Basic types

In Kotlin, everything is an object in the sense that we can call member functions and properties on any variable. Some types can have a special internal representation - for example, numbers, characters and booleans can be represented as primitive values at runtime - but to the user they look like ordinary classes. In this section we describe the basic types used in Kotlin: numbers, booleans, characters, strings, and arrays.

Numbers

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.

Type	Size (bits)	Min value	Max value
Byte	8	-128	127
Short	16	-32768	32767
Int	32	-2,147,483,648 (-2 ³¹)	2,147,483,647 (2 ³¹- 1)
Long	64	-9,223,372,036,854,775,808 (-2 ⁶³)	9,223,372,036,854,775,807 (2 ⁶³- 1)

All variables initialized with integer values not exceeding the maximum value of Int have the inferred type Int. If the initial value exceeds this value, then the type is Long. To specify the Long value explicitly, append the suffix L to the value.

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

Floating-point types

For real numbers, Kotlin provides floating-point types Float and Double. According to the IEEE 754 standard, floating point types differ by their decimal place, that is, how many decimal digits they can store. Float reflects the IEEE 754 single precision, while Double provides double precision.

Type	Size (bits)	Significant bits	Exponent bits	Decimal digits
Float	32	24	8	6-7
Double	64	53	11	15-16

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 pi = 3.14 // Double
// val one: Double = 1 // Error: type mismatch
val oneDouble = 1.0 // Double

To explicitly specify the Float type for a value, add the suffix f or F. If such a value contains more than 6-7 decimal digits, it will be rounded.

val e = 2.7182818284 // Double
val eFloat = 2.7182818284f // Float, actual value is 2.7182817

Note that 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

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

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.

Note that nullable references to the same number can be different objects:

fun main() {
//sampleStart
    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
//sampleEnd
}

All nullable references to a are actually the same object because of the memory optimization that JVM applies to Integer s 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:

fun main() {
//sampleStart
    val b: Int = 10000
    println(b == b) // Prints 'true'
    val boxedB: Int? = b
    val anotherBoxedB: Int? = b
    println(boxedB == anotherBoxedB) // Prints 'true'
//sampleEnd
}

Explicit 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.

fun main() {
//sampleStart
    val b: Byte = 1 // OK, literals are checked statically
    // val i: Int = b // ERROR
    val i1: Int = b.toInt()
//sampleEnd
}

All number types support conversions to other types:

toByte(): Byte
toShort(): Short
toInt(): Int
toLong(): Long
toFloat(): Float
toDouble(): Double
toChar(): Char

In many cases, there is no need in explicit conversions because the type is inferred from the context, and arithmetical operations are overloaded for appropriate conversions, for example:

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

Operations

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

fun main() {
//sampleStart
    println(1 + 2)
    println(2_500_000_000L - 1L)
    println(3.14 * 2.71)
    println(10.0 / 3)
//sampleEnd
}

You can also override these operators for custom classes. See Operator overloading for details.

Division of integers

Division between integers numbers always returns an integer number. Any fractional part is discarded.

fun main() {
//sampleStart
    val x = 5 / 2
    //println(x == 2.5) // ERROR: Operator '==' cannot be applied to 'Int' and 'Double'
    println(x == 2)
//sampleEnd
}

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

fun main() {
//sampleStart
    val x = 5L / 2
    println(x == 2L)
//sampleEnd
}

To return a floating-point type, explicitly convert one of the arguments to a floating-point type.

fun main() {
//sampleStart
    val x = 5 / 2.toDouble()
    println(x == 2.5)
//sampleEnd
}

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

When the operands a and b are statically known to be Float or Double or their nullable counterparts (the type is declared or inferred or is a result of a smart cast), the operations on the numbers and the range that they form follow the IEEE 754 Standard for Floating-Point Arithmetic.

However, to support generic use cases and provide total ordering, when the operands are not statically typed as floating point numbers (e.g. Any, Comparable<...>, a type parameter), the operations use the equals and compareTo implementations for Float and Double, which disagree with the standard, so that:

NaN is considered equal to itself
NaN is considered greater than any other element including POSITIVE_INFINITY
-0.0 is considered less than 0.0

Unsigned integers

In addition to integer types, Kotlin provides the following types for unsigned integer numbers:

UByte: an unsigned 8-bit integer, ranges from 0 to 255
UShort: an unsigned 16-bit integer, ranges from 0 to 65535
UInt: an unsigned 32-bit integer, ranges from 0 to 2^32 - 1
ULong: an unsigned 64-bit integer, ranges from 0 to 2^64 - 1

Unsigned types support most of the operations of their signed counterparts.

Unsigned arrays and ranges

Same as for primitives, each of unsigned type has corresponding type that represents arrays of that type:

UByteArray: an array of unsigned bytes
UShortArray: an array of unsigned shorts
UIntArray: an array of unsigned ints
ULongArray: an array of unsigned longs

Same as for signed integer arrays, they provide similar API to Array class without boxing overhead.

When you use unsigned arrays, you'll get a warning that indicates that this feature is not stable yet. To remove the warning, opt in using the @ExperimentalUnsignedTypes annotation. It's up to you to decide if your clients have to explicitly opt-in into usage of your API, but keep in mind that unsigned array are not a stable feature, so API which uses them can be broken by changes in the language. Learn more about opt-in requirements.

Ranges and progressions are supported for UInt and ULong by classes UIntRange, UIntProgression, ULongRange, and ULongProgression. Together with the unsigned integer types, these classes are stable.

Literals

To make unsigned integers easier to use, Kotlin provides an ability to tag an integer literal with a suffix indicating a specific unsigned type (similarly to Float or Long):

u and U tag unsigned literals. The exact type is determined based on the expected type. If no expected type is provided, compiler will use UInt or ULong depending on the size of literal.

val b: UByte = 1u  // UByte, expected type provided
val s: UShort = 1u // UShort, expected type provided
val l: ULong = 1u  // ULong, expected type provided

val a1 = 42u // UInt: no expected type provided, constant fits in UInt
val a2 = 0xFFFF_FFFF_FFFFu // ULong: no expected type provided, constant doesn't fit in UInt

uL and UL explicitly tag literal as unsigned long.

val a = 1UL // ULong, even though no expected type provided and constant fits into UInt

Further discussion

See language proposal for unsigned types for technical details and further discussion.

Booleans

The type Boolean represents boolean objects that can have two values: true and false.

Boolean has a nullable counterpart Boolean? that also has the null value.

Built-in operations on booleans include:

|| – disjunction (logical OR)
&& – conjunction (logical AND)
!- negation (logical NOT)

|| and && work lazily.

fun main() {
//sampleStart
    val myTrue: Boolean = true
    val myFalse: Boolean = false
    val boolNull: Boolean? = null
    
    println(myTrue || myFalse)
    println(myTrue && myFalse)
    println(!myTrue)
//sampleEnd
}

Characters

Characters are represented by the type Char. Character literals go in single quotes: '1'.

Special characters start from an escaping backslash \. The following escape sequences are supported: \t, \b, \n, \r, \', \", \\ and \$.

To encode any other character, use the Unicode escape sequence syntax: '\uFF00'.

fun main() {
//sampleStart
    val aChar: Char = 'a'
 
    println(aChar)
    println('\n') //prints an extra newline character
    println('\uFF00')
//sampleEnd
}

If a value of character variable is a digit, you can explicitly convert it to an Int number using the digitToInt() function.

Strings

Strings in Kotlin are represented by the type String. Generally, a string value is a sequence of characters in double quotes (").

val str = "abcd 123"

Elements of a string are characters that you can access via the indexing operation: s[i]. You can iterate over these characters with a for loop:

fun main() {
val str = "abcd"
//sampleStart
for (c in str) {
    println(c)
}
//sampleEnd
}

Strings are immutable. Once you initialize a string, you can't change its value or assign a new value to it. All operations that transform strings return their results in a new String object, leaving the original string unchanged.

fun main() {
//sampleStart
    val str = "abcd"
    println(str.uppercase()) // Create and print a new String object
    println(str) // the original string remains the same
//sampleEnd
}

To concatenate strings, use the + operator. This also works for concatenating strings with values of other types, as long as the first element in the expression is a string:

fun main() {
//sampleStart
val s = "abc" + 1
println(s + "def")
//sampleEnd
}

Note that in most cases using string templates or raw strings is preferable to string concatenation.

String literals

Kotlin has two types of string literals:

escaped strings that may contain escaped characters
raw strings that can contain newlines and arbitrary text

Here's an example of an escaped string:

val s = "Hello, world!\n"

Escaping is done in the conventional way, with a backslash (\). See Characters above for the list of supported escape sequences.

A raw string is delimited by a triple quote ("""), contains no escaping and can contain newlines and any other characters:

val text = """
    for (c in "foo")
        print(c)
"""

To remove leading whitespace from raw strings, use the trimMargin() function:

val text = """
    |Tell me and I forget.
    |Teach me and I remember.
    |Involve me and I learn.
    |(Benjamin Franklin)
    """.trimMargin()

By default, | is used as margin prefix, but you can choose another character and pass it as a parameter, like trimMargin(">").

String templates

String literals may contain template expressions - pieces of code that are evaluated and whose results are concatenated into the string. A template expression starts with a dollar sign ($) and consists of either a name:

fun main() {
//sampleStart
    val i = 10
    println("i = $i") // prints "i = 10"
//sampleEnd
}

or an expression in curly braces:

fun main() {
//sampleStart
    val s = "abc"
    println("$s.length is ${s.length}") // prints "abc.length is 3"
//sampleEnd
}

You can use templates both in raw and escaped strings. To insert the $ character in a raw string (which doesn't support backslash escaping) before any symbol, which is allowed as a beginning of an identifier, use the following syntax:

val price = """
${'$'}_9.99
"""

Arrays

Arrays in Kotlin are represented by the Array class. It has get and set functions that turn into [] by operator overloading conventions, and the size property, along with other useful member functions:

class Array<T> private constructor() {
    val size: Int
    operator fun get(index: Int): T
    operator fun set(index: Int, value: T): Unit

    operator fun iterator(): Iterator<T>
    // ...
}

To create an array, use the function arrayOf() and pass the item values to it, so that arrayOf(1, 2, 3) creates an array [1, 2, 3]. Alternatively, the arrayOfNulls() function can be used to create an array of a given size filled with null elements.

Another option is to use the Array constructor that takes the array size and the function that returns values of array elements given its index:

fun main() {
//sampleStart
    // Creates an Array<String> with values ["0", "1", "4", "9", "16"]
    val asc = Array(5) { i -> (i * i).toString() }
    asc.forEach { println(it) }
//sampleEnd
}

As we said above, the [] operation stands for calls to member functions get() and set().

Arrays in Kotlin are invariant. This means that Kotlin does not let us assign an Array<String> to an Array<Any>, which prevents a possible runtime failure (but you can use Array<out Any>, see Type Projections).

Primitive type arrays

Kotlin also has classes that represent arrays of primitive types without boxing overhead: ByteArray, ShortArray, IntArray, and so on. These classes have no inheritance relation to the Array class, but they have the same set of methods and properties. Each of them also has a corresponding factory function:

val x: IntArray = intArrayOf(1, 2, 3)
x[0] = x[1] + x[2]

// Array of int of size 5 with values [0, 0, 0, 0, 0]
val arr = IntArray(5)

// e.g. initialise the values in the array with a constant
// Array of int of size 5 with values [42, 42, 42, 42, 42]
val arr = IntArray(5) { 42 }

// e.g. initialise the values in the array using a lambda
// Array of int of size 5 with values [0, 1, 2, 3, 4] (values initialised to their index value)
var arr = IntArray(5) { it * 1 } 

Last modified: 19 June 2021