Integer types

Kotlin provides a set of built-in types that represent numbers.
For integer numbers, there are four types with different sizes and, hence, value ranges:

When you initialize a variable with no explicit type specification, the compiler automatically infers the type with the smallest range enough to represent the value starting from Int. If it is not exceeding the range of Int, the type is Int. If it exceeds, the type is Long. To specify the Long value explicitly, append the suffix L to the value. Explicit type specification triggers the compiler to check the value not to exceed the range of the specified type.

The Compiler infers the type when a variable doesn't have a type specification

A numeric variable without a type can be inferred in Int or Long.

val one = 1 // Int
val threeBillion = 3000000000 // Long
val oneLong = 1L // Long
val oneByte: Byte = 1

Kotlin also provides unsigned integer types.

Floating-point types

For real numbers, Kotlin provides floating-point types Float and Double that adhere to the IEEE 754 standard. Float reflects the IEEE 754 single precision, while Double reflects double precision.

These types differ in their size and provide storage for floating-point numbers with different precision:

You can initialize Double and Float variables with numbers having a fractional part. It's separated from the integer part by a period (.) For variables initialized with fractional numbers, the compiler infers the Double type:

val oneLong = 1L // Long
val pi = 3.14 // Double
// val one: Double = 1 // Error: type mismatch
val eFloat = 2.7182818284f // Float, actual value is 2.7182817

Unlike some other languages, there are no implicit widening conversions for numbers in Kotlin. For example, a function with a Double parameter can be called only on Double values, but not Float, Int, or other numeric values:

fun main() {
    fun printDouble(d: Double) { print(d) }

    val i = 1
    val d = 1.0
    val f = 1.0f

    printDouble(d)
//    printDouble(i) // Error: Type mismatch
//    printDouble(f) // Error: Type mismatch
}

To convert numeric values to different types, use explicit conversions.

Literal constants for numbers

There are the following kinds of literal constants for integral values:

• Decimals: 123

• Longs are tagged by a capital L: 123L

• Hexadecimals: 0x0F

• Binaries: 0b00001011

Octal literals are not supported in Kotlin.

Kotlin also supports a conventional notation for floating-point numbers:

• Doubles by default: 123.5, 123.5e10

• Floats are tagged by f or F: 123.5f

You can use underscores to make number constants more readable:

val oneMillion = 1_000_000
val creditCardNumber = 1234_5678_9012_3456L
val socialSecurityNumber = 999_99_9999L
val hexBytes = 0xFF_EC_DE_5E
val bytes = 0b11010010_01101001_10010100_10010010

Numbers representation on the JVM

On the JVM platform, numbers are stored as primitive types: int, double, and so on. Exceptions are cases when you create a nullable number reference such as Int? or use generics. In these cases numbers are boxed in Java classes Integer, Double, and so on.

Nullable numbers are boxed in Java classes, primitive types are not.

Nullable references to the same number can refer to different objects:

val a: Int = 100
val boxedA: Int? = a
val anotherBoxedA: Int? = a

val b: Int = 10000
val boxedB: Int? = b
val anotherBoxedB: Int? = b

println(boxedA === anotherBoxedA) // true
println(boxedB === anotherBoxedB) // false

All nullable references to a are actually the same object because of the memory optimization that JVM applies to Integers between -128 and 127. It doesn't apply to the b references, so they are different objects.

On the other hand, they are still equal:

val b: Int = 10000
println(b == b) // Prints 'true'
val boxedB: Int? = b
val anotherBoxedB: Int? = b
println(boxedB == anotherBoxedB) // Prints 'true'

Explicit number conversions

Due to different representations, smaller types are not subtypes of bigger ones. If they were, we would have troubles of the following sort:

// Hypothetical code, does not actually compile:
val a: Int? = 1 // A boxed Int (java.lang.Integer)
val b: Long? = a // Implicit conversion yields a boxed Long (java.lang.Long)
print(b == a) // Surprise! This prints "false" as Long's equals() checks whether the other is Long as well

So equality would have been lost silently, not to mention identity.

As a consequence, smaller types are NOT implicitly converted to bigger types. This means that assigning a value of type Byte to an Int variable requires an explicit conversion:

val b: Byte = 1 // OK, literals are checked statically
// val i: Int = b // ERROR
val i1: Int = b.toInt()

All number types support conversions to other types:

• toByte(): Byte

• toShort(): Short

• toInt(): Int

• toLong(): Long

• toFloat(): Float

• toDouble(): Double

In many cases, there is no need for explicit conversions because the type is inferred from the context.

Arithmetical operations are overloaded for appropriate conversions, for example:

val l = 1L + 3 // Long + Int => Long

Operations on numbers

Kotlin supports the standard set of arithmetical operations over numbers: +, -, *, /, %. They are declared as members of appropriate classes

You can also override these operators for custom classes.

Division of integers

Division between integers numbers always returns an integer number.

Any fractional part is discarded.

This is true for a division between any two integer types:

val x = 5 / 2
//println(x == 2.5) // ERROR: Operator '==' cannot be applied to 'Int' and 'Double'
println(x == 2)  // true

To return a floating-point type,

explicitly convert one of the arguments to a floating-point type:

val x = 5L / 2
println(x == 2L) // true

val x = 5 / 2.toDouble()
println(x == 2.5)  // true

Bitwise operations

Kotlin provides a set of bitwise operations on integer numbers. They operate on the binary level directly with bits of the numbers' representation. Bitwise operations are represented by functions that can be called in infix form. They can be applied only to Int and Long:

val x = (1 shl 2) and 0x000FF000

Here is the complete list of bitwise operations:

• shl(bits) – signed shift left

• shr(bits) – signed shift right

• ushr(bits) – unsigned shift right

• and(bits) – bitwise AND

• or(bits) – bitwise OR

• xor(bits) – bitwise XOR

• inv() – bitwise inversion

Floating-point numbers comparison

The operations on floating-point numbers discussed in this section are:

• Equality checks: a == b and a != b

• Comparison operators: a < b, a > b, a <= b, a >= b

• Range instantiation and range checks: a..b, x in a..b, x !in a..b

References

https://kotlinlang.org/docs/numbers.html