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This is a quick start guide to Kotlin for people with prior coding experience (in any other language). I compiled what I learned from different sources like Kotlin Koans and JetBrains Academy, as a quick reference while working on coding challenges or Android applications. This guide is in no way a complete reference to Kotlin, but it probably will be sufficient for new users. I've assumed you already know programming in some other language, so I have skipped explanations for concepts like mutability and lambda expressions. However, if you are new to such terms, a simple web search will definitely help you undertand them.

A sample Hello-World program has been included in the src directory to give you an idea of how Kotlin files and packages look like.

Table of Contents
Basic Facts
Values and Variables
Data Types
Type Conversion
Type Inference
Ranges
Regex
BigInteger
Null Safety
Conventions
Standard Input/Output
Functions and Lambda Expressions
Standard Library
Arrays
Control Flow Statements
Object Oriented Programming
Collections
Exceptions
Working With Files
References
Learn Kotlin

Basic Facts

Blocks of code are enclosed in curly braces, {...}
Each statement should start on a new line
There are three types of comments in Kotlin:

// This is a single line comment

/* This is an example of
   a multiline comment. */

/**
  * These are
  * documentation comments.
  */

&&, ||, ! and xor represent the logical operators - and, or, not and xor.

Values and Variables

val is used to declare read-only values.

val name = "Kotlin"
name = "Programming"   // won't work, as 'name' is read-only and cannot be reassigned

var is used to declare mutable variables. You can reassign the variables with values of the same type as the initial value.

var language = "English"
language = "Klingon"   // this is possible
language = 10   // won't work, as language is a String variable

The variables type is inferred by the compiler. This is called type inference.
The type of a variable can be specified, if required, while declaring it. The type should be specified if the variable is not initialized during declaration, as the compiler cannot automatically infer the type.

var name: String = "Kotlin"
val num: Int = 999
val foo   // won't work, as type can't be inferred
val character: Char
character = 'A'   // works as expected

If you create a variable and assign an object to it, the new variable can point to the same object as well. This is called copying by reference. In other words, the = sign does not copy the object itself, it only copies a reference to it.

val msg1 = "Hey"
val msg2 = msg1
// msg1 and msg2 will both point to a single object in memory, the String "Hey".

If the object is immutable, you cannot change it, but you can use another object and assign this new object to the same variable. When you reassign the variable, it will point to the new object and other variables will still point to the old object. Standard types such as strings or numbers are immutable, so it's safe to copy them by reference. The behavior of mutable objects is different. If you modify an object from one variable, the other assigned variables continue to point to that object, so they will also reflect the same changes.
If mutable types like lists or arrays are declared with the val keyword, the reference to the object is declared as read-only. However, the contents of the mutable type are still modifiable. In other words, if you declare a variable of type Array using val, you cannot reassign another Array to it, but you can change the values in the original Array.
The comparison operators == and != checks for structual equality, while === and !== checks for referential equality.

val blue = Box(3)
val green = blue   // object reference is copied
val red = Box(3)

println(blue == green)  // true
println(blue === green) // true, as both point (refer) to the same object
println(blue == red)   // true
println(blue === red)  // false, they point (refer) to different objects

var two = 2
var anotherTwo = 2

println(two === anotherTwo) // true, because the '===' equality check is equivalent to the '==' check for primitive types
two = two + 1
println(two === anotherTwo) // false

The val keyword implies an immutable reference to the object. The var keyword implies a mutable reference to a variable, so you can reassign it.
Kotlin supports prefix and postfix increment/decrement operators (++ and --) on variables.

Data Types

The names of Kotlin datatypes start with a capital letter.
Types:
Byte: 8 bits (1 byte), Range: -128 to 127
Short: 16 bits (2 bytes), Range: -32768 to 32767
Int: 32 bits (4 bytes), Range: −(2³¹) to (2³¹)−1
Long: 64 bits (8 bytes), Range: −(2⁶³) to (2⁶³)−1
Float: 32 bits (4 bytes), 6-7 significant decimal digits
Double: 64 bits (8 bytes), 15-16 significant decimal digits
Char: 16 bits (2 bytes), represents a 16-bit Unicode character
Boolean: size is machine-dependent
String: size depends on the string
Strings are enclosed in double quotes, "...".
Characters are enclosed in single quotes, '_'.
Fractional numbers are inferred as Double by default. Explicitly specify the type, or use the suffix f or F to use Float instead.
Long is used by the compiler only if the value won't fit in an Int variable. To use Long with a small value, explicitly specify the type of the variable, or use the suffix l or L with the value.
Underscores can be used to divide the a number into blocks for easier readability. 1_000_000 is the same as 1000000.
Types in Kotlin are organized into a hierarchy of subtype-supertype relationships. Supertype is a type that specifies some common characteristics and rules of behavior that every subtype will follow.
Number is a supertype for all types that represent numeric value. For example, Int, Float and Double are subtypes of Number type.
The type checker of the Kotlin compiler also enforces supertype-subtype relationships. For example, to a function waiting for an argument of type Number, you can pass its subtype, Int:

fun calc(number: Number) { /*...*/ }

val number: Int = 1
calculate(number)

Any is a supertype for all non-nullable types in Kotlin. Any? is a supertype for Any that includes every other nullable type.
Unit is the equivalent of the void type found in many other programming languages. It is the return type of a function that does not return any meaningful value. If the return type of a function is not specified, it is inferred to be Unit. A return statement is not required for such functions as they are returned implicitly. Unit is also a subtype of Any.
Nothing is a type that has no instances, and are used in functions like fail() or expressions like throw, which doesn't return control. Any code following an expression of type Nothing is unreachable. When you call a function with a Nothing return type, the compiler won't execute any code beyond this call.
If you use null to initialize a value of an inferred type and there's no other information that can be used to determine a more specific type, the compiler will infer the Nothing? type:

val x = null           // type: Nothing?
val l = listOf(null, null)   // type: List<Nothing?>

In Kotlin, 0 is not the same as false. So you cannot assign a Int value to a Boolean variable.
Small errors may accumulate after operations involving Float and Double types. Generally, avoid using == in expressions involving floating-point operations.

val one = 0.1 + 0.1 + 0.1 + 0.1 + 0.1 + 0.1 + 0.1 + 0.1 + 0.1 + 0.1
println(one) // 0.9999999999999999
// one == 1.0 will return false

A Char can be also created by using its hexadecimal code in the Unicode table. They are interconvertible with Int values.

val alphabet = '\u0041'     // represents 'A'

Int numbers can be added to characters, but the reverse is not true.

var ch = 'a'

val ch1 = ch + 1      // 'b'
val ch2 = 1 + ch      // Error

ch += 2               // 'c'

You can compare Char using relational operators according to their position in the Unicode table.
The Char class has built-in utility functions like isDigit(), isLetter(), isLetterOrDigit(), isWhitespace(), isUpperCase(), isLowerCase(), toUpperCase() and toLowerCase().
String has a property length that stores the number of characters in the string. Individual elements can be accessed using their indices. It also has functions like first(), last() and lastIndex() to improve readability.

val text = "Hello!"

println(text.length)      // 6
println(text.first())     // H
println(text.last())      // !
println(text.lastIndex()) // 5

Individual characters of a String can't be changed, because it is an immutable type.
String objects can be concatenated using the + operator. Values of other types can also be appended to a String using the + operator, but the expression must start with the String.

val firstName = "John"
val lastName = "Doe"
val fullName = lastName + ", " + firstName      // Doe, John
val userName = "johnny" + true + 69             // johnnytrue69
val nickname = 8 + "ball"                   // Error, as a string can't be added to an Int

Kotlin supports string templates. Variables can be used directly in a String by prefixing the dollar sign ($) to the variable name. Similarly, expressions can be used in a String by enclosing them in curly braces after the dollar sign.

val name = "John"

println("My name is $name.")                      // My name is John.
println("My name has ${name.length} letters.")    // My name has 4 letters.

The substring() method of a String accepts startIndex and lastIndex as arguments and returns the substring that starts at the startIndex and ends right before the lastIndex. If lastIndex is omitted, the default value (length of the original string) is used.
The substringBefore() and substringAfter() functions of String accept a delimiter as argument and return a String before/after the first occurrence of the specified delimiter. If the String does not contain the delimiter, the entire String is returned. Similarly, the substringBeforeLast() and substringAfterLast() methods return a String before or after the last occurrence of the delimiter. If a second argument is provided to these methods, it is returned if the delimiter is not found in the String.
The replace() method of a String replaces all occurrences of the first argument in the String with the second argument. Similarly, replaceFirst() replaces the first occurrence of the first argument with the second, and skips the rest.
The toLowerCase() and toUpperCase() methods of a String returns a String with its case converted accordingly.
The repeat() method of String allows you to repeat a String without using loops.

print("Hey".repeat(5))      // HeyHeyHeyHeyHey

A CharArray can be converted to a String using String(), and a String can be converted to a CharArray using the toCharArray() method. The toTypedArray() method can be used to convert an list of String to a TypedArray.

val chars = charArrayOf('A', 'B', ' ', 'C', 'D')
val text = String(chars)                                        // "AB CD"
val charsFromString = text.toCharArray()                        // { 'A', 'B', ' ', 'C', 'D' }
val words1: List<String> =  text.split(" ")                     // { "AB", "CD" }
val words2: Array<String> = text.split(" ").toTypedArray()      // { "AB", "CD" }

You can iterate through a String using a range of indices, array of indices, or using the characters of the String.

val text = "Hello World"

for (i in 0 until text.length) {
    print("${text[i]} ")              // H e l l o  W o r l d
}

for (index in text.indices) {
    print("${text[index]} ")          // H e l l o  W o r l d
}

for (i in text) {
    print("$i ")                      // H e l l o  W o r l d
}

Type Conversion

Unlike some other programming languages, there are no implicit widening conversions for numbers in Kotlin. For example, an Int variable cannot be initialized with a Short or Byte variable. A function with a Double parameter can be called only on Double values, but not Float, Int, or other numeric values. The compiler will not perform any automatic type conversion in these cases.
There are methods built into the datatypes for explicit type conversion. It will not affect the type of the original variables.
Every number type supports the following conversions:
toByte(), toShort(), toInt(), toLong(), toFloat(), toDouble() and toChar().
Numbers are converted to Char and vice versa using the Unicode Table.
When you convert a larger type to smaller type, the value may be truncated.
All datatypes can be converted to a string using toString().
Strings can be converted to numbers if they are in the correct format of the type.
If the string is "true" or "false" (case insensitive), it can be converted to a Boolean using toBoolean(). Any value other than "true" will be a false Boolean.

Type Inference

In expressions involving variables of different datatypes, the widest type in the expression is inferred by the compiler as the type of the result.
Double > Float > Long > Int > (Byte, Short)
The result of an expression involving Byte or Short variables is of the type Int (or an even larger type), unless explicitly casted.

Ranges

Kotlin lets you create ranges of values using rangeTo() from the kotlin.ranges package, and its operator form ...
a..b is a range from a to b (inclusive), and in is a keyword used to check whether a value is within a range.

if (i in 1..5) {  // equivalent of 1 <= i && i <= 5
    print(i)
}

If you need to exclude the right border, you should subtract one from it:

val within = c in a..b - 1
// equivalent of a <= c && c < b

If you need to check that a value is not within a range, use !in:

val notWithin = 100 !in 10..99   // true

You can assign a range to a variable and use it later somewhere in the code:

val range = 1..5
print(3 in range)   // true

You can also use ranges of characters and strings (in dictionary order):

println('b' in 'a'..'c') // true
println("hello" in "he".."hi") // true

Integral type ranges can be iterated over using for loops:

for (i in 1..5) print(i)   // 12345

To iterate numbers in reverse order, use the downTo function instead of ..:

for (i in 5 downTo 1) print(i)   // 54321

To iterate over numbers with an arbitrary step, use the step function:

for (i in 1..10 step 2) {
    print(i)   // 13579
}

To iterate a number range which does not include its end element, use the until function:

for (i in 1 until 10) {   // i in [1, 10), 10 is excluded
    print(i)
}

Regex

A regex instance can be created using the toRegex() method of a String or by calling the Regex constructor:

val string = "ing"
val regex1 = string.toRegex()
val regex2 = Regex("ing")
// regex1 and regex2 represent the same regular expression

matches() method of a string returns true if the string is full match for the regex specified:

val regex = Regex("ing")
val string1 = "ing"
val string2 = "running"

println(string1.matches(regex))   // true
println(string2.matches(regex))   // false

The dot character (.) can match any character except a newline. A question mark (?) denotes that the preceding character is optional. A double backslash (\\) is used to escape special characters. A single backslash (\) is used to escape double quotes (").
To match \, the regexp is \\\\.

BigInteger

The Java Class Library provides a BigInteger class for processing very large numbers, limited only by the memory available. It is immutable.

import java.math.BigInteger

val number1 = BigInteger("52955871795228763416553091")   // initialized using constructor
val number2 = BigInteger.valueOf(1000000000)   // initialized from a Long value using the valueOf() method
val number3 = 1234.toBigInteger()   // initialized from an Int value using the toBigInteger() method

// Some constants defined in the BigInteger class:
val zero = BigInteger.ZERO // 0
val one = BigInteger.ONE   // 1
val ten = BigInteger.TEN   // 10

The divideAndRemainder() function returns an array consisting of the result of integer division and its remainder.

val (result, remainder) = oneHundredTen.divideAndRemainder(nine) // 12 and 2

The abs() function returns a new BigInteger whose value is the absolute value of the initial BigInteger. The gcd() function returns the greatest common divisor of two numbers.

val number = BigInteger("-1")
println(number.abs()) // 1

val twelve = BigInteger.valueOf(12)
val fifteen = BigInteger.valueOf(15)
println(twelve.gcd(fifteen)) // 3

Null Safety

In Kotlin, the type system distinguishes between references that can hold null (nullable references) and those that can not (non-null references).
To allow nulls, we have to declare a variable as nullable by defining the type of the variable as <type>?.

var b: String? = "abc" // can be set null

You can generally handle operations using nullable variables in three ways:
- You can explicitly check whether the variable is null.
```
var a: String? = "abc"
val l = if (a != null) a.length else -1
```
- Use the safe call operator, ?..
```
val b: String? = null
println(b?.length)      // returns b.length if b is not null, and null otherwise
// the type of this expression is Int?
```
Safe calls are useful in chains. For example, if Bob (Employee) may be assigned to a Department, that in turn may have another Employee as a department head, then to obtain the name of Bob's department head (if any), we write:
```
bob?.department?.head?.name     // returns null if any of the properties is null
```
To perform an operation only for non-null values, you can use the safe call operator together with let:
```
val listWithNulls: List<String?> = listOf("Kotlin", null)
for (item in listWithNulls) {
  item?.let { println(it) } // prints Kotlin and ignores null
}
```
A safe call can also be placed on the left side of an assignment. If one of the receivers in the safe calls chain is null, the assignment is skipped, and the expression on the right is not evaluated at all:
```
// If either `person` or `person.department` is null, the function is not called
person?.department?.head = managersPool.getManager()
```
We can also use the Elvis operator (?:) in place of an if-expression. The expression to the right of ?: is evaluated only if the left-hand side is null.
```
val l = a?.length ?: -1
// Equivalent to:
// val l: Int = if (a != null) a.length else -1

// Other examples:
val parent = node.getParent() ?: return null
val name = node.getName() ?: throw IllegalArgumentException("name expected")
```
- The not-null assertion operator (!!) can be used to convert any value to a non-null type and throw an exception if the value is null.
```
val l = a!!.length
```
Regular casting may result in a ClassCastException if the object is not of the target type. You can use safe casts that return null if the attempt was unsuccessful:

var a: String? = "abc"
val n: Int? = a as? Int     // null, as the String can't be casted to an Int type

If you have a collection of elements of a nullable type and want to filter non-null elements, you can use filterNotNull():

val nullableList: List<Int?> = listOf(1, 2, null, 4)
val intList: List<Int> = nullableList.filterNotNull()     // [1, 2, 4]

Conventions

Four spaces are used for indentation. Do not use tabs.
Omit semicolons (;) wherever possible, as they are optional.
Put spaces around binary operators, for example, a + b.
Put spaces between control flow keywords (if, when, for and while) and the corresponding opening parenthesis.
If a variable or function name is a single word, it should be in lowercase; if the name has multiple words, it should be in lowerCamelCase.
For curly braces, put the opening brace in the end of the line where the construct begins, and the closing brace on a separate line aligned horizontally with the opening construct.

if (elements != null) {
    for (element in elements) {
        // ...
    }
}

More conventions can be found in the official documentation.

Standard Input/Output

print(message: Any?) can be used to print the given message to the standard output stream.
println(message: Any?) prints the given message to the standard output stream along with a newline at the end.
readLine() reads a whole line from standard input and returns it as a String. The returned string can be casted to the required type using casting function like toInt() or toBoolean(). You can use two exclamation marks at the end of the function call to force a non-null return value.

val line = readLine()   // can be null
val line2 = readLine()!!    // not null; commonly used

A Java Scanner object can be used to read input:

import java.util.Scanner      // or import java.util.*

val scanner = Scanner(System.`in`)
val line = scanner.nextLine() // read a line
val integer = scanner.nextInt()   // read a number
val word = scanner.next()   // read a string

Functions and Lambda Expressions

A function definition in Kotlin looks like:

fun functionName(param1: Type1, param2: Type2, ...): ReturnType {
    // body of the function
    return result     // optional
}
// ReturnType is optional if the return type is Unit

If the function returns a single expression, you can avoid the curly braces:

fun sum(a: Int, b: Int) = a + b
fun hello() = print("Hello World")
fun isNegative(num: Int): Boolean = num < 0
// the return types of these functions can be inferred automatically

The main() function is the entry point in a Kotlin program.

fun main(args: Array<String>) {
    // body
}
// args is optional

Kotlin supports default arguments in function declarations and named arguments in function calls:

fun horizontalRule(symbol: Char = '-', count: Int = 10, end: Char = '\n') {
    for (i in 1..count) {
        print(symbol)
    }
    print(end)  
}

fun main() {
    horizontalRule()                // ----------
    horizontalRule('_', end = '?')  // __________?
}

The default value for a parameter of a function can also be another named argument, or a function:

fun sum(a: Int, b: Int = a) = a + b

Kotlin uses the call stack (execution stack) of the JVM to store information about functions that are being executed. Each function and its associated data (like function address, argument references and local variables) is stored as a stack frame in the stack. A StackOverflowError occurs when the number function invocations exhaust the memory available to the stack.
Functions are first-class citizens in Kotlin. They can be stored in variables, and passed to or returned by other functions.
The type of a function is given as:
(types of parameters, comma separated) -> return type of the function

fun sum(a: Int, b: Int): Int = a + b
// Type is:
// (Int, Int) -> Int

fun hello() = print("Hello World")
// Type is:
// () -> Unit
}

Reference to a function can be obtained by prefixing a double colon (::) to the function name.

fun sum(a: Int, b: Int): Int = a + b
var newSum = ::sum
// newSum(1, 2) will return 3

The filter() method of String or collections can accept references to other functions to act as the filtering criteria.
Anonymous functions in Kotlin are of the form fun(arguments): ReturnType { body }
Lambda expressions in Kotlin are of the form { arguments -> body }.
If the lambda doesn't have arguments, it's written in the form { body }.
When the lambda is the last argument to a function, it can be placed outside the parenthesis. If the function has no other arguments other than the lambda, you can omit the parenthesis altogether. If there is only a single argument for the lambda, it can be used to refer to the parameter rather than assigning a name to it.

val someText = "L-o-r-e-m I-p-s-u-m"

print(   someText.filter({ ch -> ch != '-' })    )    // Lorem Ipsum
print(   someText.filter() { ch -> ch != '-' }   )    // Lorem Ipsum
print(   someText.filter { ch -> ch != '-' }     )    // Lorem Ipsum
print(   someText.filter { it != '-' }           )    // Lorem Ipsum

If the lambda has multiple lines of code, its last line is treated as the return value. The return keyword is not required in this case.
If you use return in a lambda expression, a non-local return happens to the nearest enclosing function.

fun foo() {
    listOf(1, 2, 3, 4, 5).forEach {
        if (it == 3) return              // non-local return directly to the caller of foo()
        print(it)
    }
    println("this point is unreachable")
}

To return just from a lambda expression and not the enclosing function, you need to use a fully qualified return statement.

fun foo() {
    listOf(1, 2, 3, 4, 5).forEach lit@{
        if (it == 3) return@lit           // local return to the caller of the lambda, the forEach loop
        print(it)
    }
    print(" done with explicit label")
}

fun bar() {
    listOf(1, 2, 3, 4, 5).forEach {
        if (it == 3) return@forEach       // local return to the caller of the lambda, the forEach loop
        print(it)
    }
    print(" done with implicit label")
}

A return statement in an anonymous function will return from the anonymous function itself.

fun foo() {
    listOf(1, 2, 3, 4, 5).forEach(fun(value: Int) {
        if (value == 3) return            // local return to the caller of the anonymous function, the forEach loop
        print(value)
    })
    print(" done with anonymous function")
}

The variables and values visible where the lambda is created are also visible inside the lambda, i.e. lambda captures variables. So if a variable is changed inside the lambda, the changes are also reflected outside it.

fun placeArgument(value: Int, f: (Int, Int) -> Int): (Int) -> Int {
    return { i -> f(value, i) }
}

val mul = { a: Int, b: Int -> a * b }
val double = placeArgument(2, mul)
print(double(5))      // 10

Standard Library

The Math (inherited from Java) and kotlin.math libraries contains implementation of common mathematical functions and constants.

val sqrt = Math.sqrt(2.0)   // 1.4142...

Random numbers can be generated using the Random class in the kotlin.random package.

import kotlin.random.Random

fun main() {
    println( Random.nextInt() )       // generates a random Int number
    println( Random.nextLong() )      // generates a random Long number
    println( Random.nextFloat() )     // generates a random Float number between 0 (inclusive) and 1.0 (exclusive)
    println( Random.nextDouble() )    // generates a random Double number between 0 (inclusive) and 1.0 (exclusive)
    println( Random.nextInt(10) )     // generates a non-negative Int value less than 10
    println( Random.nextInt(1, 10) )  // generates an Int value between 1 (inclusive) and 10 (exclusive)
    // ranges can be specified for all these functions except nextFloat()
}

You can set the seed for the pseudorandom number generator using the Random() class constructor.

val generator = Random(42)      // setting the seed 42
for (i in 0..4) {
    print(generator.nextInt(10).toString() + " ")     // "3 0 1 2 1 " will always be the sequence generated for this seed if the same Kotlin runtime and architecture is used
}

A generator initialized using Random.Default will have a different seed each time it is used.

Arrays

Kotlin supports the following native arrays for primitive types: IntArray, LongArray, DoubleArray, FloatArray, CharArray, ShortArray, ByteArray and BooleanArray. There is no StringArray.
To create an array of a specified type, pass the elements as parameters to functions like intArrayOf(), charArrayOf() and booleanArrayOf(). To initialize an array with a single value, use a lambda expression along with the type constructor.

val numbers1 = intArrayOf(1, 2, 3, 4, 5)      // Array<Int>: { 1, 2, 3, 4, 5 }
println(numbers.joinToString())               // 1, 2, 3, 4, 5

val numbers2 = IntArray(5)                    // { 0, 0, 0, 0, 0 }
val numbers3 = IntArray(5) {1}                // { 1, 1, 1, 1, 1 }

You cannot change the size of an array, but you can modify its elements.
The size of an array can be obtained using the size property.

val numbers = intArrayOf(1, 2, 3, 4, 5)
println(numbers.size)       // 5

Arrays also have the first(), last() and lastIndex() methods, and it works in the same way it does for String.
Arrays can be compared using the contentEquals() method, as == only compares the references and not the contents of the object.

val numbers1 = intArrayOf(1, 2, 3, 4)
val numbers2 = intArrayOf(1, 2, 3, 4)

println(numbers1.contentEquals(numbers2))     // true
println(numbers1 == numbers2)                 // false

The Array<T> class lets you create an array of any object. The arrayOf() and emptyArray() functions can be used to create an Array.

val array1 = arrayOf("Hello", "World")
val array2 = arrayOf<String>("Hello", "World")      // if you want to enforce the type
val array3 = emptyArray<String>()

The joinToString() method returns a String containing the elements of the array, separated using commas. The contentEquals() method can be used to compare the Array with another Array.
The size of an array can be changed (by adding or removing elements) only if it was declared using var, and not val. However, you can change the elements in both cases.
Arrays can be nested to create multidimensional arrays. The nested arrays can even be of different types.

val array = arrayOf(
    arrayOf(0),
    arrayOf(1, 2),
    arrayOf(3, 4, 5))

The contentDeepToString() method can be used to get the entire content of a multidimensional array as a single String.

Control Flow Statements

In Kotlin, if is also treated as an expression, and not just a statement. It can return the result of some computation, by specifying the result as the last expression in the body. If if is used as an expression, it must have an else branch.

val max = if (a > b) {
    println("a wins!")
    a
} else {
    println("b wins!")
    b
}

The braces can be omitted if the body has only one statement.

println( if (a == b) "a equal b"
         else if (a > b) "a is greater than b"
         else "a is less than b" )

The when construct is the alternative to the switch-case statement found in many languages. The else keyword specifies the default (fallback) argument for the construct. You can use multiple statements in a branch by enclosing them in curly braces. They can also be used as expressions, and the else branch is required in such cases.

when (alphabet) {
        "a", "A" -> println("Alpha")
        "b", "B" -> println("Beta")
        "c" -> { println("Gamma")
                 alphabet = "C"
               }
        else -> println("Unknown")
    }

The branch conditions of a when construct can also be expressions. It can also be used without an argument; in this case, the branch conditions are evaluated as boolean expressions.

val a = 5
val b = 6
val c = 11    

println(when (c) {
        a + b -> "$c equals $a plus $b"
        a - b -> "$c equals $a minus $b"
        a * b -> "$c equals $a times $b"
        else -> "We do not know how to calculate $c"
        })                                            // "11 equals 5 plus 6"
        
when (c) {
        in 0..10 -> println("c is between 0 and 10 (inclusive).")
        else -> println("c is not between 0 and 10.")
        }

The repeat block simply executes its body over and over again for the specified number of times.

import java.util.*

val scanner = Scanner(System.`in`)
val n = 10
val sum = 0

repeat(n) {
        val num = scanner.nextInt()
        sum += num
    }

A for loop is used to iterate through ranges, arrays or collections of elements.

for (i in 1..4) {
    println(i)    
}

Kotlin supports the standard while and do..while loops. The break and continue statements also work as expected.
Any expression in Kotlin may be marked with a label. Labels have the form of an identifier followed by the @ sign. You can use break, continue and return statements using these labels.

loop@ for (i in 1..100) {
    for (j in 1..100) {
        if (i == 10 && j == 10) break@loop      // breaks out from both loops
    }
}

fun foo() {                                     // prints "1245"
    listOf(1, 2, 3, 4, 5).forEach {
        if (it == 3) return@forEach // local return to the forEach loop
        print(it)
    }
}

fun foobar() {                                  // prints "12"
    run loop@{
        listOf(1, 2, 3, 4, 5).forEach {
            if (it == 3) return@loop // non-local return from the lambda passed to run
            print(it)
        }
    }
}

Object Oriented Programming

A class is defined using the class keyword. If the body of the class is empty, the curly braces can be omitted.

class ClassName {
  // class properties and methods
}

val object = ClassName()

The default values of the properties can be specified in the class definition itself. The properties can be accessed by using the object.property format.

class Solid {
  var color = "red"
  var count = 10
  var shape = "sphere"
}

val object = Solid()
println(object.shape)     // sphere

A class in Kotlin has one primary constructor, and zero or more secondary constructors. The primary constructor is a part of the class header, and does not have to be defined separately. Optionally, the primary constructor can also take parameters.

class Person(name: String) { /*...*/ }
// Same as:
// class Person constructor(name: String) { /*...*/ }

Any initialization code for the primary constructor has to be put in one or more init blocks. These init blocks are executed in the order in which they appear.

class InitOrderDemo(name: String) {
    val firstProperty = "First property: $name".also(::println)

    init {
        println("First initializer block that prints ${name}")
    }

    val secondProperty = "Second property: ${name.length}".also(::println)

    init {
        println("Second initializer block that prints ${name.length}")
    }
}

The parameters passed to the primary constructor can be used as class properties by adding val or var keywords to the parameters list.

class Name(val firstName: String, val lastName: String)     // example of a data class, used to store structured data

The primary constructor also takes default values in the arguments list.

class Solid(val shape = "sphere", val color = "red") { /*...*/ }
val object = Solid(color = "blue")

Class methods are called member functions in Kotlin. They are defined inside the class definition block. The this keyword can be used to refer to the current instance.

class Color(var color = "red") {
  fun printColor() {
    print(this.color)     // you can omit the 'this' keyword if the property name is unique
  }
}

val obj = Color("green")
obj.printColor()      // prints "green"

When you access a property, a getter function is invoked implicitly. Similarly, a setter function is invoked when the value of a property is changed. Each property has their own getter and setter functions. They are automatically defined by the compiler, but you can define custom functions if required. Inside the getter and setter function, the keyword field has to be used in place of the actual property name to avoid an infinite recursive call of getter/setter functions.

class Person {
    var name: String = "John"
        get() = field
        set(value) {
          print("Changed name from $field to $value.")
          field = value
        }
}

val guy = Person()
guy.name = "Ron"      // prints "Changed name from John to Ron"

If a class is to be inherited (extended), it has to be declared with the open keyword. We can pass parameters to the constructor of the parent class, if required.

open class Person(var name: String, var age: Int = 25)

class Employee(name: String, dept: String) : Person(name) {
    var department = dept
}

Any function that accepts a parent class also accepts a child class as its parameter.
To override a function from the parent class in a child class, the open modifier has to be used in the function definition in the parent class, and the override modifier has to be used in the function definition in the child class. An overridden method is automatically open to all subclasses (if any). The final modifier can be used to close such implicitly open functions. Overriding properties works in a similar way.

open class Shape {
    open val vertices: Int = 0
}

class Rectangle : Shape() {
    final override val vertices = 4
}

Code in a derived class can call its superclass functions and property accessors implementations using the super keyword.

open class Rectangle {
    open fun draw() { println("Drawing a rectangle") }
}

class FilledRectangle : Rectangle() {
    override fun draw() {
        super.draw()
        println("Filling the rectangle")
    }
}

Secondary constructors are declared as constructor(). A class can have more than one secondary constructors, provided they all have unique function signatures.

class Person {
    var children: MutableList<Person> = mutableListOf()
    constructor(parent: Person) {
        parent.children.add(this)
    }
}

If the class has a primary constructor, each secondary constructor needs to delegate to the primary constructor, either directly or indirectly through another secondary constructor(s). The this keyword can be used to delegate to another constructor of the same class. Delegation to the primary constructor happens as the first statement of a secondary constructor, so the code in init blocks and property initializers is executed before the secondary constructor body.

class Person(val name: String) {
    var children: MutableList<Person> = mutableListOf()
    init {                                        
        println("Initializing..")     // always runs before the secondary constructor
    }
    constructor(name: String, parent: Person) : this(name) {
        parent.children.add(this)
    }
}

A singleton class is a special class with a only single instance globally. Singletons are implemented in Kotlin by using object declarations. The singleton is declared like a regular class declaration, but with the keyword object in place of class. Such classes doesn't need instantiation, and can be accessed directly using the class name.

object President {
  val name = "Abraham Lincoln"
  fun printMsg() {
    println("$name was a former US President.")
  }
}

President.printMsg()

Classes can be nested. As a singleton is also a special type of class, objects declarations can also be nested in class or object declarations.
Extensions are used to extend a class with new functionality without having to inherit from the class. You can declare methods and properties for a class without modifying the actual class declaration. To declare an extension, we prefix its name with the receiver type (the class being extended). Extensions do not actually modify the classes being extended, and are resolved statically.

fun MutableList<Int>.swap(index1: Int, index2: Int) {
    val tmp = this[index1] // 'this' corresponds to the list
    this[index1] = this[index2]
    this[index2] = tmp
}

val list = mutableListOf(1, 2, 3)
list.swap(0, 2) // [3, 2, 1]

If an extension for a class is defined with the same name (and signature) as a member, the member always wins. However, functions can be overloaded by using a different signature.

class Example {
    fun printFunctionType() { println("Class method") }
}

fun Example.printFunctionType() = println("Extension function")

Example().printFunctionType()     // Class method

Extensions can also be defined with a nullable receiver type.

fun Any?.toString(): String {
    if (this == null) return "null"
    // after the null check, 'this' is autocast to a non-null type, so the toString() below
    // resolves to the member function of the Any class
    return toString()
}

Extension properties can have getters and setters, but cannot have initializers.

  val <T> List<T>.lastIndex: Int
    get() = size - 1

The toString() method of classes are invoked implicitly by when you try to print them. You can override the toString() method of a class to change the default behavior (which is a string of the form <class_name>@). This modified function is inherited by the child classes, if any.

open class Toy(val type: String = "ball", val color: String = "blue") {
  override fun toString(): String {
    return "$color $type"
  }
}

class ToyOwner(val name: String, type: String = "ball", color: String = "blue") : Toy(type, color) {
  override fun toString(): String {
    return "${name}'s ${super.toString()}"
  }
}

val blueBall = ToyOwner("John")
print(blueBall)   // John's blue ball

An object declaration inside a class can be marked with the companion keyword. Members of the companion object can be called by using simply the class name as the qualifier. The name of the companion object can be omitted, in which case the name Companion will be automatically.

class SampleClass {
    companion object {
        fun create(): SampleClass = SampleClass()
    }
}

val instance1 = SampleClass.create()
val instance2 = SampleClass.Companion.create()
// both function calls refer to the same function, and creates different instances of the class

Properties from the outer class will shadow the properties of the companion if they have the same name. Only one companion object can be created for a class. A companion class cannot be nested inside another companion class or object declaration.
The enum keyword is used to create enumeration classes.

enum class Rainbow {
    RED, ORANGE, YELLOW, GREEN, BLUE, INDIGO, VIOLET
}

Enum classes can be used to store multiple parameters per member, and even add member functions.

enum class Rainbow(val color: String, val rgb: String) {
    RED("Red", "#FF0000"),
    ORANGE("Orange", "#FF7F00"),
    YELLOW("Yellow", "#FFFF00"),
    GREEN("Green", "#00FF00"),
    BLUE("Blue", "#0000FF"),
    INDIGO("Indigo", "#4B0082"),
    VIOLET("Violet", "#8B00FF");

    fun printFullInfo() {
        println("Color: $color --- RGB: $rgb")
    }
}

val rgb = Rainbow.RED.rgb

Enum classes also contain some methods and properties implicitly defined by Kotlin.

val color: Rainbow = Rainbow.BLUE
println(color.name)     // returns the enum instance's name, "BLUE"
println(color.ordinal)      // returns the postion (index) of the instance in the class declaration, 4
fun isRainbowColor(color: String) : Boolean {
    for (enum in Rainbow.values()) {      // returrns a list of instances in the enum class
        if (color.toUpperCase() == enum.name) return true
    }
    return false
}

println(isRainbowColor("Pink"))     // false

Data classes declared with the modifier data is used to store data in an organized manner. These classes automatically support methods like equals(), hashCode(), toString() and copy(). However, such built-in methods only consider the properties listed in the constructor, and ignores any properties found in the body of the class. All the methods except copy() can be overridden if required.

data class Toy(var type = "Ball", var color = "Blue")

val blueBall = Toy()
val redBall = blueBall.copy(color = "Red")

Data classes can be unpacked, or destructured. A destructuring declaration uses a componentN() operator that returns the n-th element in the data class.

data class Toy(var type = "Ball", var color = "Blue")

val blueBall = Toy()
val (toyType, toyColor) = blueBall
// Same as:
// val toyType = blueBall.component1()
// val toyColor = blueBall.component2()

println(toyType)      // Ball
println(toyColor)     // Blue

Regular classes can also be destructured if you explicitly define componentN() operators.

class Toy(var type = "Ball", var color = "Blue") {
    operator fun component1(): String = type
    operator fun component2(): String = color
}

Destructuring also works with arrays. If there are less variables than the size of the array, it will skip the remaining elements of the array. If there are more variables, the expression will result in an error. If you don't need a variable in the destructuring declaration, you can place an underscore instead of its name.

Collections

The following properties and methods are supported by all the standard collections in Kotlin:
- size
- contains(element) → checks if element is in the collection
- containsAll(col) → checks whether all the elements in the collection col is in the current collection
- isEmpty()
- joinToString() returns a string consisting of the elements of the collection.
- indexOf(element) returns the index of the first occurrence of element in the collection, and -1 if element is not present in the collection.
Mutable collections have some more common methods:
- clear() removes all the elements in the collection
- remove(element) removes the first occurrence of element
- removeAll(col) removes all elements of the collection col from the current collection
List<T> stores elements in a specified order and provides indexed access to them. A List can contain duplicate elements, even null.

val numbers = listOf("one", "two", "three", "four")
val empty = emptyList<String>()     // to initialize an empty list

println("Number of elements: ${numbers.size}")
println("Third element: ${numbers.get(2)}")
println("Fourth element: ${numbers[3]}")
println("Index of element \"two\" ${numbers.indexOf("two")}")

A MutableList<T> is a mutable List, i.e supports adding or removing elements. A List can be converted to a MutableList using the toMutableList() method of a List.

val numbers = mutableListOf(1, 2, 3, 4)
numbers.add(5)    // 1, 2, 3, 4, 5
numbers.removeAt(1)   // 1, 3, 4, 5
numbers[0] = 0    // 0, 3, 4, 5
numbers.shuffle()   // list is shuffled in a random order
println(numbers)

A Set<T> is used an unordered collection of unique elements. Even null can occur only once in a Set. Two sets are equal if they have the same size, and for each element of a set there is an equal element in the other set. The default implementation of a Set is a LinkedHashSet, which remembers the order of insertion of elements. So it also supports methods like first() and last().

val numbers = setOf(1, 2, 3, 4)
println("Number of elements: ${numbers.size}")

if (numbers.contains(1)) println("1 is in the set")

val numbersBackwards = setOf(4, 3, 2, 1)
println("${numbers == numbersBackwards}")

A MutableSet allows you to add or remove values.

val empty = mutableSetOf<Int>()     // declare an empty mutable set for Int elements

Map<K, V> stores key-value (K-V) pairs. The keys in a Map should be unique. Two maps containing the equal pairs are equal regardless of the order in which the pairs are found in the maps.

val numbersMap = mapOf<String, Int>("key1" to 1, "key2" to 2, "key3" to 3, "key4" to 1)     // the type specification can be omitted as it is automatically inferred
println("All keys: ${numbersMap.keys}")
println("All values: ${numbersMap.values}")
if ("key2" in numbersMap) println("Value of \"key2\": ${numbersMap["key2"]}")    
if (1 in numbersMap.values) println("The value 1 is in the map")
if (numbersMap.containsValue(1)) println("The value 1 is in the map") // same as previous

A MutableMap allows you to add new key-value pairs and update existing ones.

val numbersMap = mutableMapOf("one" to 1, "two" to 2)
numbersMap.put("three", 3)
numbersMap["one"] = 11
println(numbersMap)     // {one=11, two=2, three=3}
numbersMap.remove("four")     // nothing happens
numbersMap.remove("three", 4)     // removes the pair only if the value associated with "three" is 4
println(numbersMap.containsValue(3))      // true
println(numbersMap.containsKey(3))      // false

Collections support iteration. When iterating over maps, a pair of values is returned in each iteration, which can be destructured into two separate variables.
A description of all the properties and methods supported by lists, mutable lists, sets, mutable sets, maps and mutable maps can be found in the official documentation.

Exceptions

You can throw exceptions using the throw keyword.

throw ArithmeticException

The Exception class is used to create custom exceptions.

val customException = Exception("A custom error message.")
throw customException

// Same as:
// throw Exception("A custom error message.")

Kotlin supports try-catch blocks. They can have multiple handlers (catch blocks) if required. The message property stores the error message related to an Exception. When an exception occurs in the try block, the suitable handler is determined by trying to match the exception types from the first catch block to the last. So specialized handlers like IOException should come before general handlers like Exception (which can catch every type of exception). After the exception is handled, code execution resumes from the line immediately after the try-catch block.

try {
    print(10 / 0)
}
catch (e: IOException) {
    // handles IOException and its subtypes
}
catch (e: Exception) {
    // handles all subtypes of Exception
    println(e.message)      // "/ by zero"
}

A finally block is executed irrespective of whether an exception was encounter in a try or catch block.

try {
    // code that may throw an exception
}
catch (e: Exception) {
    // exception handler
}
finally {
    // always executed
}

A try-catch block can also be used as an expression in Kotlin.

val number: Int = try { "abc".toInt() } catch (e: NumberFormatException) { 0 }
println(number)     // 0

Working With Files

Files are handled using the File class from the java.io package. A File object contains a pointer to the specified file. When a method (except write methods) is called and the file does not exist, and a NoSuchFileException is thrown.
The readText() method returns the entire contents of the file as a String.

import java.io.File

val file = File("file.txt")       // file is not opened, only a reference to the file is returned
val content = file.readText()     // file is opened and closed automatically by the method
print(content)                    // entire content of the text file is displayed

The readLines() method returns a List with each line of the file as an element of the list.
The readBytes() method returns the entire content of the file as a byte array.
The forEachLine() method executes an action for each line in the file. This is recommended for very large files, as directly trying to read the entire file in one go may exhaust the system memory.

val fileName = "file.txt"
File(fileName).forEachLine { println(it) }

The writeText() method is used to write a String to file. If the file does not exists, it is created automatically. If the file already exists, its contents are overwritten.
The appendText() method is used to append a String to an existing file.
The writeBytes() and appendBytes() methods are used to write a ByteArray to file.
The length() method returns the number of characters in the file.
The exists() method of the File class can be used to check whether a file exists.
A list of all methods supported by the File class can be found in the official documentation.

References

Basic Syntax - Kotlin Official Documentation
Kotlin Programming Language Reference - Kotlin Official Documentation
NOTE: A lot of code samples have been taken directly from the official documentations.

Learn Kotlin

JetBrains Academy has a Kotlin track that helps you learn by working on a project in Kotlin. The course is current in a public beta phase, and available for free at the time of writing.
Kotlin Koans is great way for Java developers to learn Kotlin, by working on a curated series of exercises in Kotlin.

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