What is class in Kotlin? Example: class MyBook { ... }

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What is class in Kotlin?

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обновлено: 3 February 2025

Class in Kotlin can contain:
• Constructor (primary)
• Constructor (secondary)
• Initialization block init
• fields
• Properties
• Functions
• Nested Classes
There is no destructor in the class in Kotlin.

In general terms:
The class in Kotlin describes the object.
Kotlin program can be represented as interrelated objects.

Syntax

To define a class

Use the word class

Kotlin

class MyBook
{
...
}

To create a class object:

используем Class Name(parameters or no parameters)

Kotlin

MyBook("Hello!")

Note!
In other languages, the word new has always been used
In Java we wrote

Java

new MyBook("Hello!")

Example 1 with secondary constructor

Kotlin

class MyBook
{
     var bookText:String // field

     constructor(text:String) // constructor
     {
          bookText = text;
     }

     fun Show() // method
     {
          println(bookText.toString())
     }
}

fun main() {
     val book1 = MyBook("Hello!") // create an object

     book1.Show() // call the method
}

Note!
It's a secondary constructor, that's how the theory describes it, because it's inside a class

Example 2 with primary constructor

Kotlin

class MyBook(var bookText:String) // constructor and field in the same expression
{
     fun Show() // method
     {
          println(bookText.toString())
     }
}

fun main() {
     val book1 = MyBook("Hello!") // create an object

     book1.Show() // call the method
}

Notes
It is primary constructor

Example 3 with primary constructor and init block

Kotlin

class MyShop(name: String, street:String) {
     val shopName = "Welcome to $name"

     init {
          println("first block of initialization ... if need use $name")
     }

     val streetName = "good $street"

     init {
          println("second block of initialization ... if need use streetName")
     }
}

fun main() {

     val shop1 = MyShop("Book store", "Orange avenu") // create an object

}

Note!
Primary constructor cannot contain executable code.
If we need to execute some initializing code, it can be placed in the appropriate blocks, which are marked with the word init.
When you create a class object, the initialization blocks are executed in the order in which they go in the body of the class, interspersed with the initialization of properties.

Example 4 with primary constructor and secondary constructor

Kotlin

class MyShop(val shopName:String) {

     private val shopFruits:MutableList<String> = mutableListOf()

     constructor(fruits:List<String>, name:String) : this(name) {

          shopFruits.addAll(0, fruits)
     }

     fun Show()
     {
          println("$shopName has fruits:")
          for (item in shopFruits)
               println(item)
     }
}

fun main() {

     val shop1:MyShop = MyShop(listOf("orange", "banana"), "Market")
     shop1.Show()

}

Let's run the program and see the result:

Market has fruits:
orange
banana

Note!
If the class has a primary constructor and a secondary constructor.
In this case, the secondary constructor must call the primary constructor using the word this.

this(Parameters of the main constructor)

Example 5 There are no constructors, but there is a block init

Kotlin

class MyShop {

     private val shopFruits:MutableList<String> = mutableListOf()

     init {
          shopFruits.addAll(listOf("orange", "banana"))
     }

     fun Show()
     {
          for (item in shopFruits)
               println(item)
     }
}

fun main() {
     val shop1:MyShop = MyShop()
     shop1.Show()
}

Let's run the program and see the result:

orange
banana

Note!
If a class does not have a primary constructor and a secondary constructor, the initialization block will still execute.

Example 6 no constructors and no block init

Kotlin

class MyShop {

     var shopFruits:MutableList<String> = mutableListOf()

     fun Show()
     {
          for (item in shopFruits)
               println(item)
     }
}

fun main() {

     val shop1:MyShop = MyShop()
     shop1.shopFruits.addAll(listOf("orange", "banana"))
     shop1.Show()

}

Note!
If a class does not have a primary constructor and does not have a secondary constructor, that class will automatically generate an empty, invisible constructor with no parameters.
The visibility of this constructor will be public.

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