CODEWITHSUNDEEP
intellij-ideanullkotlin
Dims
Dims

Reputation: 51199

Why "Smart cast to kotlin.String" in IntelliJ

I wrote my class with nullable argument

class MyClass(a: String? = null) {
    var b: String

    init {
        if( a == null ) {
            b = "N/A"
        }
        else {
            b = a
        }
    }
}

and noticed, that IntelliJ warns about b = a assignment

Smart cast to kotlin.String

Why is this warning and how to aoid it?

Upvotes: 6

Views: 4650

Answers (2)

zsmb13
zsmb13

Reputation: 89608

This is not something to avoid, this is a language feature called a smart cast.

The compiler has detected that a is a non-null value on a given code path, and therefore it's available to you with a non-nullable String type, which lets you use it directly without having to worry about its nullability. If you didn't get a smart cast, you'd have to manually cast it from a String? to a String to call methods on it - or to assign it to a non-nullable variable, like in your case.

Another, perhaps easier example to understand when smart cast kicks in is with subclasses. Let's say you have Person and Customer classes, with only the latter having a getPurchases() method.

In Java, you'd have to do the following to call said method:

if (p instanceof Customer) {
    ((Customer)p).getPurchases();
}

In Kotlin, you get a smart cast inside the if block to the subtype that you've already made the check for:

if (p is Customer) {
    p.getPurchases()
}

If you think about it, the cast from String? to String is the same mechanism at work - you're getting a smart cast to a more concrete type too. 

if (a != null)

given an a of type String? is basically the same as 

if (a is String)

Upvotes: 6

s1m0nw1
s1m0nw1

Reputation: 82017

It’s not a warning, just an information that the compiler knows your variable a to be not null. As a result, it can be used as a String instead of nullable String? and therefore without safe-operator e.g.

Otherwise, you needed to explicitly cast a: String? to String in order to make it assignable to the String variable b, as shown on the following screenshot:

enter image description here

Upvotes: 11

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