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Intermediate Topic 18 of 50 Software Engineering

SOLID Principles

Five guidelines for maintainable object-oriented design.

How it works

SOLID is a set of five object-oriented design principles: Single Responsibility, a class should have one reason to change; Open-Closed, software entities should be open for extension but closed for modification; Liskov Substitution, subtypes must be usable wherever their base type is expected; Interface Segregation, clients should not depend on methods they do not use; and Dependency Inversion, high-level modules should depend on abstractions, not on concrete low-level details.

Step-by-step walkthrough

  1. S: a class should have one reason to change.
  2. O: open for extension, closed for modification.
  3. L: subtypes must work wherever the base type is expected.
  4. I: prefer small specific interfaces.
  5. D: depend on abstractions, not concrete classes.

Where it is used

They guide the design of maintainable, testable code in frameworks, services and libraries, and appear often in design discussions and code reviews.

Common pitfall

Applying them dogmatically creates needless abstraction and tiny classes. Violating Liskov, for example by overriding a method to throw an exception, breaks polymorphism. Single responsibility is about reasons to change, not the number of methods. Use them as heuristics when code becomes hard to change, rather than as rules to apply upfront.

Where it fits in the course

SOLID Principles is topic 18 of 50 in Software Engineering, rated Intermediate. It follows Software Maintenance. The next topic is DRY, KISS and YAGNI.

Practice and explore

How to study this topic

  1. Read the explanation above and restate the definition in your own words.
  2. Open the interactive lesson and answer its mini quiz.
  3. Read the common pitfall, then write a short example that avoids it.
  4. Check your understanding in the Software Engineering quiz, and revisit the topic if you miss a question.

Frequently asked questions

What is SOLID Principles?
Five guidelines for maintainable object-oriented design. It belongs to Software Engineering and is an intermediate-level topic.
Where do people use SOLID Principles?
They guide the design of maintainable, testable code in frameworks, services and libraries, and appear often in design discussions and code reviews.
What is the most common mistake with SOLID Principles?
Applying them dogmatically creates needless abstraction and tiny classes.
What should I learn before this topic?
Start with Software Maintenance, the topic just before it in Software Engineering.
How can I practice it?
Answer the mini quiz in the interactive lesson, then take the Software Engineering quiz.
Is SOLID Principles hard to learn?
It is an Intermediate topic. Be comfortable with the Beginner topics in this area first, then read the common pitfall carefully, since it is where most mistakes happen.
How long does it take to learn SOLID Principles?
Most learners can read the explanation and finish the interactive lesson in under an hour. Fluency comes from applying the idea, so plan a few short practice sessions with the quiz.
Which topics are related to SOLID Principles?
Software Maintenance, DRY, KISS and YAGNI, Bug Tracking are closely connected. The full list is in the related topics section of this page.
What should I study after SOLID Principles?
Continue with DRY, KISS and YAGNI, the next topic in Software Engineering.

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