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Basic Syntax of Kotlin Programming Language

Package definition and imports

Package specification should be at the top of the source file:

package my.demo

import kotlin.text.*

// ...

It is not required to match directories and packages: source files can be placed arbitrarily in the file system.

Program entry point

An entry point of a Kotlin application is the main function.

fun main() {
    println("Hello world!")
}

Functions

Function having two Int parameters with Int return type:

fun sum(a: Int, b: Int): Int {
    return a + b
}

Function with an expression body and inferred return type:

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

Function returning no meaningful value:

fun printSum(a: Int, b: Int): Unit {
    println("sum of $a and $b is ${a + b}")
}

Unit return type can be omitted:

fun printSum(a: Int, b: Int) {
    println("sum of $a and $b is ${a + b}")
}

Variables

Read-only local variables are defined using the keyword val. They can be assigned a value only once.

val a: Int = 1  // immediate assignment
val b = 2   // `Int` type is inferred
val c: Int  // Type required when no initializer is provided
c = 3       // deferred assignment

Variables that can be reassigned use the var keyword:

var x = 5 // `Int` type is inferred
x += 1

Top-level variables:

val PI = 3.14
var x = 0

fun incrementX() { 
    x += 1 
}

Comments

Just like most modern languages, Kotlin supports single-line (or end-of-line) and multi-line (block) comments.

// This is an end-of-line comment

/* This is a block comment
   on multiple lines. */

Block comments in Kotlin can be nested.

/* The comment starts here
/* contains a nested comment */     
and ends here. */

String templates

var a = 1
// simple name in template:
val s1 = "a is $a" 

a = 2
// arbitrary expression in template:
val s2 = "${s1.replace("is", "was")}, but now is $a"

Conditional expressions

fun maxOf(a: Int, b: Int): Int {
    if (a > b) {
        return a
    } else {
        return b
    }
}

In Kotlin, if can also be used as an expression:

fun maxOf(a: Int, b: Int) = if (a > b) a else b

Nullable values and null checks

A reference must be explicitly marked as nullable when null value is possible.

Return null if str does not hold an integer:

fun parseInt(str: String): Int? {
    // ...
}

Use a function returning nullable value:

fun printProduct(arg1: String, arg2: String) {
    val x = parseInt(arg1)
    val y = parseInt(arg2)

    // Using `x * y` yields error because they may hold nulls.
    if (x != null && y != null) {
        // x and y are automatically cast to non-nullable after null check
        println(x * y)
    }
    else {
        println("'$arg1' or '$arg2' is not a number")
    }    
}

or

// ...
if (x == null) {
    println("Wrong number format in arg1: '$arg1'")
    return
}
if (y == null) {
    println("Wrong number format in arg2: '$arg2'")
    return
}

// x and y are automatically cast to non-nullable after null check
println(x * y)

Type checks and automatic casts

The is operator checks if an expression is an instance of a type. If an immutable local variable or property is checked for a specific type, there’s no need to cast it explicitly:

fun getStringLength(obj: Any): Int? {
    if (obj is String) {
        // `obj` is automatically cast to `String` in this branch
        return obj.length
    }

    // `obj` is still of type `Any` outside of the type-checked branch
    return null
}

or

fun getStringLength(obj: Any): Int? {
    if (obj !is String) return null

    // `obj` is automatically cast to `String` in this branch
    return obj.length
}

or even

fun getStringLength(obj: Any): Int? {
    // `obj` is automatically cast to `String` on the right-hand side of `&&`
    if (obj is String && obj.length > 0) {
        return obj.length
    }

    return null
}

for loop

val items = listOf("apple", "banana", "kiwifruit")
for (item in items) {
    println(item)
}

or

val items = listOf("apple", "banana", "kiwifruit")
for (index in items.indices) {
    println("item at $index is ${items[index]}")
}

while loop

val items = listOf("apple", "banana", "kiwifruit")
var index = 0
while (index < items.size) {
    println("item at $index is ${items[index]}")
    index++
}

when expression

fun describe(obj: Any): String =
    when (obj) {
        1          -> "One"
        "Hello"    -> "Greeting"
        is Long    -> "Long"
        !is String -> "Not a string"
        else       -> "Unknown"
    }

Ranges

Check if a number is within a range using in operator:

val x = 10
val y = 9
if (x in 1..y+1) {
    println("fits in range")
}

Check if a number is out of range:

val list = listOf("a", "b", "c")

if (-1 !in 0..list.lastIndex) {
    println("-1 is out of range")
}
if (list.size !in list.indices) {
    println("list size is out of valid list indices range, too")
}

Iterating over a range:

for (x in 1..5) {
    print(x)
}

or over a progression:

for (x in 1..10 step 2) {
    print(x)
}
println()
for (x in 9 downTo 0 step 3) {
    print(x)
}

Collections

Iterating over a collection:

for (item in items) {
    println(item)
}

Checking if a collection contains an object using in operator:

when {
    "orange" in items -> println("juicy")
    "apple" in items -> println("apple is fine too")
}

Using lambda expressions to filter and map collections:

val fruits = listOf("banana", "avocado", "apple", "kiwifruit")
fruits
  .filter { it.startsWith("a") }
  .sortedBy { it }
  .map { it.toUpperCase() }
  .forEach { println(it) }

Creating basic classes and their instances

val rectangle = Rectangle(5.0, 2.0)
val triangle = Triangle(3.0, 4.0, 5.0)

Source : Kotlin Lang