2. Kotlin Basics

Variable vs Value

var nameofvar:Int =5 // Variable declaration syntax
val nameofval:Int =5 // Value declaration syntax
// Variables can be changed while values can not be

Ranges inside loop

for(i in 1..5){
  }
for(i in 1.rangeTo(5)){
  }
// above loops are in range 1 to 5
for(i in 10.downTo(1)){
  }
// the above loop ranges in decreasing order from 10 to 1
for(i in 1..10  step 2 ) // 1, 3, 5, 7, 9

Null Safety

Kotlin is a null safe language! It can not hold null values by default unless explicitly mentioned.

val x:Int = null //Throws an error
val x:Int ?= null //Suffixing ? to a type accepts null
// Since the '?' operator is used for specifying nullability,
// conditional operator does not exist in kotlin

Safe call

val b: String? = null
println(b?.length)// '?.' safe call operator

This returns b.length if b is not null, and null otherwise. The type of this expression is Int?.

Elvis

When we have a nullable reference b, we can say "if b is not null, use it, otherwise use some non-null value":

val l = b?.length ?: -1 // '?:' elvis operator
// here b will use -1 if it is null

Not-null assertion (!!)

val l = b!!.length

The not-null assertion operator (!!) converts any value to a non-null type and throws a NPE if the value is null. 😈

Collections of Nullable Type

If you have a collection of elements of a nullable type and want to filter non-null elements, you can do so by using filterNotNull:

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

Conditions

if...else

Unlike other languages, the if..else statement in Kotlinhas the ability to assign a variable from the returned value of the if..else statement.

val number = 13
val result = if (number % 2 == 0) {
print("$number is divisible by 2")
} else {
print("$number is not divisible by 2")// 13 is not divisible by 2
} // result has the value "13 is not divisible by 2"

when

Alternate of switch in other languages.

val number = 2
when (number) { 
    1 -> println("number is 1")
    2 -> { //enclosed in braces for multiline body
        println("number is 2") 
        println("it is an even number")
        //this will be executed
    }
    3,4 -> println("number is 3 OR 4") //values can also be combined
}

// when can also work without any argument like this
when {
number == 1 -> println("it is one")
number == 2 -> println("it is two")
}

Functions

Above function can also be written as

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

Extension Functions

• An extension function is a member function of a class that is defined outside the class.

fun String.removeFirstLastChar(): String = this.substring(1, this.length - 1)