Kotlin Basics

Basically, Kotlin attempts to resolve NullPointerException(NPE) by making the data types as nou nullable

Variable Declaration

If you try to assign a null value to a variable, the following compile error will be thrown by IDE

To make the variable nullable, we can declare as follows

Accessing null Variable

If you access the null variable, you will get the following compile error

Safe Operator -?

? - using this operator, we can access the null variable as follows


    val bookName: String? = null

    println(bookName?.length) //  prints null

bookName?.length does the following

 println(bookName?.length) //  prints null

    if (bookName == null) {
        println(null)
    } else {
        println(bookName.length)
    }

Elvis Operator - ?:

The Elvis operator is used to simplify the code while using the nullable variable as follows

    val bookName: String? = null

    // elvis operator
    println(bookName ?: "bookName is null") // prints bookName is null

    /*
      Elvis operator does the following
     */
    if (bookName != null) {
        println(bookName)
    } else {
        println("bookName is null")
    }

The Left side of Elvis operator is executed if the bookName is not null

The Right side of Elvis operator is executed if the bookName is null

!! Operator

!! operator executes the null variable without checking the variable is null which is if you want to have NPE, then you can use the variable as follows

    val bookName: String? = null

    println(bookName!!.length) //  throws NPE

Nullable Receiver

you can specify behavior for null values without the need to use null-checking logic at each call-site.

For example, let's take a look at toString() extension function

fun Any?.toString(): String {
    if (this == null) return "null"
    // after the null check, 'this' is autocast to a non-null type, so the toString() below
    // resolves to the member function of the Any class
    return toString()
}

So finally, I hope I made somebody understand how Kotlin handles null

Please leave your comments to improve.

Happy and Enjoy coding

Vignesh Prabhu