the violation and the refactoring that fixes it
- CourseFreeMIT 6.102: Software Construction
MIT · Intermediate
Free: the Specifications and Interfaces and Subtyping readings explain substitution as contracts