What is static property? When should we use static property?

Have you seen a static property in a class?

What is static property? When should we use static property?
If you are searching the answers of these questions than this article is for you. I have used PHP to explain this concept. But, I believe anyone from any other programming language, will be able to understand.

I am sure that you have heard the line while learning object oriented programming(OOP) that "a class is a blueprint or design of objects", right? So,if we have a class of a Car, we can programmatically make an instance of Car. Same as, if we have an architectural design(blueprint) of house we can build one or multiple houses using that blueprint. In OOP the architectural design is class and the house(s) we have build is object. Then, what is property? A property is something that define the state of the created object. For better understanding, let's see the code below:

class Car{
  public $color;
  
  public function __construct($color) {
    $this->color = $color;
  }
}

$car1 = new Car('Blue');
$car2 = new Car('Black');

echo $car1->color; // Blue
echo $car2->color; // Black

This is obvious, right? But why? color seems a property of one single class but still color of $car1 and $car2 is Blue and Black respectively. No! color is not a property of Car class but property of created objects. It is because, this property is not state of the Class, it's state of Object. Let's verify this little bit further:

$car1->color = 'Red';

echo $car1->color; // Red
echo $car2->color; // Black

Here, we have changed the color of $car1 from Black to Red but this operation has not effect on the color of another car object $car2. Now, we are quite sure that the property color is associated with Object not with the Class. In other words, the color not the state of architectural design of Car but it is state of an specific instance.

Now, let's say we are in a situation where we need to count the number of created car(eg. $totalCarCreated) using the Car class. Is it a property of a specific car object? Let's think about it...

class Car{
    public $color;
    public $totalCarCreated = 0;
    
    public function __construct($color) {
        $this->color = $color;
        $this->totalCarCreated++;
    }
}

$car1 = new Car('Blue');
$car2 = new Car('Black');

echo $car1->totalCarCreated; // 1
echo $car2->totalCarCreated; // 1

We have created 2 car using our class but $car1 and $car2 is returning 1! Is it logical to have the $totalCarCreated property in a specific car. Is it logical to write $car1->totalCarCreated? I mean should we ask $car1, how many car have been created? Should our Black car know how many car have been created using this same design? NO! Here, we are trying to associate a property with an object which is not a state of that object but an state of the Class itself. Here comes our static property. When a property is not associated with an object but with the class, we define that property as static property or class property.

Let's modify our previous code as below to implement what we have just learned:

class Car{
    public $color;
    public static $totalCarCreated = 0;
    
    public function __construct($color) {
        $this->color = $color;
        self::$totalCarCreated++; // static::totalCarCreated++;
    }
} 

$car1 = new Car('Blue');
$car2 = new Car('Black');

echo Car::$totalCarCreated; // 2

Have you noticed?

1. The static property $totalCarCreated is defined using the keyword static.
2. In the constructor we have used self(or static) keyword to access the static property.
3. $totalCarCreated is not property of $car1 or $car2 so we accessed it as Car::$totalCarCreated

I hope this article explains clearly what is static property and when we should use static property. My initial plan was to explain both static property and method in this very same article. But I think it is already a long one. So, in my next article I will try to explain what is static method and when we should use static method. Till then, Happy Programming!