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Basic Types

In Kotlin, everything is an object in the sense that we can call member functions and properties on any variable. Some types are built-in, because their implementation is optimized, but to the user they look like ordinary classes. In this section we describe most of these types: numbers, characters, booleans and arrays.

Numbers

Kotlin handles numbers in a way close to Java, but not exactly the same. For example, there are no implicit widening conversions for numbers, and literals are slightly different in some cases.

Kotlin provides the following built-in types representing numbers (this is close to Java):

Type Bit width
Double 64
Float 32
Long 64
Int 32
Short 16
Byte 8

Note that characters are not numbers in Kotlin.

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

NOTE: Octal literals are not supported.

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

Representation

On the Java platform, numbers are physically stored as JVM primitive types, unless we need a nullable number reference (e.g. Int?) or generics are involved. In the latter cases numbers are boxed.

Note that boxing of numbers does not preserve identity:

val a: Int = 10000
print(a === a) // Prints 'true'
val boxedA: Int? = a
val anotherBoxedA: Int? = a
print(boxedA === anotherBoxedA) // !!!Prints 'false'!!!

On the other hand, it preserves equality:

val a: Int = 10000
print(a == a) // Prints 'true'
val boxedA: Int? = a
val anotherBoxedA: Int? = a
print(boxedA == anotherBoxedA) // Prints 'true'

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(a == b) // Surprise! This prints "false" as Long's equals() check for other part to be Long as well

So not only identity, but even equality would have been lost silently all over the place.

As a consequence, smaller types are NOT implicitly converted to bigger types. This means that we cannot assign a value of type Byte to an Int variable without an explicit conversion

val b: Byte = 1 // OK, literals are checked statically
val i: Int = b // ERROR

We can use explicit conversions to widen numbers

val i: Int = b.toInt() // OK: explicitly widened

Every number type supports the following conversions:

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

Absence of implicit conversions is rarely noticeable 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, which are declared as members of appropriate classes (but the compiler optimizes the calls down to the corresponding instructions). See Operator overloading.

As of bitwise operations, there’re no special characters for them, but just named functions that can be called in infix form, for example:

val x = (1 shl 2) and 0x000FF000

Here is the complete list of bitwise operations (available for Int and Long only):

  • shl(bits) – signed shift left (Java’s <<)
  • shr(bits) – signed shift right (Java’s >>)
  • ushr(bits) – unsigned shift right (Java’s >>>)
  • and(bits) – bitwise and
  • or(bits) – bitwise or
  • xor(bits) – bitwise xor
  • inv() – bitwise inversion

Characters

Characters are represented by the type Char. They can not be treated directly as numbers

fun check(c: Char) {
  if (c == 1) { // ERROR: incompatible types
    // ...
  }
}

Character literals go in single quotes: '1'. Special characters can be escaped using a backslash. The following escape sequences are supported: \t, \b, \n, \r, \', \", \\ and \$. To encode any other character, use the Unicode escape sequence syntax: '\uFF00'.

We can explicitly convert a character to an Int number:

fun decimalDigitValue(c: Char): Int {
  if (c !in '0'..'9')
    throw IllegalArgumentException("Out of range")
  return c.toInt() - '0'.toInt() // Explicit conversions to numbers
}

Like numbers, characters are boxed when a nullable reference is needed. Identity is not preserved by the boxing operation.

Booleans

The type Boolean represents booleans, and has two values: true and false.

Booleans are boxed if a nullable reference is needed.

Built-in operations on booleans include

  • || – lazy disjunction
  • && – lazy conjunction
  • ! - negation

Arrays

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

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

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

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

Another option is to use a factory function that takes the array size and the function that can return the initial value of each array element given its index:

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

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

Note: unlike Java, 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).

Kotlin also has specialized classes to 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]

Strings

Strings are represented by the type String. Strings are immutable. Elements of a string are characters that can be accessed by the indexing operation: s[i]. A string can be iterated over with a for-loop:

for (c in str) {
  println(c)
}

String Literals

Kotlin has two types of string literals: escaped strings that may have escaped characters in them and raw strings that can contain newlines and arbitrary text. An escaped string is very much like a Java 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)
"""

You can remove leading whitespace with 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

Strings may contain template expressions, i.e. 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 simple name:

val i = 10
val s = "i = $i" // evaluates to "i = 10"

or an arbitrary expression in curly braces:

val s = "abc"
val str = "$s.length is ${s.length}" // evaluates to "abc.length is 3"

Templates are supported both inside raw strings and inside escaped strings. If you need to represent a literal $ character in a raw string (which doesn’t support backslash escaping), you can use the following syntax:

val price = """
${'$'}9.99
"""