Learning Kotlin Basic Syntax
Defining packages
Package specification should be at the top of the source file:
package my.demo
import java.util.*
// …
It is not required to match directories and packages: source files can be placed arbitrarily in the file system.
Defining functions
Function having two Int parameters with Int return type:
fun sum(a: Int, b: Int): Int {
return a + b
}
Target platform: JVMRunning on kotlin v. 1.1.2
Function with an expression body and inferred return type:
fun sum(a: Int, b: Int) = a + b
Target platform: JVMRunning on kotlin v. 1.1.2
Function returning no meaningful value:
fun printSum(a: Int, b: Int): Unit {
println(“sum of $a and $b is ${a + b}”)
}
Target platform: JVMRunning on kotlin v. 1.1.2
Unit return type can be omitted:
fun printSum(a: Int, b: Int) {
println(“sum of $a and $b is ${a + b}”)
}
Target platform: JVMRunning on kotlin v. 1.1.2
Defining local variables
Assign-once (read-only) local variable:
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
Target platform: JVMRunning on kotlin v. 1.1.2
Mutable variable:
var x = 5 // `Int` type is inferred
x += 1
Target platform: JVMRunning on kotlin v. 1.1.2
Comments
Just like Java and JavaScript, Kotlin supports end-of-line and block comments.
// This is an end-of-line comment
/* This is a block comment
on multiple lines. */
Unlike Java, block comments in Kotlin can be nested.
Using 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 $a”
Target platform: JVMRunning on kotlin v. 1.1.2
Using conditional expressions
fun maxOf(a: Int, b: Int): Int {
if (a > b) {
return a
} else {
return b
}
}
Target platform: JVMRunning on kotlin v. 1.1.2
Using if as an expression:
fun maxOf(a: Int, b: Int) = if (a > b) a else b
Target platform: JVMRunning on kotlin v. 1.1.2
Using nullable values and checking for null
A reference must be explicitly marked as nullable when null value is possible.
Return null if str does not hold an integer:
fun parseInt(str: String): Int? {
// …
}
Use a function returning nullable value:
fun printProduct(arg1: String, arg2: String) {
val x = parseInt(arg1)
val y = parseInt(arg2)
// Using `x * y` yields error because they may hold nulls.
if (x != null && y != null) {
// x and y are automatically cast to non-nullable after null check
println(x * y)
}
else {
println(“either ‘$arg1’ or ‘$arg2’ is not a number”)
}
}
Target platform: JVMRunning on kotlin v. 1.1.2
or
// …
if (x == null) {
println(“Wrong number format in arg1: ‘${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)
Target platform: JVMRunning on kotlin v. 1.1.2
Using 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
}
Target platform: JVMRunning on kotlin v. 1.1.2
or
fun getStringLength(obj: Any): Int? {
if (obj !is String) return null
// `obj` is automatically cast to `String` in this branch
return obj.length
}
Target platform: JVMRunning on kotlin v. 1.1.2
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
}
Target platform: JVMRunning on kotlin v. 1.1.2
Using a for loop
val items = listOf(“apple”, “banana”, “kiwi”)
for (item in items) {
println(item)
}
Target platform: JVMRunning on kotlin v. 1.1.2
or
val items = listOf(“apple”, “banana”, “kiwi”)
for (index in items.indices) {
println(“item at $index is ${items[index]}”)
}
Target platform: JVMRunning on kotlin v. 1.1.2
Using a while loop
val items = listOf(“apple”, “banana”, “kiwi”)
var index = 0
while (index < items.size) { println(“item at $index is ${items[index]}”) index++ } Target platform: JVMRunning on kotlin v. 1.1.2
Using when expression
fun describe(obj: Any): String = when (obj) { 1 -> “One”
“Hello” -> “Greeting”
is Long -> “Long”
!is String -> “Not a string”
else -> “Unknown”
}
Target platform: JVMRunning on kotlin v. 1.1.2
Using 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”)
}
Target platform: JVMRunning on kotlin v. 1.1.2
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”)
}
Target platform: JVMRunning on kotlin v. 1.1.2
Iterating over a range:
for (x in 1..5) {
print(x)
}
Target platform: JVMRunning on kotlin v. 1.1.2
or over a progression:
for (x in 1..10 step 2) {
print(x)
}
for (x in 9 downTo 0 step 3) {
print(x)
}
Target platform: JVMRunning on kotlin v. 1.1.2
Using collections
Iterating over a collection:
for (item in items) {
println(item)
}
Target platform: JVMRunning on kotlin v. 1.1.2
Checking if a collection contains an object using in operator:
when {
“orange” in items -> println(“juicy”)
“apple” in items -> println(“apple is fine too”)
}
Target platform: JVMRunning on kotlin v. 1.1.2
Using lambda expressions to filter and map collections:
fruits
.filter { it.startsWith(“a”) }
.sortedBy { it }
.map { it.toUpperCase() }
.forEach { println(it) }
fruits
.filter { it.startsWith(“a”) }
.sortedBy { it }
.map { it.toUpperCase() }
.forEach { println(it) }
Last Updated on March 21, 2021 by Code Guru