Properties
- Kotlin course
- Beginner
- Lesson 108/116
- 5 min
Learn to declare properties in a class.
Properties: val and var
A class's properties are declared with val or var. val is immutable, var is mutable.
class Box {
val capacity: Int = 10
var item: String = "empty"
}
val b = Box()
b.item = "pen" // ok
// b.capacity = 20 // compile errorA val property is set once; it cannot change. A var property can be changed any time. Kotlin generates a getter (and a setter for var) automatically.
Automatic getters and setters
Kotlin generates a getter for every property behind the scenes; for var it also generates a setter. You do not write them explicitly; it is handled for you.
class Person(var name: String)
val p = Person("Alex")
println(p.name) // getter called
p.name = "Ayşe" // setter calledp.name actually calls getName(). This concise syntax creates a full getter/setter pair behind the scenes. Later you will see how to customise them.
Practice Add an item property to Box
Define class Box(var item: String). In main create val b = Box("pen"); change the property with b.item = "notebook"; print b.item.
Starter code
fun main() {
// Define a Box class with a mutable item property
// Create an instance, change item, print the result
}Sample solution
class Box(var item: String)
fun main() {
val b = Box("pen")
b.item = "notebook"
println(b.item)
}Expected output
defter
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Match keyword to meaning
| Term | Meaning |
|---|---|
val |
Immutable |
var |
Mutable |
getter |
Reading |
setter |
Writing |
2 Fill in the blank: declare with var
class Box {
____ item: String = ""
}class Box {
var item: String = ""
}Explanation: var item is a mutable property.
3 What is the difference between val and var?
- val is immutable, var is mutable Correct answer
- val is mutable, var is immutable
- they are the same
- val does not exist
Explanation: val is set once, var can change later.
4 Fill in the blank: assign to the property
val b = Box()
b.____ = "pen"val b = Box()
b.item = "pen"Explanation: b.item = "pen" changes the var property.