By Samuel Oloruntoba, Anish Singh Walia and Manikandan Kurup
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()
{
$area = [];
foreach ($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 AreaCalculator
class, as designed above, has several issues regarding the Single Responsibility Principle:
sum()
method. This means if you introduce a new shape (e.g., Triangle), you have to modify AreaCalculator
. This violates SRP (the AreaCalculator
has a reason to change for every new shape type).output()
method. This is a separate concern from calculation.Let’s apply SRP in two stages to address these issues.
In a truly object-oriented design and aligning with SRP, a shape’s single responsibility should include knowing its own properties and how to perform operations intrinsically related to those properties, such as calculating its own area. The AreaCalculator
should not need to know the specific formula for every shape.
To address this, we move the area()
calculation logic into each individual shape 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 = $radius;
}
public function area()
{
return pi() * pow($this->radius, 2);
}
}
Now, the sum
method for AreaCalculator
can be rewritten to simply ask each shape for its area, without knowing the specific calculation formula:
class AreaCalculator{
protected $shapes;
public function __construct($shapes = [])
{
$this->shapes = $shapes;
}
public function sum()
{
$area = [];
foreach ($this->shapes as $shape) {
// Each shape is now responsible for its own area calculation
$area[] = $shape->area();
}
return array_sum($area);
}
public function output()
{
// This method still handles output, which we'll address next
return implode('', [
'',
'Sum of the areas of provided shapes: ',
$this->sum(),
'',
]);
}
}
Now, adding a new shape type (e.g., Triangle
) will not require modifying the AreaCalculator
’s sum()
method. We’ve applied SRP by giving shapes the single responsibility of knowing their own area.
Even after Stage 1, the AreaCalculator
class still has two primary responsibilities:
Consider a scenario where the output should be converted to another format like JSON. The AreaCalculator
class would currently handle all of this output logic directly within its output()
method. If we needed a JSON output, we’d add more logic to output()
, or create outputJson()
, etc. This means the AreaCalculator
would change not only if the calculation logic changed, but also if the output format changed.
The AreaCalculator
class should primarily be concerned with the sum of the areas of provided shapes and should not care whether the user wants JSON or HTML.
To address this, you can apply SRP by separating the concerns. You can create a separate SumCalculatorOutputter
class (or classes) and use that new class to handle the logic you need to output the data to the user:
class SumCalculatorOutputter
{
protected $calculator;
public function __construct(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();
Now, the AreaCalculator
class has a single responsibility: calculating the sum of areas and SumCalculatorOutputter
has a single responsibility of formatting and outputting calculation results. This separation satisfies the Single Responsibility Principle for these two classes.
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 = $radius;
}
public function area()
{
return pi() * pow($this->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 __construct(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, a double or an 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.
This principle emphasizes that large, general-purpose interfaces should be broken down into smaller, more specific ones. This way, client classes only need to know about the methods that are relevant to them.
Continuing from the previous ShapeInterface
example, let’s consider a scenario where you need to support new three-dimensional shapes of Cuboid
and Spheroid
, and these shapes will need to also calculate both area
and 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 doesn’t need.
This would violate the interface segregation principle. Instead of having one large, monolithic interface, we should create separate, more granular interfaces that define specific capabilities.
We keep ShapeInterface
for two-dimensional shapes that only have an area:
interface ShapeInterface
{
public function area();
}
And we create a new interface specifically for three-dimensional shapes that can calculate volume:
interface ThreeDimensionalShapeInterface
{
public function volume();
}
Now, concrete shape classes can implement only the interfaces that are relevant to their capabilities.
For 2D shapes:
class Square implements ShapeInterface { // Only implements area()
public $length;
public function __construct($length) {
$this->length = $length;
}
public function area() {
return pow($this->length, 2);
}
}
For 3D Shapes (e.g., Cuboid) which have both surface area and volume, the class implements both relevant interfaces.
class Cuboid implements ShapeInterface, ThreeDimensionalShapeInterface
{
public function area()
{
// calculate the surface area of the cuboid
}
public function volume()
{
// calculate the volume of the cuboid
}
}
The Square
class (and any other 2D shape) is no longer forced to implement a volume()
method it doesn’t need. It only implements ShapeInterface
and its area()
method.
This approach ensures that clients are not forced to depend on interfaces (or methods within interfaces) that they do not use, leading to cleaner, more cohesive code.
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 (e.g., from MySQL to PostgreSQL or an API service), you would also have to edit the PasswordReminder
class, which would violate the open-close principle
, as the class would need modification for extension.
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.
First, define an interface for database connections. This interface (DBConnectionInterface
) serves as the 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.
This interface simply declares what a database connection object should be able to do (connect()
), without specifying how it does it.
Next, your concrete MySQLConnection
class will implement this interface, providing the specific details of how to connect to a MySQL database:
class MySQLConnection implements DBConnectionInterface
{
public function connect()
{
// handle the database connection
return 'Database connection established';
}
}
Now, in the constructor of the PasswordReminder
class, instead of directly type-hinting the concrete MySQLConnection
class, you type-hint the DBConnectionInterface
.
class PasswordReminder{
private $dbConnection;
public function __construct(DBConnectionInterface $dbConnection) // Type-hinting the interface
{
$this->dbConnection = $dbConnection;
}
public function remind() {
$connectionStatus = $this->dbConnection->connect();
return "Password reminder process initiated. Connection status: " . $connectionStatus;
}
}
When you use PasswordReminder
, you provide it with an instance of a class that implements DBConnectionInterface
. The PasswordReminder
doesn’t need to know the specific type of connection (MySQL, PostgreSQL, etc.); it just knows it has an object that guarantees it can call a connect()
method.
Here’s how you would use it in your application:
// Create a concrete MySQL connection object
$mysqlConnector = new MySQLConnection();
// Inject the concrete MySQL connection object into the PasswordReminder
// The PasswordReminder only sees it as a DBConnectionInterface
$passwordReminder = new PasswordReminder($mysqlConnector);
echo $passwordReminder->remind(); // Output: Password reminder process initiated. Connection status: MySQL Database connection established.
If you later decide to switch to a PostgreSQL database, you would simply create a PostgreSQLConnection
class that also implements DBConnectionInterface
and, in your application’s setup, simply change which concrete class you instantiate and inject:
$pgConnector = new PostgreSQLConnection();
$passwordReminder = new PasswordReminder($pgConnector);
echo $passwordReminder->remind();
This demonstrates how easily you can swap implementations thanks to dependency inversion.
Notice that the PasswordReminder
class itself did not need to be changed or modified when the underlying database technology changed. Both the high-level PasswordReminder
module and the low-level MySQLConnection
(or PostgreSQLConnection
) module now depend on the DBConnectionInterface
abstraction, not on each other’s concretions. This makes the system more flexible, easier to test (you can inject mock database connections), and adheres to the Open-Closed Principle.
SOLID is an acronym representing five fundamental object-oriented design principles formulated by Robert C. Martin, also known as Uncle Bob. These principles are:
Together, these principles establish a set of best practices for developing software that is robust, adaptable, and easy to maintain and extend as projects evolve.
SOLID principles are critically important in object-oriented programming because they directly address common challenges in software development, such as rigidity, fragility, immobility, and viscosity. By adhering to SOLID, developers can build systems that are:
Ultimately, SOLID helps in creating high-quality software that is easier to evolve and has a longer lifespan.
Applying the Single Responsibility Principle (SRP) involves understanding the “responsibilities” of a class or module. If you can identify more than one distinct “reason to change” for a given class, it likely violates SRP. For instance, a class that both processes data and saves it to a database has two reasons to change: one if the data processing logic changes, and another if the database interaction method changes.
To apply SRP, you should:
This separation leads to smaller, more focused, and highly cohesive components that are easier to understand, test, and maintain.
While both the Open-Closed Principle (OCP) and the Dependency Inversion Principle (DIP) aim to reduce coupling and increase flexibility, they address different aspects of design:
In essence, DIP is often a key enabler for OCP. By depending on abstractions (DIP), you can easily substitute different concrete implementations, allowing you to extend functionality (OCP) without altering the core logic that depends on those abstractions. OCP is a design goal, and DIP is a powerful pattern to achieve that goal.
While the SOLID principles were originally articulated in the context of Object-Oriented Programming (OOP) and use terms like “class” and “interface,” their underlying philosophies and benefits extend far beyond strict OOP. The core ideas of managing dependencies, isolating changes, promoting modularity, and enabling extensibility are universal to good software design.
By now, you’ve gained a solid understanding of the SOLID principles. These aren’t just abstract concepts; they’re powerful tools for improving the design of your object-oriented systems. Implementing them consistently leads to code that’s more maintainable, easier to extend, simpler to test, and less prone to breaking when requirements shift. Master SOLID, and you’ll elevate your development skills, creating software that stands the test of time.
Continue your learning by reading about other practices for Agile vs. Waterfall and Most Common Design Patterns in Java.
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Would you kindly clear dependency inversion for me. As I caught it, in the first example real MySQLConnection instance injected into PasswordReminder. But if we inject interface, which is actually nothing, will things work? Or we will have to implement it later? What’s then use of all this stuff?
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IMO the examples are not thought through. For example: does it make sense to have an object that is of ThreeDimensionalShapeInterface type and is not of type ShapeInterface? And the proposition here is to make developer remember to always implement the third interface ManageShapeInterface.
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