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Why a separate

isInitialized

property instead of a null check? Is it to allow for initialization to a nullable value? (since

T

is allowed to be nullable)

It’s just so it covers some edge-cases. Theoretically, when the variable is lazily initialized, the initializer might do something and set variable to

null

. And then that initialization would happen every other time we reference it, since the value will again be

null

. This won’t happen if used correctly but with boolean we keep the implementation very precise. If you check the Kotlin source, they don’t use

isInitialized

but the property value is defined as

Any

and it holds some predefined object (

UNINITIALIZED_VALUE

), and then once it’s initialized, it will cast to

T

(https://github.com/JetBrains/kotlin/blob/master/libraries/stdlib/src/kotlin/util/Lazy.kt#L76-L85). For the sake of simplicity, I used boolean value.