Difference between ArrayList with safe call operator and Not Null Assertion Operator in Kotlin-CodePudding

I have four ArrayList below, I am confusing ArrayList with safe call operator and Not Null Assertion Operator like that, even I know what is the meaning of ? and !! in Kotlin, Any one can explain it for me?, how can I explain those four ArrayList situation? thanks.

ArrayList<User?>?, ArrayList<User?>, ArrayList<User>? and ArrayList<User>

ArrayList<String?>?,ArrayList<String?>,ArrayList<String>? and ArrayList<String>

CodePudding user response：

Hope that'll help you clear that out:

var nullableArrayOfNullableItems: ArrayList<User?>?
// you can do that
nullableArrayOfNullableItems = null
// you also can do that
nullableArrayOfNullableItems = arrayOf(null)

var arrayOfNullableItems: ArrayList<User?>
// you CANT do that
arrayOfNullableItems = null
// you can do that
arrayOfNullableItems = arrayOf(null)

var nullableArrayOfNonnullableItems: ArrayList<User>?
// you can do that
nullableArrayOfNonnullableItems = null
// you CANT do that
nullableArrayOfNonnullableItems = arrayOf(null)

var arrayOfNonnullableItems: ArrayList<User>
// you CANT do that
arrayOfNonnullableItems = null
// you CANT do that
arrayOfNonnullableItems = arrayOf(null)

CodePudding user response：

This may explain the four type declarations:

val input = listOf("ArrayList<User?>?",
"ArrayList<User?>",
"ArrayList<User>?", 
"ArrayList<User>")
input.forEach {
    val arr = it.split(Regex("[<>]"))
    val null1 = if (it.endsWith('?')) "nullable" else "not-null"
    val null2 = if (arr[1].endsWith('?')) "nullable" else "not-null"
        println("$it : $null1 ${arr[0]} with $null2 elements in type ${arr[1].replace(Regex("[?]"),"")}")

}

It prints:

ArrayList<User?>? : nullable ArrayList with nullable elements in type User
ArrayList<User?> : not-null ArrayList with nullable elements in type User
ArrayList<User>? : nullable ArrayList with not-null elements in type User
ArrayList<User> : not-null ArrayList with not-null elements in type User

CodePudding user response：

These uses of ? in a type have nothing to do with safe calls or non-null assertions.

Succinctly:

A ? inside the list type <> like ArrayList<User?> means the list can hold null values.

A ? after the complete type of the list like ArrayList<User>? means the list itself can be null.

Having ? in both places like ArrayList<User?>? means both. It's a list that can hold null values and might be null itself.

Without any question marks, it's a List that is not null and cannot hold null values.