Primary constructor
- Kotlin course
- Intermediate
- Lesson 1/116
- 5 min
Learn the primary constructor and init block.
Primary constructor
In Kotlin the primary constructor is declared in the class header, inside parentheses. By marking parameters with val or var you turn them directly into properties.
Example: class User(val name: String). Here name is both a constructor parameter and a class property. The body starts with an init block where initialization runs. This gives a short and readable definition.
The init block
The init block runs when an instance is created. Think of it as the body of the primary constructor. Multiple init blocks are allowed; they run top to bottom.
Example: class User(val name: String) { init { println("Ready: $name") } }. Properties are already assigned, so they can be used safely inside init. Parameter validation and side effects go here.
Practice Define the User class
Define a class named User. Its primary constructor takes a val name: String parameter. Add an init block that writes println("Created: $name"). In main create a User instance and print its name property.
Starter code
// Define the User class
// primary constructor: val name: String
// print inside init
// instantiate and print in mainSample solution
class User(val name: String) {
init {
println("Created: $name")
}
}
fun main() {
val u = User("Alex")
println(u.name)
}Expected output
Created: Alex Alex
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Which is a valid primary constructor definition?
- function User(name: String)
- class User(val name: String) Correct answer
- User class { name: String }
- def User(name):
Explanation: The class name, a parameter list in parentheses with val gives the primary constructor.
2 Open the class body
class User(val name: String) ____
init { println(name) }
}class User(val name: String) {
init { println(name) }
}Explanation: The curly brace opens the class body.
3 Match the parts of a primary constructor
| Term | Meaning |
|---|---|
class User |
Class header |
(val name: String) |
Parameter list |
init { ... } |
Initialization block |
val name |
Property parameter |