Null Safety in Kotlin - Kotlin Assignment Help

Nullable and Non-Nullable Types in Kotlin

There are two types of references that can hold null (nullable references) and those that can not (non-null references).

A variable of type String can not hold null. If we try to assign null to the variable, it gives a compiler error.

var s1: String = "Abhay"
s1 = null // compilation error

To allow a variable to hold null, we can declare a variable as a nullable string, written String?

var s2: String? = "Abhay"
s2 = null // ok
print(s2)

Kotlin program of non-nullable type:-

fun main(args: Array<String>){
	// variable is declared as non-nullable
	var s1 : String = "Abhay"

	//s1 = null // gives compiler error
	
	print("The length of string s1 is: "+s1.length)
}

Output:

The length of string s1 is: 5

Kotlin program of nullable type-

fun main(args: Array<String>) {
    // variable is declared as nullable 
    var s2: String? = "Abhay" 
      
    s2 = null    // no compiler error 
      
    println(s2.length)  // compiler error because string can be null
}

Output:

Error:(8, 15) Kotlin: Only safe (?.) or non-null asserted (!!.) calls are allowed on a nullable receiver of type String?

Checking for null in conditions –

The most common way of checking null reference is using if-else expression. We can explicitly check if variable is null, and handle the two options separately.

Kotlin program of checking null in conditions –

fun main(args: Array<String>) {
	// variable declared as nullable
	var s: String? = "Abhay"
	println(s)
	if (s != null) {
		println("String of length ${s.length}")
	} else {
		println("Null string")
	}
	// assign null
	s = null
	println(s)
	if (s != null) {
		println("String of length ${s.length}")
	} else {
		println("Null String")
	}
}

Output:

Abhay
String of length 5
null
Null String


Hope now you are able to understand the nullablity or Null saftey in kotlin in next blog we are going to learn about loops in kotlin. 

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