Becoming a better developer by using the SOLID design principles

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Becoming a better developer by using the SOLID design principles How to write code that’s easy to maintain, extend and understand

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Katerina Trajchevska ● Senior Software Engineer & co-founder of Adeva ● Remote Work Advocate ● Community Volunteer ● Consultant with startups and Fortune 500 companies Software Engineer and co-founder of Adeva

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@ktrajchevska Working on legacy code ● Re-reading code multiple times to get to the part you need to change ● Hard to understand what a method does ● Spending a lot of time to fix a minor bug ● You spend more time reading than writing code.

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@ktrajchevska Working on a startup product ● In charge of the development process ● Constantly adding new features ● No formal process ● Very dynamic environment, no time to worry about code structure ● What is it like to go back to your code after 2 years?

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@ktrajchevska What will we talk about today? ● What is SOLID and how it can make our lives easier ● What’s the purpose of each of the SOLID design principles ● How to not use SOLID

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@ktrajchevska The purpose of SOLID design principles ● To make the code more maintainable. ● To make it easier to quickly extend the system with new functionality without breaking the existing ones. ● To make the code easier to read and understand, thus spend less time figuring out what it does and more time actually developing the solution. ● Introduced by Robert Martin (Uncle Bob), named by Michael Feathers.

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Single Responsibility Principle A class should have one, and only one, reason to change.

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@ktrajchevska Single Responsibility Principle ● A class should only be responsible for one thing. ● There’s a place for everything and everything is in its place. ● Find one reason to change and take everything else out of the class. ● Very precise names for many small classes > generic names for large classes.

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Open/Closed Principle An entity should be open for extension, but closed for modification.

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@ktrajchevska Open/Closed Principle ● An entity should be open for extension but closed for modification. ● Extend functionality by adding new code instead of changing existing code. ● Separate the behaviors, so the system can easily be extended, but never broken. ● Goal: get to a point where you can never break the core of your system.

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Liskov Substitution Principle Let φ(x) be a property provable about objects x of type T. Then φ(y) should be true for objects y of type S where S is a subtype of T.

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@ktrajchevska Liskov Substitution Principle ● Any derived class should be able to substitute its parent class without the consumer knowing it. ● Every class that implements an interface, must be able to substitute any reference throughout the code that implements that same interface. ● Every part of the code should get the expected result no matter what instance of a class you send to it, given it implements the same interface.

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Image credits: Derick Bailey

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Interface Segregation Principle No client should be forced to depend on methods it does not use.

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@ktrajchevska Interface Segregation Principle ● A client should never be forced to depend on methods it doesn't use. ● Or, a client should never depend on anything more than the method it’s calling. ● Changing one method in a class shouldn’t affect classes that don’t depend on it. ● Replace fat interfaces with many small, specific interfaces.

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Dependency Inversion Principle High-level modules should not depend on low-level modules. Both should depend on abstractions.

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Image credits: Derick Bailey

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@ktrajchevska Dependency Inversion Principle ● Never depend on anything concrete, only depend on abstractions. ● High level modules should not depend on low level modules. They should depend on abstractions. ● Able to change an implementation easily without altering the high level code.

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@ktrajchevska ⚠ Don’t get trapped by SOLID ● SOLID design principles are principles, not rules. ● Always use common sense when applying SOLID. ● Avoid over-fragmenting your code for the sake of SRP or SOLID. ● Don’t try to achieve SOLID, use SOLID to achieve maintainability.

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@ktrajchevska Final Thoughts ● The purpose of SOLID principles is to make your code more maintainable, easy to extend and reason about. ● It requires spending more time writing code, so you can spend less reading it later. ● SOLID principles are principles, not rules. ● Always know your trade-offs and use common sense. ● SOLID is your tool, not your goal.

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Thank you @ktrajchevska Connecting tech companies with world-class developers