How to check "instanceof " class in kotlin?

Question

In kotlin class, I have method parameter as object (See kotlin doc here ) for class type T. As object I am passing different classes when I am calling method. In Java we can able to compare class using instanceof of object which class it is.

So I want to check and compare at runtime which Class it is?

How can I check instanceof class in kotlin?

score 446 · Accepted Answer · edited May 23 '17 at 18:11

446

Use is.

if (myInstance is String) { ... }

or the reverse !is

if (myInstance !is String) { ... }

edited May 23 '17 at 18:11

Jayson Minard

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answered May 21 '17 at 15:52

nhaarman

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3

This only works when the instance object is created from a Kotlin class. This will not work if the instance was created using newInstance() from something like SomeClass::class.java – Johann Aug 22 '21 at 15:30
1

@androiddev why is that? – nhaarman Aug 22 '21 at 21:12
2

No idea. I tried it but it doesn't work. Some quirk with the way Kotlin interacts with Java. However, you can use SomeClass.isInstance(someObject), which does work. – Johann Aug 23 '21 at 07:08

score 69 · Answer 2 · answered Jan 11 '18 at 18:08

69

Combining when and is:

when (x) {
    is Int -> print(x + 1)
    is String -> print(x.length + 1)
    is IntArray -> print(x.sum())
}

copied from official documentation

answered Jan 11 '18 at 18:08

methodsignature

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1

Yes this is the idiomatic way. – WestCoastProjects Apr 06 '20 at 18:35

Avijit Karmakar · Answer 3 · 2018-01-27T07:40:22.963

34

We can check whether an object conforms to a given type at runtime by using the is operator or its negated form !is.

Example:

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

if (obj !is String) {
    print("Not a String")
}

Another Example in case of Custom Object:

Let, I have an obj of type CustomObject.

if (obj is CustomObject) {
    print("obj is of type CustomObject")
}

if (obj !is CustomObject) {
    print("obj is not of type CustomObject")
}

edited Jan 27 '18 at 07:40

answered May 21 '17 at 15:53

Avijit Karmakar

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Note one other nice thing here: inside the block of the `if`, `obj` is automatically cast to `String`. So you can use properties such as `length` directly, without the need to explicitly cast `obj` to `String` inside the block. – Jesper May 22 '17 at 06:20
Also, nice opportunity to use an else statement ;) – Juan Nunez Jan 29 '21 at 15:20
What if i want to differentiate two classes where one extends the other e.g LinearLayoutManager & GridLayoutManager – Isaac Obella Apr 19 '21 at 09:45

score 9 · Answer 4 · edited Oct 05 '17 at 11:56

9

Try using keyword called is Official page reference

if (obj is String) {
    // obj is a String
}
if (obj !is String) {
    // // obj is not a String
}

edited Oct 05 '17 at 11:56

Rashi Karanpuria

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answered May 21 '17 at 15:53

Terril Thomas

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It is great thing to give answer with official doc. But it is better practices to add sample code in answer, it is helpful if link was dead. Thanks for the answer. – pRaNaY May 21 '17 at 17:03

score 9 · Answer 5 · answered May 21 '17 at 15:55

9

You can use is:

class B
val a: A = A()
if (a is A) { /* do something */ }
when (a) {
  someValue -> { /* do something */ }
  is B -> { /* do something */ }
  else -> { /* do something */ }
}

answered May 21 '17 at 15:55

ice1000

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score 5 · Answer 6 · answered Mar 27 '19 at 07:56

5

You can check like this

 private var mActivity : Activity? = null

then

 override fun onAttach(context: Context?) {
    super.onAttach(context)

    if (context is MainActivity){
        mActivity = context
    }

}

answered Mar 27 '19 at 07:56

The Bala

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score 3 · Answer 7 · answered Aug 23 '19 at 07:21

You can read Kotlin Documentation here https://kotlinlang.org/docs/reference/typecasts.html . We can check whether an object conforms to a given type at runtime by using the is operator or its negated form !is, for the example using is:

fun <T> getResult(args: T): Int {
    if (args is String){ //check if argumen is String
        return args.toString().length
    }else if (args is Int){ //check if argumen is int
        return args.hashCode().times(5)
    }
    return 0
}

then in main function i try to print and show it on terminal :

fun main() {
    val stringResult = getResult("Kotlin")
    val intResult = getResult(100)

    // TODO 2
    println(stringResult)
    println(intResult)
}

This is the output

6
500

Álvaro Agüero · Answer 8 · 2019-05-10T16:10:28.807

-8

How to check "instanceof " class in kotlin?

8 Answers8

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