What is the difference between instance.let and instance?.let

Question

What is the difference between two line of codes:

1.

 viewHolderRef?.get()?.adapterPosition.let {

viewHolderRef?.get()?.adapterPosition?.let

I know that it can affect on the type of the argument that the lambda expression gets but in difference i mean, is there a situation when in one of the expression the let will run and in the second know (with refer to null issue)

Answer 1

v?.let means: if v is null, then just make the entire expression null (the let method won't even be called because it'll be terminated early), and if it's not null, then change the type of the expression to be non-nullable then keep going with the property chain. So, v?.let will only call the function if v is non-null.

On the contrary, v.let will always call let, regardless if v is null or not, so the parameter is nullable (since there's no guarantee it isn't null like with v?.let. The difference is best demonstrated with this code:

fun test(v: String?) {
    v.let { it: String? -> println("v.let: $it") }
    v?.let { it: String -> println("v?.let: $it") }
}

fun main () {
    test(null)
    println("----------")
    test("foo")
}

Playground link

This outputs:

v.let: null
----------
v.let: foo
v?.let: foo

As you can see, when calling it on a null output, only v.let runs and prints null, whereas on a non-null output, both let statements run. You can also see a difference in the types of the parameters: v.let takes in a String?, whereas v?.let takes in a String.