Learn Kotlin

Note

no semicolons at end of lines
Data types are declared after a variable or function name
Question mark after a datatype means the variable can expect a null value

Syntax

(i) package definition -top of .kt file

		package my.demo
		import kotlin.text.*

(ii) program entry point-through main function

    fun main() {
		    //code//
		}

Variables

we can use :

val – read-only variable that is assigned data once
var – variable whose data can change during execution
TODO(): Marking code as incomplete

		…
		var a: Int
		var a: Int = 5
		var name = “Dickson”

		val a: Int
		val a: Int = 5
		…

Swapping two variables

		var a = 1
		var b = 2
		a = b.also { b = a }

Constants

const val MAX_COUNT = 8

constants are named in capital letters

Array

use listOf()

val fruits = listOf("apple", "banana", "kiwifruit")

values can't be changed - "val"

loop through an Array

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

Checking if a collection contains an object using in operator:

val fruits = listOf("banana", "avocado", "apple", "kiwifruit")
when {
		"orange" in fruits -> println("juicy")
		"apple" in fruits -> println("apple is fine too")
}

map and filter data in array using lambda expression

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

check if an array contains a specific object

use 'in' - this checks if an instance of the object exists in the array

val fruits = listOf("banana", "avocado", "apple", "kiwifruit")
when {
    "orange" in items -> println("juicy")
    "apple" in items -> println("apple is fine too")
}

in code(for other checks):

if ("apple" in fruits) {
			//code
	 }

Map

data is added in value key pair

val map = mapOf("a" to 1, "b" to 2, "c" to 3)
	println(map["key"])
	map["key"] = value

Functions

can be declared with parameters e.g

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

data type comes after the parameter is declared in the function

String template

				var a = 1
				val s1 = "a is $a"
				a = 2
				val s2 = "${s1.replace("is", "was")}, but now is $a"

$a - get value of the variable a

Conditional expression

if...else

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

or one-liner if..else

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

			syntax: fun Name(parameters) = if(condtion) trueReturn else falseReturn

check for null

  if(Name != null){
    //code
  }

e.g

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

    if (x != null && y != null) {
        println(x * y)
      }
      else {
        println("'$arg1' or '$arg2' is not a number")
      }
}

null check shorthand in a variable

val value = ...
	value?.let {
			 //execute this block if not null
	}

casting a datatype to an object

'is' is used

fun getStringLength(obj: Any): Int? {
    if (obj is String && obj.length > 0) {
    return obj.length
    }

    if (obj !is String) {
    return null
    }

    return null
}

* question mark only comes in datatype any
* A datatype is then casted to it

Loops

for loop

  val fruits = listOf("apple", "banana", "kiwifruit")
    for (fruit in fruits) {
      println(fruit)
    }

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

when expression

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

acts like switch case
in this function a String is expected that's why after the Datatype return of the statements in the function are Strings

when (x) {
	is "Foo" -> ...
	is "Bar" -> ...
	else 	 -> ...
}

checks like a variable named x
return when statements

fun transform(color: String): Int {
	return when (color) {
				"Red" -> 0
				"Green" -> 1
				"Blue" -> 2
				else -> throw IllegalArgumentException("Invalid color param value")
		}
}

loop with a Ranges

e.g print all numbers between the range of 1 to 5

closed range that includes 5

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

checks if x is within the range of 1 - 5 then prints the number
half-open range doesn't include 100

for (x in 1 until 100) {
	...
 }

loop with ranges over a progression

acts like a for loop till a condition is met

counts up by 2

for (x in 1..10 step 2) {
    print(x)
}

counts down by 3

println()
for (x in 9 downTo 0 step 3) {
    print(x)
}

loop over a collection

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

Ranges

used to check if a number is within a range

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

checks if x(10) is within the range of 1 to 10

y=9 so y+1 = 10

Classes

Class Name{

}

Writing Test

class MyTestCase {
		@Test fun ensureEverythingWorks_onAndroid() {
				//code
			}
}

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