Learning Kotlin: the Difference Between the Notnull Delegate and Lateinit

Before we look at the two solutions, let us look at a non-working example. Here, we have a class to represent a physical building, and we want to store the number of floors which we get from calling an API (not shown). The problem here is that there is no good default value. Let's look at a house with just a ground floor at 0, a multistory office building that could have 10 floors, and an underground bunker that could have -30 floors.

class Building() {
    var numberOfFloors : Int

    fun getBuildingInfo(erfNumber : String){
        // call municpality web service to get details
        numberOfFloors = 0; // 0 cause 0 = G like in elevators
    }
}

fun main(args:Array<String>) {
    val house = Building()
    house.getBuildingInfo("123")
    println("My house, up the road, has ${house.numberOfFloors}")
}

This will not compile as it states Property must be initialized or be abstract for the number of floors.

Notnull

Fom our previous delegates, we have the notnull delegate. We can use the notnull delegate to allow the property to not be set initially. And, then:

import kotlin.properties.Delegates

class Building() {
var numberOfFloors : Int by Delegates.notNull()

fun getBuildingInfo(erfNumber : String){
// call municpality web service to get details
this.numberOfFloors = 10;
}
}

fun main(args:Array<String>) {
val house = Building()
house.getBuildingInfo("123")
println("My house, down the street, has ${house.numberOfFloors}")
}

This example will print out that we have 10 floors. If we were to comment out line 14, we would get the following exception: Exception in thread "main" java.lang.IllegalStateException: Property numberOfFloors should be initialized before get So, this is not exactly a null exception but close enough that it makes no difference.

Lateinit

Another option is lateinit, which we can add before the var keyword. But, we cannot use it with Int or other primitive types, so we need to change that as well to a class. This is a really nice and simple solution.

data class Floors(val aboveGround: Int, val belowGround: Int)

class Building() {
    lateinit var numberOfFloors : Floors

    fun getBuildingInfo(erfNumber : String){
        // call municpality web service to get details
        this.numberOfFloors = Floors(2, 40);
    }
}

fun main(args:Array<String>) {
    val house = Building()
    house.getBuildingInfo("123")
    println("My house, in the middle of the avenue, has ${house.numberOfFloors}")
}

Once again, if we comment out line 14, we get an exception as expected: 'Exception in thread "main" kotlin. UninitializedPropertyAccessException : lateinit  property  numberOfFloors has not been initialized'.

Lateinit Vs. Notnull

As we can see in these simple examples, both methods achieve the same goal, but they both have their own limitations and advantages:

• notnull being a delegate needs an extra object instance for each property, so there is a small additional memory/performance load.
• The notnull delegate hides the getting and setting in a separate instance, which means anything that works with the field, for example, Java DI tools, will not work with it.
• lateinit doesn't work with primitive types (Int, Char, Boolean etc...) as we saw above.
• lateinit only works with var and not val.
• When using lateinit, you gain access to the .isInitialized method, which you can use to check if it was initialized.