Presentation that introduces that concepts of STUPID code and SOLID code. Also if this two acronyms have been covered multiple times in Software Engineering history that are still sometimes not well known. It is always worth the spread the word!

1. How to build SOLID code
Sebastiano (eTr) Merlino

2. ZenConf
04/Jun/2014 - Catania

3. Almost entirely copied from inspired by the presentation “From
STUPID to SOLID” by William Durand.
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4. DISCLAIMER:
I give you an opinion not rules.
Please, consider these just as principles, not laws!
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5. Only 2 types of code do exist:
Your code: Other people code:
It’s REASSURING and you
understand it
It INTIMIDATES you and it’s ~99.
999% of world’s code
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6. Why should my code be
clear?
Because we READ much more than we WRITE
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7. STUPID code, seriously?
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8. What makes code STUPID?
● Singleton
● Tight Coupling
● Untestability
● Premature Optimization
● Indescriptive Naming (yeah! It’s not misspelled)
● Duplication
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9. Singleton
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10. Singleton
● It represents a GLOBAL STATE in your code
● Programs using global state are VERY difficult to test and debug
● Programs relying on global state HIDE their dependencies
Links:
- Why singletons are controversial?
- Why is singleton an antipattern?
- So Singletons are bad, then what?
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11. Tight Coupling
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12. Tight Coupling
● Generalization of the singleton ISSUE
● If changing something in a module FORCES you to change also
another module
● It makes code difficult to REUSE and also difficult to TEST
● To avoid it, favor COMPOSITION over inheritance and try to use
DEPENDENCY INJECTION where possible.
Links:
- Reducing Coupling (Martin Fowler)
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13. Untestability
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14. Untestability
● Testing should not be HARD
● Whenever you don’t write UNIT TESTS because you DON’T HAVE
TIME, the real issue is that your code is BAD
● Most of the time untestability is caused by TIGHT COUPLING
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15. Premature Optimization
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16. Premature Optimization
PREMATURE OPTIMIZATION is the root of all evil - DONALD KNUTH
● Do not OVER-COMPLICATE your system in order to optimize it
● There is only COST and no BENEFIT
● MEASURE the performances before you start to optimize
DON’T DO IT.
For experts only: DON’T DO IT NOW!
Links:
- Premature Optimization Anti-Pattern
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17. Indescriptive Naming
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18. Indescriptive Naming
● Name your classes, methods, attributes and variables properly
● Don’t abbreviate, NEVER!
● If you are not able to give your class a short name, maybe its
responsibilities are not WELL DEFINED
● Programming languages are for HUMANS
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19. Duplication
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20. Duplication
● If you do it, you will find yourself changing your code in multiple
places.
● Don’t Repeat Yourself (DRY)
● Keep It Simple, Stupid! (KISS)
● Be DRY not WET (We Enjoy Typing)
Links:
- No, seriously. Don’t repeat yourself
- DRY principle
- KISS principle
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22. SOLID
Term describing a collection of design principles for GOOD CODE
that was coined by ROBERT C. MARTIN aka UNCLE BOB
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23. What makes code SOLID?
● Single Responsibility Principle
● Open/Closed Principle
● Liskov Substitution Principle
● Interface Segregation Principle
● Dependency Inversion Principle
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24. Single Responsibility Principle
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25. Single Responsibility Principle
● There should NEVER be more than ONE reason for a class to
change
● Split big classes
● Use LAYERS
● Avoid GOD classes
● Write STRAIGHTFORWARD comments
Links:
- Single Responsibility Principle
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26. Open/Closed Principle
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27. Open/Closed Principle
● Software entities should be OPEN for extension but CLOSED for
manipulation
● Make ALL member variables private
● NO global variables, EVER
● Avoid SETTERS (as much as possible)
● Builder Pattern + Immutable Objects
Links:
- Open/Closed Principle
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28. Liskov Substitution Principle
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29. Liskov Substitution Principle
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public class Rectangle {
protected int width;
protected int height;
public int getArea() { return width * height; }
public void setWidth(int width) { this.width = width; }
public void setHeight(int height) { this.height = height; }
}
public class Square extends Rectangle {
public void setWidth(int width) { this.width = width; this.height = width; }
public void setHeight(int height) { this.width = height; this.height = height; }
}
public class Main {
public static void main(String[] args) {
Rectangle rectangle = new Square();
rectangle.setWidth(10); rectangle.setHeight(20);
assert rectangle.getArea() == 200; // this will fail!
}
}

30. Liskov Substitution Principle
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● Objects in a program should be replaceable with instances of
their subtypes WITHOUT ALTERING THE CORRECTNESS of the
program
Links:
● Liskov Substitution Principle
● Circle-ellipse problem
● Should sets inherit from bags?

31. Interface Segregation Principle
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32. Interface Segregation Principle
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● MANY client-specific interfaces are BETTER THAN ONE general-
purpose interface.
● You should not have to implement methods you don’t use
● Enforcing ISP gives you LOW COUPLING and HIGH COHESION
● KEEP COMPONENTS FOCUSED and MINIMIZE DEPENDENCIES BETWEEN
THEM
Links:
● Interface Segregation Principle
● Coupling and Cohesion: Principles of Orthogonal OOP

33. Dependency Inversion Principle
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34. Dependency Inversion Principle
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● High level modules SHOULD NOT DEPEND upon low level modules.
Both SHOULD DEPEND upon abstractions
● Abstractions should not depend upon details. Details should
depend upon abstractions
● Use the same level of abstraction at a given level
● It REDUCES DEPENDENCY on implementation specifics and makes
code MORE REUSABLE
Links:
● Dependency Inversion Principle
● Programming to the interface

35. Rule of thumb:
USE YOUR BRAIN!
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36. Thank you!
https://github.com/etr
https://twitter.com/electrictwister
http://www.slideshare.net/electrictwister
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37. Credits:
http://williamdurand.fr/2013/07/30/from-stupid-to-solid-
code/
http://lostechies.com/derickbailey/2009/02/11/solid-
development-principles-in-motivational-pictures/
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38. License:
Creative Commons Attribution-ShareAlike 3.0 Unported License
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