Author: Saim Khalid

Kotlin data type is a classification of data which tells the compiler how the programmer intends to use the data. For example, Kotlin data could be numeric, string, boolean etc.

Kotlin treats everything as an object which means that we can call member functions and properties on any variable.

Kotlin is a statically typed language, which means that the data type of every expression should be known at compile time.

Kotlin built in data type can be categorized as follows:

• Number
• Character
• String
• Boolean
• Array

(a) Kotlin Number Data Types

Kotlin number data types are used to define variables which hold numeric values and they are divided into two groups: (a) Integer types store whole numbers, positive or negative (b) Floating point types represent numbers with a fractional part, containing one or more decimals.

Following table list down all the Kotlin number data types, keywords to define their variable types, size of the memory taken by the variables, and a value range which can be stored in those variables.

Data Type	Size (bits)	Data Range
Byte	8 bit	-128 to 127
Short	16 bit	-32768 to 32767
Int	32 bit	-2,147,483,648 to 2,147,483,647
Long	64 bit	-9,223,372,036,854,775,808 to +9,223,372,036,854,775,807
Float	32 bit	1.40129846432481707e-45 to 3.40282346638528860e+38
Double	64 bit	4.94065645841246544e-324 to 1.79769313486231570e+308

If we will try to store a value more than permitted value in a variable of particular data type, the Kotlin compiler will complain because an overflow would occur at runtime.

Example

Following example shows how to define and access different Kotlin number data types:

funmain(args: Array<String>){val a: Int =10000val d: Double =100.00val f: Float =100.00fval l: Long =1000000004val s: Short =10val b: Byte =1println("Int Value is "+ a)println("Double  Value is "+ d)println("Float Value is "+ f)println("Long Value is "+ l )println("Short Value is "+ s)println("Byte Value is "+ b)}

When you run the above Kotlin program, it will generate the following output:

Int Value is 10000
Double  Value is 100.0
Float Value is 100.0
Long Value is 1000000004
Short Value is 10
Byte Value is 1

(b) Kotlin Character Data Type

Kotlin character data type is used to store a single character and they are represented by the type Char keyword. A Char value must be surrounded by single quotes, like ‘A’ or ‘1’.

Example

Following example shows how to define and access a Kotlin Char data type:

funmain(args: Array<String>){val letter: Char    // defining a Char variable
   letter ='A'// Assigning a value to itprintln("$letter")}

When you run the above Kotlin program, it will generate the following output:

A

Kotlin supports a number of escape sequences of characters. When a character is preceded by a backslash (\), it is called an escape sequence and it has a special meaning to the compiler. For example, \n in the following statement is a valid character and it is called a new line character

println('\n')//prints a newline characterprintln('\$')//prints a dollar $ characterprintln('\\')//prints a back slash \ character

The following escape sequences are supported in Kotlin: \t, \b, \n, \r, \’, \”, \\ and \$.

(c) Kotlin String Data Type

The String data type is used to store a sequence of characters. String values must be surrounded by double quotes (” “) or triple quote (“”” “””).

We have two kinds of string available in Kotlin – one is called Escaped String and another is called Raw String.

• Escaped string is declared within double quote (” “) and may contain escape characters like ‘\n’, ‘\t’, ‘\b’ etc.
• Raw string is declared within triple quote (“”” “””) and may contain multiple lines of text without any escape characters.

funmain(args: Array<String>){val escapedString : String  ="I am escaped String!\n"var rawString :String  ="""This is going to be a
   multi-line string and will
   not have any escape sequence""";print(escapedString)println(rawString)}

When you run the above Kotlin program, it will generate the following output:

I am escaped String!
This is going to be a
   multi-line string and will
   not have any escape sequence

(d) Kotlin Boolean Data Type

Boolean is very simple like other programming languages. We have only two values for Boolean data type – either true or false.

funmain(args: Array<String>){val A: Boolean =true// defining a variable with true valueval B: Boolean =false// defining a variable with false valueprintln("Value of variable A "+ A )println("Value of variable B "+ B )}

When you run the above Kotlin program, it will generate the following output:

Value of variable A true
Value of variable B false

Boolean has a nullable counterpart Boolean? that can store a null value as below:

val boolNull: Boolean?=null

(e) Kotlin Array Data Type

Kotlin arrays are a collection of homogeneous data. Arrays are used to store multiple values in a single variable, instead of declaring separate variables for each value.

We will study array in a separate chapter, for now let’s look at one example to define an array of integers and then access its one of the elements.

funmain(args: Array<String>){val numbers: IntArray =intArrayOf(1,2,3,4,5)println("Value at 3rd position : "+ numbers[2])}

When you run the above Kotlin program, it will generate the following output:

Value at 3rd position : 3

Kotlin Data Type Conversion

Type conversion is a process in which the value of one data type is converted into another type. Kotlin does not support direct conversion of one numeric data type to another, For example, it is not possible to convert an Int type to a Long type:

funmain(args: Array<String>){val x: Int =100val y: Long = x  // Not valid assignmentprintln(y)}

When you run the above Kotlin program, it will generate the following output:

main.kt:3:18: error: type mismatch: inferred type is Int but Long was expected
   val y: Long = x  // Not valid assignment

To convert a numeric data type to another type, Kotlin provides a set of functions:

• toByte()
• toShort()
• toInt()
• toLong()
• toFloat()
• toDouble()
• toChar()

Now let’s rewrite above example once again and try to run it:

funmain(args: Array<String>){val x: Int =100val y: Long = x.toLong()println(y)}

When you run the above Kotlin program, it will generate the following output:

100

Variables are an important part of any programming. They are the names you give to computer memory locations which are used to store values in a computer program and later you use those names to retrieve the stored values and use them in your program.

Kotlin variables are created using either var or val keywords and then an equal sign = is used to assign a value to those created variables.

Syntax

Following is a simple syntax to create two variables and then assign them different values:

var name ="Zara Ali"var age =19var height =5.2

Examples

Once a variable is created and assigned a value, later we can access its value using its name as follows:

funmain(){var name ="Zara Ali"var age =19println(name)println(age)}

When you run the above Kotlin program, it will generate the following output:

Zara Ali
19

Let’s see one more example where we will access variable values using dollar sign $:

funmain(){var name ="Zara Ali"var age =19println("Name = $name")println("Age = $age")}

When you run the above Kotlin program, it will generate the following output:

Name = Zara Ali
Age = 19

Let’s see one more example to display the variable values without using a dollar sign as below:

funmain(){var name ="Zara Ali"var age =19println("Name = "+ name)println("Age = "+ age)}

When you run the above Kotlin program, it will generate the following output:

Name = Zara Ali
Age = 19

Kotlin Mutable Variables

Mutable means that the variable can be reassigned to a different value after initial assignment. To declare a mutable variable, we use the var keyword as we have used in the above examples:

funmain(){var name ="Zara Ali"var age =19println("Name = $name")println("Age = $age")
   
   name ="Nuha Ali"
   age =11println("Name = $name")println("Age = $age")}

When you run the above Kotlin program, it will generate the following output:

Name = Zara Ali
Age = 19
Name = Nuha Ali
Age = 11

Kotlin Read-only Variables

A read-only variable can be declared using val (instead of var) and once a value is assigned, it can not be re-assigned.

funmain(){val name ="Zara Ali"val age =19println("Name = $name")println("Age = $age")
   
   name ="Nuha Ali"// Not allowed, throws an exception
   age =11println("Name = $name")println("Age = $age")}

When you run the above Kotlin program, it will generate the following output:

main.kt:8:4: error: val cannot be reassigned
   name = "Nuha Ali" // Not allowed, throws an exception
   ^
main.kt:9:4: error: val cannot be reassigned
   age = 11
   ^

Read-only vs Mutable

The Mutable variables will be used to define variables, which will keep charging their values based on different conditions during program execution.

You will use Read-only variable to define different constant values i.e. the variables which will retain their value throughout of the program.

Kotlin Variable Types

Kotlin is smart enough to recognise that “Zara Ali” is a string, and that 19 is a number variable. However, you can explicitly specify a variable type while creating it:

funmain(){var name: String ="Zara Ali"var age: Int =19println("Name = $name")println("Age = $age")
   
   name ="Nuha Ali"
   age =11println("Name = $name")println("Age = $age")}

When you run the above Kotlin program, it will generate the following output:

Name = Zara Ali
Age = 19
Name = Nuha Ali
Age = 11

Soon we will learn more about different data types available in Kotlin which can be used to create different type of variables.

Kotlin Variable Naming Rules

There are certain rules to be followed while naming the Kotlin variables:

• Kotlin variable names can contain letters, digits, underscores, and dollar signs.
• Kotlin variable names should start with a letter or underscores
• Kotlin variables are case sensitive which means Zara and ZARA are two different variables.
• Kotlin variable can not have any white space or other control characters.
• Kotlin variable can not have names like var, val, String, Int because they are reserved keywords in Kotlin.

Kotlin keywords are predefined, reserved words used in Kotlin programming that have special meanings to the compiler. These words cannot be used as an identifier (variables names, package names, function names etc.) and if used then compiler will raise an exception.

Kotlin uses fun keyword to define a function, so if we we will try to use it as a variable name then it will be an exception. For example:

funmain(){varfun="Zara Ali"// Not allowed, throws an exceptionvar age =19// Valid variable nameprintln("Name = $fun")println("Age = $age")}

When you run the above Kotlin program, it will generate the following output:

main.kt:2:7: error: expecting property name or receiver type
   var fun = "Zara Ali"  // Not allowed, throws an exception
  ^
main.kt:2:11: error: expecting '('
   var fun = "Zara Ali"  // Not allowed, throws an exception
      ^
main.kt:5:21: error: keyword cannot be used as a reference
   println("Name = $fun")
                ^

Kotlin keywords have been categorised into three broad categories: (a) Hard Keywords (b) Soft Keywords (c) Modifier Keywords

As a good programming practice, it is highly recommended not to use any of the mentioned keywords to name any identifiers while coding in Kotlin.

(a) Kotlin Hard Keywords

Following is a list of hard keywords and they cannot be used as identifiers:

as	as?	break	class
continue	do	else	false
for	fun	if	in
!in	interface	is	!is
null	object	package	return
super	this	throw	true
try	typealias	typeof	val
var	when	while

(b) Kotlin Soft Keywords

Following is the list of keywords (soft) in the context when they are applicable and can be used as identifiers in other contexts:

by	catch	constructor	delegate
dynamic	field	file	finally
get	import	init	param
property	receiver	set	setparam
value	where

(c) Kotlin Modifier Keywords

Following is the list of tokens which act as keywords in modifier lists of declarations and can be used as identifiers in other contexts:

actual	abstract	annotation	companion
const	crossinline	data	enum
expect	external	final	infix
inline	inner	internal	lateinit
noinline	open	operator	out
override	private	protected	public
reified	sealed	suspend	tailrec
vararg

A comment is a programmer-readable explanation or annotation in the Kotlin source code. They are added with the purpose of making the source code easier for humans to understand, and are ignored by Kotlin compiler.

Just like most modern languages, Kotlin supports single-line (or end-of-line) and multi-line (block) comments. Kotlin comments are very much similar to the comments available in Java, C and C++ programming languages.

Kotlin Single-line Comments

Single line comments in Kotlin starts with two forward slashes // and end with end of the line. So any text written in between // and the end of the line is ignored by Kotlin compiler.

Following is the sample Kotlin program which makes use of a single-line comment:

// This is a commentfunmain(){println("Hello, World!")}

When you run the above Kotlin program, it will generate the following output:

Hello, World!

A single line comment can start from anywhere in the program and will end till the end of the line. For example, you can use single line comment as follows:

funmain(){println("Hello, World!")// This is also a comment}

Kotlin Multi-line Comments

A multi-line comment in Kotlin starts with /* and end with */. So any text written in between /* and */ will be treated as a comment and will be ignored by Kotlin compiler.

Multi-line comments also called Block comments in Kotlin.

Following is the sample Kotlin program which makes use of a multi-line comment:

/* This is a multi-line comment and it can span
 * as many lines as you like 
 */funmain(){println("Hello, World!")}

When you run the above Kotlin program, it will generate the following output:

Hello, Word!

Kotlin Nested Comments

Block comments in Kotlin can be nested, which means a single-line comment or multi-line comments can sit inside a multi-line comment as below:

/* This is a multi-line comment and it can span
 * as many lines as you like 
 /* This is a nested comment */// Another nested comment */funmain(){println("Hello, World!")}

When you run the above Kotlin program, it will generate the following output:

Hello, World!

Kotlin Program Entry Point

An entry point of a Kotlin application is the main() function. A function can be defined as a block of code designed to perform a particular task.

Let’s start with a basic Kotlin program to print “Hello, World!” on the standard output:

funmain(){var string: String  ="Hello, World!"println("$string")}

When you run the above Kotlin program, it will generate the following output:

Hello, World!

Entry Point with Parameters

Another form of main() function accepts a variable number of String arguments as follows:

funmain(args: Array<String>){println("Hello, world!")}

When you run the above Kotlin program, it will generate the following output:

Hello, World!

If you have observed, its clear that both the programs generate same output, so it is very much optional to pass a parameter in main() function starting from Kotlin version 1.3.

print() vs println()

The print() is a function in Kotlin which prints its argument to the standard output, similar way the println() is another function which prints its argument on the standard output but it also adds a line break in the output.

Let’s try the following program to understand the difference between these two important functions:

funmain(args: Array<String>){println("Hello,")println(" world!")print("Hello,")print(" world!")}

When you run the above Kotlin program, it will generate the following output:

Hello, 
 world!
Hello, world!

Both the functions (print() and println()) can be used to print numbers as well as strings and at the same time to perform any mathematical calculations as below:

funmain(args: Array<String>){println(200)println("200")println(2+2)print(4*3)}

When you run the above Kotlin program, it will generate the following output:

200
200
4
12

Semicolon (;) in Kotlin

Kotlin code statements do not require a semicolon (;) to end the statement like many other programming languages, such as Java, C++, C#, etc. do need it.

Though you can compile and run a Kotlin program with and without semicolon successfully as follows:

funmain(){println("I'm without semi-colon")println("I'm with semi-colon");}

When you run the above Kotlin program, it will generate the following output:

I'm without semi-colon
I'm with semi-colon

So as a good programming practice, it is not recommended to add a semicolon in the end of a Kotlin statement.

Packages in Kotlin

Kotlin code is usually defined in packages though package specification is optional. If you don’t specify a package in a source file, its content goes to the default package.

If we specify a package in Kotlin program then it is specified at the top of the file as follows:

package org.tutorialspoint.com

funmain(){println("Hello, World!")}

When you run the above Kotlin program, it will generate the following output:

Hello, World!

Kotlin is a programming language and has its own architecture to allocate memory and produce a quality output to the end user.

Following are the different scenarios where Kotlin compiler will work differently.

• Compile Kotlin into bytecode which can run on JVM. This bytecode is exactly equal to the byte code generated by the Java .class file.
• Whenever Kotlin targets JavaScript, the Kotlin compiler converts the .kt file into ES5.1 and generates a compatible code for JavaScript.
• Kotlin compiler is capable of creating platform basis compatible codes via LLVM.
• Kotlin Multiplatform Mobile (KMM) is used to create multiplatform mobile applications with code shared between Android and iOS.

Whenever two byte coded files ( Two different programs from Kotlin and Java) runs on the JVM, they can communicate with each other and this is how an interoperable feature is established in Kotlin for Java.

Kotlin Native

Kotlin/Native is a technology for compiling Kotlin code to native binaries, which can run without a virtual machine. Kotlin/Native supports the following platforms:

• macOS
• iOS, tvOS, watchOS
• Linux
• Windows (MinGW)
• Android NDK
• Many more…

Kotlin/Native is primarily designed to allow compilation for platforms where virtual machines are not desirable or possible, for example, embedded devices or iOS.

It is easy to include a compiled Kotlin code into existing projects written in C, C++, Swift, Objective-C, and other languages.