What is difference between "as" and "is" operator in Kotlin?

Question

In Java, I can write code like:

    void cast(A a)  {
    if(a  instanceof  Person)  {
        Person p = (Person) a;
    }
}

In Kotlin, what should I do? Use as operator or is operator?

Solution 1

is X is the equivalent of instanceof X

foo as X is the equivalent of ((X) foo)

Additionally, Kotlin performs smart casting where possible, so no additional cast needed after you check the type using is:

open class Person : A() {
    val foo: Int = 42
}

open class A

and then:

if (p is Person) {
    println(p.foo) // look, no cast needed to access `foo`
}

Solution 2

is is type checking. But Kotlin has smart cast which means you can use a like Person after type check.

if(a is Person) {    
    // a is now treated as Person
}

as is type casting. However, as is not recommended because it does not guarantee run-time safety. (You may pass a wrong object which cannot be detected at compiled time.)

Kotlin has a safe cast as?. If it cannot be casted, it will return null instead.

val p = a as? Person
p?.foo()

Solution 3

"Kotlin in action" by Dmitry Jemerov and Svetlana Isakova has a good example of as and is:

Solution 4

is - To check if an object is of a certain type

Example:

if (obj is String) {
    print(obj.length)
}

as - To cast an object to a potential parent type

Example:

val x: String = y as String
val x: String? = y as String?
val x: String? = y as? String

Reference: https://kotlinlang.org/docs/reference/typecasts.html

Solution 5

as is used for explicit type casting

val p = a as Person;

is is exactly the same as instanceof in Java. Which is used to check if an object is an instance of a class

if(a  is  Person)  {
      // a is an instance of Person
}

You can also used !is as is it not an object of a class

fun cast(a: A)  {
    if(a  is  Person)  {
        val p = a as Person;
    }
}

View more solutions

Comments

[Shawn Plus]

so if i always use 'is', that mean I can never use 'as' right? because IDE has smart cast

[Joshua]

@ShawnPlus It depends on your use case. as? is better if you use the variable in multiple place. Smart cast only work within the bracket only.

[Shawn Plus]

we can use 'if' Statement to avoid ClassCastException, why need 'as'? , I can always use 'if' and 'is'

[juzraai]

Please edit your answer and add explanation to your code.

[wonsuc]

can I see the any evidence reference of as is not recommended?

[Sourav Kannantha B]

@Joshua smart cast works outside bracket, if inside bracket returns. Eg: if(a !is String) return //after this line, a is smart casted to string

[Adeel Ahmad]

@wonsuc ClassA as ClassB will throw exception if cast is not possible. However ClassA as? ClassB will return null if cast is not possible