Kotlin Tutorial
Kotlin Classes
Constructor
A constructor is a special method of a class in OOP that initializes a newly created object of that type.
In Kotlin, There are two types of constructor – Primary and Secondary.
Primary Constructor:
Primary constructor is the part of the class header. It can be created by using () after the class name and optional type parameters (like regular parameters in functions).
Example:
class Human(var name: String,var role: String)
Notice: The var keyword before each parameter. It means these are the properties of the class.
Let’s get things more interesting by creating the objects of the class
val h1 = Human("Steve Jobs", "Founder")
val h2 = Human("Tim Cook", "CEO")
Done? Let’s print it now:
println("I'm ${h1.name}; ${h1.role} of Apple")
println("I'm ${h2.name}; ${h2.role} of Apple")
The code above will outputs:
I'm Steve Jobs; Founder of Apple
I'm Tim Cook; CEO of Apple
Secondary Constructor:
Kotlin classes can have one or more secondary constructors. They are created by the keyword constructor.
Example
class Human {
constructor()
}
Now the question is how to use it?
Unlike the primary constructor, you have to create a variable inside the class that will be the property of the class.
Example:
class Human {
var name = ""
constructor()
}
Good, now add it inside the secondary constructor.
Example:
class Human {
var name = ""
constructor(name: String) {
// Body
}
}
Notice: Unlike the primary constructor, we have a body {} for secondary constructor also.
One last thing remaining here, initialize the variable (property) inside the constructor body.
Example:
class Human {
var name = ""
constructor(name: String) {
// Body
this.name = name
}
}
This Means: The name declared inside the secondary constructor
constructor(name: String)
initialized to the variable
var name = ""
(property).
Create an object and access it.
Example:
val h1 = Human("Steve Jobs");
println(h1.name)
RUN your code; above code will outputs:
Steve Jobs
Want to know? There is one more situation 💡.
What if we have an property in primary constructor age: Int: You will see a compile time error (primary constructor call expected) below the secondary constructor right?.
class Human(var age: Int) {
var name = ""
constructor(name: String) {
// Body
this.name = name
}
}
In this case you have to put the same property inside the secondary constructor, modify your constructor accordingly:
constructor(name: String, age: Int)
Done? But still the same compile time error; OK now with the help of this keyword call the primary constructor and pass the property inside that will simply instantiate the field variable (property) of age:
constructor(name: String, age: Int) : this(age)
With this done, If you have the object already created then you will see the compile time error again below the object otherwise create the object:
Change the object from this :
val h1 = Human("Steve Jobs");
To
val h1 = Human("Steve Jobs", 23);
Or whatever age you want to put 😁.
One more thing:
println(h1.age)
The code above outputs:
Steve Jobs
23
Solution Code:
fun main(args: Array<String>) {
val h1 = Human("Steve Jobs", 23);
println(h1.name)
println(h1.age)
}
class Human(var age: Int) {
var name = ""
constructor(name: String, age: Int) : this(age) {
// Body
this.name = name
}
}
