How can one add static methods to Java classes in Kotlin

Question

Is it possible to add a new static method to the java.lang.Math class in Kotlin? Usually, such things are possible in Kotlin thanks to Kotlin Extensions.

I already tried doing the following in a file I made called Extensions.kt:

fun Math.Companion.clamp(value:Double,minValue:Double,maxValue:Double):Double
{
    return Math.max(Math.min(value,maxValue),minValue)
}

but Math.Companion could not be resolved...

Why not add the function to the Double class? fun Double.clamp(min: Double, max Double), to be called like 1.0.clamp(2.0, 3.0). — nhaarman
– nhaarman, Commented Nov 25, 2015 at 8:35
Doesn't coerceIn (kotlinlang.org/api/latest/jvm/stdlib/kotlin/coerce-in.html) what you need? — Sergey Mashkov
– Sergey Mashkov, Commented Nov 26, 2015 at 7:28
Yes! Thanks @SergeyMashkov, that is what I needed in this case. However, I should rephrase my question as: "How can one add static methods to Java classes in Kotlin" — Eric
– Eric, Commented Nov 27, 2015 at 17:54
@Eric then only you can do is to declare top-level function (not extension) and put it to the specific package — Sergey Mashkov
– Sergey Mashkov, Commented Nov 30, 2015 at 18:45

Ky - · Accepted Answer · 2022-09-06 20:37:32Z

54

As of Kotlin 1.3, this is not possible. However, it's being considered for a future release!

To help this feature get implemented, go vote on this issue: https://youtrack.jetbrains.com/issue/KT-11968

Because all proposals are basically in limbo right now, I wouldn't hold my breath that this will get in any time soon

edited Sep 6, 2022 at 20:37

answered Mar 8, 2017 at 14:43

Ky -

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5 Comments

Rafael Ruiz Muñoz Over a year ago

To whoever falls in this answer, please vote up the jetbrains post so we can have that feature earlier!

Giszmo Over a year ago

As of today the feature request remains open.

apouche Over a year ago

This idea is very popular in the Kotlin community, so I bet it'll be in soon enough . Aug 2022 Issue is still open.

Ky - Over a year ago

@apouche YUP! Basically all movement was halted once Roman Elizarov took over. I'm still salty.

Will Pierce Over a year ago

This is now a proposal for Kotlin 2.0 at github.com/Kotlin/KEEP/issues/348

Jayson Minard · Accepted Answer · 2016-09-04 01:27:46Z

15

I think this is not possible. Documentation says the following:

If a class has a companion object defined, you can also define extension functions and properties for the companion object.

The Math class is a Java class, not a Kotlin one and does not have a companion object in it. You can add a clamp method to the Double class instead.

edited Sep 4, 2016 at 1:27

Jayson Minard

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answered Nov 25, 2015 at 8:37

aga

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Comments

Miguel A. Gabriel · Accepted Answer · 2018-03-12 19:55:48Z

3

As of Kotlin 1.2 it is still not possible.

As a workaround, to statically "extend" Environment class I am currently using:

Class EnvironmentExtensions {
    companion object {
        @JvmStatic
        fun getSomething(): File {
            ...
            return Environment.something()
        }
    }
}

It is not an ideal solution but IntelliJ/Android Studio code completion helps with the usage:

val something = EnvironmentExtensions.getSomething()

edited Mar 12, 2018 at 19:55

answered Mar 12, 2018 at 9:19

Miguel A. Gabriel

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Comments

Hrudhay · Accepted Answer · 2023-05-09 14:00:05Z

2

Update on top of other answers: Static extensions will be available right after 2.0

Official announement: kotlin conf 2023

answered May 9, 2023 at 14:00

Hrudhay

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