SOLID is an acronym for the first five object-oriented design (OOD) principles by Robert C. Martin (also known as Uncle Bob).
Note: While these principles can apply to various programming languages, the sample code contained in this article will use PHP.
These principles establish practices for developing software with considerations for maintaining and extending it as the project grows. Adopting these practices can also help avoid code smells, refactor code, and develop Agile or Adaptive software.
SOLID stands for:
In this article, you will be introduced to each principle individually to understand how SOLID can help make you a better developer.
Single-responsibility Principle (SRP) states:
A class should have one and only one reason to change, meaning that a class should have only one job.
For example, consider an application that takes a collection of shapes—circles and squares—and calculates the sum of the area of all the shapes in the collection.
First, create the shape classes and have the constructors set up the required parameters.
For squares, you will need to know the length
of a side:
class Square
{
public $length;
public function construct($length)
{
$this->length = $length;
}
}
For circles, you will need to know the radius
:
class Circle
{
public $radius;
public function construct($radius)
{
$this->radius = $radius;
}
}
Next, create the AreaCalculator
class and write the logic to sum up the areas of all provided shapes. The area of a square is calculated by length squared, and the area of a circle is calculated by pi times radius squared.
class AreaCalculator
{
protected $shapes;
public function __construct($shapes = [])
{
$this->shapes = $shapes;
}
public function sum()
{
foreach ($this->shapes as $shape) {
if (is_a($shape, 'Square')) {
$area[] = pow($shape->length, 2);
} elseif (is_a($shape, 'Circle')) {
$area[] = pi() * pow($shape->radius, 2);
}
}
return array_sum($area);
}
public function output()
{
return implode('', [
'',
'Sum of the areas of provided shapes: ',
$this->sum(),
'',
]);
}
}
To use the AreaCalculator
class, you will need to instantiate it, pass in an array of shapes, and display the output at the bottom of the page.
Here is an example with a collection of three shapes:
$shapes = [
new Circle(2),
new Square(5),
new Square(6),
];
$areas = new AreaCalculator($shapes);
echo $areas->output();
The problem with the output method is that the AreaCalculator
handles the logic to output the data.
Consider a scenario where the output should be converted to another format like JSON.
The AreaCalculator
class would handle all of the logic, violating the single responsibility principle. The AreaCalculator
class should only be concerned with the sum of the areas of provided shapes. It should not care whether the user wants JSON or HTML.
To address this, you can create a separate SumCalculatorOutputter
class and use that new class to handle the logic you need to output the data to the user:
class SumCalculatorOutputter
{
protected $calculator;
public function __constructor(AreaCalculator $calculator)
{
$this->calculator = $calculator;
}
public function JSON()
{
$data = [
'sum' => $this->calculator->sum(),
];
return json_encode($data);
}
public function HTML()
{
return implode('', [
'',
'Sum of the areas of provided shapes: ',
$this->calculator->sum(),
'',
]);
}
}
The SumCalculatorOutputter
class would work like this:
$shapes = [
new Circle(2),
new Square(5),
new Square(6),
];
$areas = new AreaCalculator($shapes);
$output = new SumCalculatorOutputter($areas);
echo $output->JSON();
echo $output->HTML();
The SumCalculatorOutputter
class handles the logic needed to output the data to the user.
That satisfies the single-responsibility principle.
Open-closed Principle (OCP) states:
Objects or entities should be open for extension but closed for modification.
This means that a class should be extendable without modifying the class itself.
Let’s revisit the AreaCalculator
class and focus on the sum
method:
class AreaCalculator
{
protected $shapes;
public function __construct($shapes = [])
{
$this->shapes = $shapes;
}
public function sum()
{
foreach ($this->shapes as $shape) {
if (is_a($shape, 'Square')) {
$area[] = pow($shape->length, 2);
} elseif (is_a($shape, 'Circle')) {
$area[] = pi() * pow($shape->radius, 2);
}
}
return array_sum($area);
}
}
Consider a scenario where the user would like the sum
of additional shapes like triangles, pentagons, hexagons, etc. You would have to constantly edit this file and add more if
/else
blocks. That would violate the open-closed principle.
A way you can make this sum
method better is to remove the logic to calculate the area of each shape out of the AreaCalculator
class method and attach it to each shape’s class.
Here is the area
method defined in Square
:
class Square
{
public $length;
public function __construct($length)
{
$this->length = $length;
}
public function area()
{
return pow($this->length, 2);
}
}
And here is the area
method defined in Circle
:
class Circle
{
public $radius;
public function construct($radius)
{
$this->radius = $radius;
}
public function area()
{
return pi() * pow($shape->radius, 2);
}
}
The sum
method for AreaCalculator
can then be rewritten as:
class AreaCalculator
{
// ...
public function sum()
{
foreach ($this->shapes as $shape) {
$area[] = $shape->area();
}
return array_sum($area);
}
}
Now, you can create another shape class and pass it in when calculating the sum without breaking the code.
However, another problem arises. How do you know that the object passed into the AreaCalculator
is actually a shape or if the shape has a method named area
?
Coding to an interface is an integral part of SOLID.
Create a ShapeInterface
that supports area
:
interface ShapeInterface
{
public function area();
}
Modify your shape classes to implement
the ShapeInterface
.
Here is the update to Square
:
class Square implements ShapeInterface
{
// ...
}
And here is the update to Circle
:
class Circle implements ShapeInterface
{
// ...
}
In the sum
method for AreaCalculator
, you can check if the shapes provided are actually instances of the ShapeInterface
; otherwise, throw an exception:
class AreaCalculator
{
// ...
public function sum()
{
foreach ($this->shapes as $shape) {
if (is_a($shape, 'ShapeInterface')) {
$area[] = $shape->area();
continue;
}
throw new AreaCalculatorInvalidShapeException();
}
return array_sum($area);
}
}
That satisfies the open-closed principle.
Liskov Substitution Principle states:
Let q(x) be a property provable about objects of x of type T. Then q(y) should be provable for objects y of type S where S is a subtype of T.
This means that every subclass or derived class should be substitutable for their base or parent class.
Building off the example AreaCalculator
class, consider a new VolumeCalculator
class that extends the AreaCalculator
class:
class VolumeCalculator extends AreaCalculator
{
public function construct($shapes = [])
{
parent::construct($shapes);
}
public function sum()
{
// logic to calculate the volumes and then return an array of output
return [$summedData];
}
}
Recall that the SumCalculatorOutputter
class resembles this:
class SumCalculatorOutputter {
protected $calculator;
public function __constructor(AreaCalculator $calculator) {
$this->calculator = $calculator;
}
public function JSON() {
$data = array(
'sum' => $this->calculator->sum();
);
return json_encode($data);
}
public function HTML() {
return implode('', array(
'',
'Sum of the areas of provided shapes: ',
$this->calculator->sum(),
''
));
}
}
If you tried to run an example like this:
$areas = new AreaCalculator($shapes);
$volumes = new VolumeCalculator($solidShapes);
$output = new SumCalculatorOutputter($areas);
$output2 = new SumCalculatorOutputter($volumes);
When you call the HTML
method on the $output2
object, you will get an E_NOTICE
error, informing you of an array-to-string conversion.
To fix this, instead of returning an array from the VolumeCalculator
class sum method, return $summedData
:
class VolumeCalculator extends AreaCalculator
{
public function construct($shapes = [])
{
parent::construct($shapes);
}
public function sum()
{
// logic to calculate the volumes and then return a value of output
return $summedData;
}
}
The $summedData
can be a float, double or integer.
That satisfies the Liskov substitution principle.
The interface segregation principle states:
A client should never be forced to implement an interface that it doesn’t use, or clients shouldn’t be forced to depend on methods they do not use.
Still building from the previous ShapeInterface
example, you will need to support the new three-dimensional shapes of Cuboid
and Spheroid
, and these shapes will need to also calculate volume
.
Let’s consider what would happen if you were to modify the ShapeInterface
to add another contract:
interface ShapeInterface
{
public function area();
public function volume();
}
Now, any shape you create must implement the volume
method, but you know that squares are flat shapes and that they do not have volumes, so this interface would force the Square
class to implement a method that it has no use of.
This would violate the interface segregation principle. Instead, you could create another interface called ThreeDimensionalShapeInterface
that has the volume
contract, and three-dimensional shapes can implement this interface:
interface ShapeInterface
{
public function area();
}
interface ThreeDimensionalShapeInterface
{
public function volume();
}
class Cuboid implements ShapeInterface, ThreeDimensionalShapeInterface
{
public function area()
{
// calculate the surface area of the cuboid
}
public function volume()
{
// calculate the volume of the cuboid
}
}
This is a much better approach, but a pitfall to watch out for is when type-hinting these interfaces. Instead of using a ShapeInterface
or a ThreeDimensionalShapeInterface
, you can create another interface, maybe ManageShapeInterface
, and implement it on both the flat and three-dimensional shapes.
This way, you can have a single API for managing the shapes:
interface ManageShapeInterface
{
public function calculate();
}
class Square implements ShapeInterface, ManageShapeInterface
{
public function area()
{
// calculate the area of the square
}
public function calculate()
{
return $this->area();
}
}
class Cuboid implements ShapeInterface, ThreeDimensionalShapeInterface, ManageShapeInterface
{
public function area()
{
// calculate the surface area of the cuboid
}
public function volume()
{
// calculate the volume of the cuboid
}
public function calculate()
{
return $this->area();
}
}
Now in AreaCalculator
class, you can replace the call to the area
method with calculate
and also check if the object is an instance of the ManageShapeInterface
and not the ShapeInterface
.
That satisfies the interface segregation principle.
Dependency inversion principle states:
Entities must depend on abstractions, not on concretions. It states that the high-level module must not depend on the low-level module, but they should depend on abstractions.
This principle allows for decoupling.
Here is an example of a PasswordReminder
that connects to a MySQL database:
class MySQLConnection
{
public function connect()
{
// handle the database connection
return 'Database connection';
}
}
class PasswordReminder
{
private $dbConnection;
public function __construct(MySQLConnection $dbConnection)
{
$this->dbConnection = $dbConnection;
}
}
First, the MySQLConnection
is the low-level module while the PasswordReminder
is high level, but according to the definition of D in SOLID, which states to Depend on abstraction, not on concretions. This snippet above violates this principle as the PasswordReminder
class is being forced to depend on the MySQLConnection
class.
Later, if you were to change the database engine, you would also have to edit the PasswordReminder
class, which would violate the open-close principle
.
The PasswordReminder
class should not care what database your application uses. To address these issues, you can code to an interface since high-level and low-level modules should depend on abstraction:
interface DBConnectionInterface
{
public function connect();
}
The interface has a connect method, and the MySQLConnection
class implements this interface. Also, instead of directly type-hinting the MySQLConnection
class in the constructor of the PasswordReminder
, you type-hint the DBConnectionInterface
. No matter the type of database your application uses, the PasswordReminder
class can connect to the database without any problems, and the open-close principle is not violated.
class MySQLConnection implements DBConnectionInterface
{
public function connect()
{
// handle the database connection
return 'Database connection';
}
}
class PasswordReminder
{
private $dbConnection;
public function __construct(DBConnectionInterface $dbConnection)
{
$this->dbConnection = $dbConnection;
}
}
This code establishes that both the high-level and low-level modules depend on abstraction.
This article introduced the five principles of the SOLID Code. Projects that adhere to these principles can be shared with collaborators, extended, modified, tested, and refactored with fewer complications.
Continue your learning by reading about other practices for Agile and Adaptive software development.
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Amazing content! It was a great help, thanks!
the exact same article on Medium by a different author https://pratikraj.medium.com/solid-principles-of-oops-e85db3b3a897
Great content
I’m sorry to say it, but that first example is a terrible one. Why would Shape classes outsource the calculation of their areas to an external class? I get what you were going for, but this example violates the OO principles of encapsulation and polymorphism to demonstrate single responsibility, which is kind of like taking one step forward and three steps backward.
The article’s exploration of Liskov Substitution Principle (LSP) and Interface Segregation Principle (ISP) was equally enlightening. Understanding how to design classes and interfaces that adhere to these principles ensures smoother collaboration among components and reduces the likelihood of unexpected issues down the line.
Dependency Inversion Principle (DIP) can be a bit abstract, but the article does an excellent job of simplifying it. The explanations and examples provided make it easier to grasp the idea of inverting the traditional dependency hierarchy for improved flexibility and maintainability.
I believe that what is written about the SRP is wrong. You are saying that “A class should have one and only one reason to change, meaning that a class should have only one job.” You are confusing the rule “Do one thing and do it well” with the SRP. The example you are showing has nothing to do with the SRP. Please read the blog from Robert C. Martin himself: http://blog.cleancoder.com/uncle-bob/2014/05/08/SingleReponsibilityPrinciple.html
The SRP is more correctly: “A class should only be responsible to one stakeholder” or “A class should only be responsible to one actor”. (From Robert C. Martins book: Clean Architecture)
Very informative indeed. Thanks for this.
Very informative indeed. Thanks for this.
Great Content! In my opinion,
ManageShapeInterface
is unnecessary when trying to calculate area(), becauseAreaCalculator
needs just theShapeInterface
to be able to calculate area().This comment has been deleted