SOLID
- Dart course
- Expert
- Lesson 35/47
- 5 min
Learn SOLID principles.
SOLID
SOLID is a set of five design principles: Single Responsibility, Open/Closed, Liskov Substitution, Interface Segregation, Dependency Inversion.
Single Responsibility: each class does one thing. Report carries data, ReportPrinter produces output. A change affects one class.
Dependency Inversion: high-level modules depend on interfaces, not low-level modules. OrderService depends on the PaymentService interface, not the concrete StripePayment.
The Open/Closed principle
Open/Closed: classes are open for extension, closed for modification. New behavior is added via a new subclass or interface implementation, without changing existing code.
Example: adding CryptoPayment to the PaymentService interface does not change OrderService. The class stays closed; the system stays open.
This principle uses inheritance and polymorphism. You define interfaces instead of concrete classes; new behavior comes with a new class.
Practice Define Report and ReportPrinter
Define a Report class with final String title; final String content; Report(this.title, this.content);. Define a ReportPrinter class with String format(Report r) => '${r.title}: ${r.content}';. In main create final report = Report('Aylık', 'Satışlar arttı');; create final printer = ReportPrinter(); and add print(printer.format(report));.
Starter code
void main() {
// define a Report class (title, content fields)
// define a ReportPrinter class (format method)
// create a report and a printer; print via format
}Sample solution
class Report {
final String title;
final String content;
Report(this.title, this.content);
}
class ReportPrinter {
String format(Report r) => '${r.title}: ${r.content}';
}
void main() {
final report = Report('Aylık', 'Satışlar arttı');
final printer = ReportPrinter();
print(printer.format(report));
}Expected output
Aylık: Satışlar arttı
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Liskov substitution
class SavingsAccount ____ Account { }class SavingsAccount extends Account { }Explanation: a subclass extends and substitutes for its parent (Liskov).
2 Match the SOLID principles
| Term | Meaning |
|---|---|
S |
Single responsibility |
O |
Open/closed |
L |
Liskov substitution |
I |
Interface segregation |
D |
Dependency inversion |
3 What does the Single Responsibility Principle say?
- Each class should implement an interface
- Each class should be extensible
- Each class should be closed
- Each class should do one thing Correct answer
Explanation: each class should have a single responsibility.