Every variable and data structure in Kotlin has a type. Types are important because they tell the compiler what you are allowed to do with that variable or data structure. In other words, what functions and properties it has.
In the last chapter, Kotlin was able to tell in the previous example that customers has type Int. Kotlin's ability to infer the type is called type inference. customers is assigned an integer value. From this, Kotlin infers that customers has a numerical type Int. As a result, the compiler knows that you can perform arithmetic operations with customers:
fun main() {
//sampleStart
var customers = 10
// Some customers leave the queue
customers = 8
customers = customers + 3 // Example of addition: 11
customers += 7 // Example of addition: 18
customers -= 3 // Example of subtraction: 15
customers *= 2 // Example of multiplication: 30
customers /= 3 // Example of division: 10
println(customers) // 10
//sampleEnd
}
In total, Kotlin has the following basic types:
Category
Basic types
Example code
Integers
Byte, Short, Int, Long
val year: Int = 2020
Unsigned integers
UByte, UShort, UInt, ULong
val score: UInt = 100u
Floating-point numbers
Float, Double
val currentTemp: Float = 24.5f, val price: Double = 19.99
Booleans
Boolean
val isEnabled: Boolean = true
Characters
Char
val separator: Char = ','
Strings
String
val message: String = "Hello, world!"
For more information on basic types and their properties, see Basic types.
With this knowledge, you can declare variables and initialize them later. Kotlin can manage this as long as variables are initialized before the first read.
To declare a variable without initializing it, specify its type with :. For example:
fun main() {
//sampleStart
// Variable declared without initialization
val d: Int
// Variable initialized
d = 3
// Variable explicitly typed and initialized
val e: String = "hello"
// Variables can be read because they have been initialized
println(d) // 3
println(e) // hello
//sampleEnd
}
If you don't initialize a variable before it is read, you see an error:
fun main() {
//sampleStart
// Variable declared without initialization
val d: Int
// Triggers an error
println(d)
// Variable 'd' must be initialized
//sampleEnd
}
Now that you know how to declare basic types, it's time to learn about collections.
Practice
Exercise
Explicitly declare the correct type for each variable:
fun main() {
val a: Int = 1000
val b = "log message"
val c = 3.14
val d = 100_000_000_000_000
val e = false
val f = '\n'
}
fun main() {
val a: Int = 1000
val b: String = "log message"
val c: Double = 3.14
val d: Long = 100_000_000_000_000
val e: Boolean = false
val f: Char = '\n'
}