Constructor
- Kotlin course
- Beginner
- Lesson 107/116
- 5 min
Learn to define the class constructor.
Constructor: initialising with a primary constructor
In Kotlin the primary constructor is written next to the class name. If you mark a parameter with val or var it becomes both a property and a constructor parameter.
class Person(val name: String)
val p = Person("Alex")
println(p.name) // AlexThe constructor forces a value to be set when an instance is created. val name declares both the parameter and an immutable property in one line. This is the core of Kotlin's concise class syntax.
Calling the constructor
To call the constructor you write the values in parentheses after the class name. You access properties with a dot.
val p = Person("Alex")
println(p.name)Person("Alex") creates a new instance and sets name. You then read it via p.name. If you used var the value could change later; val stays fixed.
Practice Add a name property to Person
Define class Person(val name: String). In main create val p = Person("Alex") and print p.name with println.
Starter code
fun main() {
// Define a Person class with a name property
// Create an instance and print its name
}Sample solution
class Person(val name: String)
fun main() {
val p = Person("Alex")
println(p.name)
}Expected output
Ayşe
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Fill in the blank: declare with val
class Person(____ name: String)class Person(val name: String)Explanation: val name becomes both a property and a constructor parameter.
2 What does class Person(val name: String) do?
- Only declares a constructor
- Only declares a property
- Defines the name property and a constructor Correct answer
- Declares a method
Explanation: val name is both a property and a constructor parameter.
3 Match piece to role
| Term | Meaning |
|---|---|
class Person |
Class |
val name |
Property |
Person("Alex") |
Instance |
p.name |
Access |