Null safety

In the Kotlin tutorial of the previous LinuxFormat issue, we mentioned the term Null safety. Now it’s time to learn more about this important Kotlin characteristic, which makes your code easier to read and much safer. First, look at the following example of Kotlin code: var aString = “Tsoukalos” aString = null This code won’t compile because the default Kotlin behaviour doesn’t permit you to assign a null value to a variable. The generated error message will be null can not be a value of a nonnull type String , which shows exactly what’s

going on. However, you can change that behaviour with the use of the safe null access operator, which is a question mark: >>> var anotherString: String? = “Tsoukalos” >>> anotherString = null

Kotlin goes a step further and requires you to check a variable that can be null before using it. Bearing this in mind, we know that the following Kotlin program won’t compile: fun printA(printMe: String) { println(printMe) }

fun main(args: Array<String>) { val a: String? = “Tsoukalos” printA(a) }

However, if you replace printA(a) with the following snippet of code, then the nSafety.kt program will compile just fine: if (a != null) printA(a)

If you’re an amateur programmer, you might not immediately appreciate the null safety feature of Kotlin, but it does save you from NullPointerExceptions… and this is a great thing!