Classes in Kotlin

Kotlin is an object oriented programming language. Almost everything in Kotlin is an object, with its attributes and methods.

Class creates a user-defined data structure, which holds its own data members (attributes) and member functions (methods), which can be accessed and used by creating an instance of that class.

Classes are like a blueprint for objects outlining possible behaviors and states that every object of a certain type could have.

Classes are defined via the keyword class, like this:

// Define an empty class Employee
class Employee {

}

Objects/Instances

An Object is an instance of a Class. A class is like a blueprint while an instance is a copy of the class with actual values. It’s not an idea anymore, it’s an actual employee, like a person named "Andrew" who is 30 years old.

You can have many employees to create many different instances, but without the class as a guide, you would be lost, not knowing what information is required.

After a class is defined, we can create objects of the class by calling the class name like a function (there is no new keyword in Kotlin), like this:

// Define an empty class Employee
class Employee {

}

// inside main function:
val employee = Employee()

Attributes/Properties

A class attribute is a variable that belongs to a class and is shared between all the objects of this class. Let's add some attributes to our Employee class:

class Employee {
	var name: String = ""
	var age: Int = 0
}

// inside main function:
val emp1 = Employee()
	
emp1.name = "Andrew"
println(emp1.name) // prints "Andrew"
	
val emp2 = Employee()
	
emp2.age = 40
println(emp2.age) // prints 40

We added two properties: the String name and the Int age. Then, we created two different Employee objects emp1 and emp2.

The attributes of objects can be accessed and modified using the dot operator (.), like we did above with emp1.name and emp2.age.

In Kotlin, members are public by default, so name and age are public attributes, which means they can be accessed from anywhere in the code, not just inside the class code.

Constructors

In real life, we want to set the attributes specific to each object when creating it. For example, an employee named Andrew of 30 years old and another one named Mary of 25 years old. Here the constructor comes to rescue.

A constructor is a special type of method (function) which is used to initialize the instance members of the class.

In Kotlin a secondary constructor is declared with the constructor keyword, and it is always called when an object is created.

Let's add a constructor to our Employee class:

class Employee {
	var name: String = ""
	var age: Int = 0
	
	// constructor:
	constructor(name: String, age: Int) {
		this.name = name
		this.age = age
	}
}

// inside main function:
val emp1 = Employee("Andrew", 30)
	
println(emp1.name)
	
val emp2 = Employee("Mary", 25)

println(emp2.age)

// prints "Andrew" and 25 on different lines

The "this"

Every object in Kotlin has access to itself through an important reference called this.

The this reference is an implicit parameter to all member functions. Therefore, inside a member function, this may be used to access and manipulate the attributes of that object, like we did above with this.name = name.

Member functions

Member functions are functions that belong to the class:

class Employee {
	var name: String = ""
	var age: Int = 0
	
	// constructor:
	constructor(name: String, age: Int) {
		this.name = name
		this.age = age
	}
	
	//member functions:
	fun printName() {
		println(name) // it is not mandatory to use "this" to access an attribute
	}
		
	fun printEmployee() {
		printName()
		println(age)
	}
}

// inside main function:
val emp1 = Employee("Andrew", 30)

emp1.printEmployee() // prints "Andrew", 30
	
val emp2 = Employee("Mary", 25)

emp2.printEmployee() // prints "Mary", 25

As you can see, we access functions just like we access attributes: using the dot syntax (.).

The function call uses the attributes of the object it was called on. That's why the first printEmployee() prints 'Andrew' and 30 and the second prints 'Mary' and 25.

Assignment
Follow the Coding Tutorial and let's write some classes.

Hint
Look at the examples above if you get stuck.