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 |
---|---|---|---|
| 8 | -128 | 127 |
| 16 | -32768 | 32767 |
| 32 | -2,147,483,648 (-231) | 2,147,483,647 (231 - 1) |
| 64 | -9,223,372,036,854,775,808 (-263) | 9,223,372,036,854,775,807 (263 - 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.
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 |
---|---|---|---|---|
| 32 | 24 | 8 | 6-7 |
| 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.
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.
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.
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
orF
:123.5f
You can use underscores to make number constants more readable:
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:
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:
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:
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.
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 for explicit conversions because the type is inferred from the context, and arithmetical operations are overloaded for appropriate conversions, for example:
Operations
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. See Operator overloading for details.
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.
To return a floating-point type, explicitly convert one of the arguments to a floating-point type.
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
.
Here is the complete list of bitwise operations:
shl(bits)
– signed shift leftshr(bits)
– signed shift rightushr(bits)
– unsigned shift rightand(bits)
– bitwise andor(bits)
– bitwise orxor(bits)
– bitwise xorinv()
– bitwise inversion
Floating-point numbers comparison
The operations on floating-point numbers discussed in this section are:
Equality checks:
a == b
anda != 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 itselfNaN
is considered greater than any other element includingPOSITIVE_INFINITY
-0.0
is considered less than0.0
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 255UShort
: an unsigned 16-bit integer, ranges from 0 to 65535UInt
: an unsigned 32-bit integer, ranges from 0 to 2^32 - 1ULong
: 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
As with primitives, each unsigned type has a corresponding type that represents arrays of that type:
UByteArray
: an array of unsigned bytesUShortArray
: an array of unsigned shortsUIntArray
: an array of unsigned intsULongArray
: an array of unsigned longs
Like signed integer arrays, they provide an API similar to the 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 an 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
andU
tag unsigned literals. The exact type is determined based on the expected type. If no expected type is provided, compiler will useUInt
orULong
depending on the size of literal.
uL
andUL
explicitly tag literal as unsigned long.
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.
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'
.
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 ("
).
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:
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.
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:
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:
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:
To remove leading whitespace from raw strings, use the trimMargin()
function:
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:
or an expression in curly braces:
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:
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:
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:
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: