SOLID in Practice
Five principles as tools for change, not commandments.
The five SOLID principles as working engineers actually use them - heuristics for code that's easy to change, applied to a messy but real ledger codebase rather than toy examples. Single responsibility, open/closed, Liskov substitution, interface segregation, and dependency inversion - each shown as the fix for a concrete pain.
Lessons in this stage
- 01
Principles, Not Rules
IntermediateWhat SOLID is really for - designing for change - why the principles are heuristics not laws, and the messy ledger codebase we'll improve throughout the path.
13 min - 02
Single Responsibility
IntermediateA class should have one reason to change. Cohesion, the many jobs a 'god class' secretly does, and how splitting by responsibility makes code safe to touch.
14 min - 03
Open/Closed
AdvancedOpen for extension, closed for modification: add behavior without editing tested code, using polymorphism and the strategy pattern instead of growing if/else chains.
14 min - 04
Liskov Substitution
AdvancedSubtypes must be usable through the base type without surprises. The classic square/rectangle trap, behavioral subtyping, and why 'is-a' isn't enough.
14 min - 05
Interface Segregation
IntermediateNo client should depend on methods it doesn't use. Fat interfaces, the empty method smell, and splitting into focused role interfaces.
12 min - 06
Dependency Inversion
AdvancedDepend on abstractions, not concretions - so policy doesn't hard-wire to details. The principle that makes code testable and sets up hexagonal architecture.
15 min