CODEWITHSUNDEEP
kotlincastingscopevariable-assignment
Eitanos30
Eitanos30

Reputation: 1439

What is the scope of casted variable in Kotlin?

When casting a variable at the right side of the assign, i'm surprisely realize that the variable still behave as the casted type and not as it was original defined. Am i doing something wrong or it's a compiler issue?

Code:

val hippoList = listOf<Hippo>(Hippo())
val hippoMutableList : MutableList<Hippo> = hippoList as MutableList<Hippo>
hippoList.add(Hippo())

since hippoList is from a List type, it is immutable. So how does trying to run add function on an immutable type isn't cause to compilation error?

Upvotes: 1

Views: 171

Answers (1)

szymon_prz
szymon_prz

Reputation: 560

If you're doing casting it means that you know more than a compiler about this context of execution and you're telling the compiler that this hippoList is a MutableList so on every next usage of hippoList compiler already knows that this have to be a MutableList and allows you to use add method because you casted it to MutableList previously. In fact you will get a runtime error UnsupportedOperationException which means that you didn't really know more about this context of execution and you did something wrong. So instead of using casting on your own allow compiler to do it's work.

In your case instead of a casting to MutableList, transform hippoList to MutableList with hippoList.toMutableList()

The same happens when you're using !! from nullable type to not null type, when you're using it when you know more than a compiler about the context of execution. Here's a little example

val someNullableType: String? = null
val thisStringIsNotNull = someNullableType!!

by using !! on someNullableType we're telling the compiler that someNullableType is not null as well, so we're allowed to write (as in you're case where you're telling that your List is a MutableList as well)

someNullableType.length

but we will receive exception earlier (in place where we used !! to tweak the compiler)

Upvotes: 3

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