Accessing Kotlin extension functions from Java

Question

Is it possible to access extension functions from Java code?

I defined the extension function in a Kotlin file.

package com.test.extensions

import c         


        

Answer 1

You need to duplicate your functions in the class files:

Create Kotlin file , for ex Utils.kt

Enter the code

  class Utils {
                companion object {
                    @JvmStatic
                    fun String.getLength(): Int {//duplicate of func for java
                        return this.length
                    }
                }
            }

        fun String.getLength(): Int {//kotlin extension function
            return this.length
        }

OR

class Utils {
    companion object {

        @JvmStatic
        fun getLength(s: String): Int {//init func for java
            return s.length
        }
    }
}

fun String.getLength(): Int {//kotlin extension function
    return Utils.Companion.getLength(this)//calling java extension function in Companion
}

In kotlin use:

val str = ""
val lenth = str.getLength()

In Java use this:

String str = "";
 Integer lenth = Utils.getLength(str);

Answer 2

As far as I can tell this isn't possible. From my reading of the extensions docs, it appears that

public fun MyModel.bar(): Int {
    return this.name.length()
}

creates a new method with the signature

public static int MyModelBar(MyModel obj) {
    return obj.name.length();
}

Then, Kotlin maps that function to calls of the form myModel.bar(), where if bar() isn't found in the MyModel class it looks for static methods matching the signature and naming scheme it outputs. Note that this is just an assumption from their statements about extensions being statically imported and not overriding defined methods. I haven't gotten far enough in their source to know for sure.

So, assuming the above is true there's no way for Kotlin extensions to be called from plain old java code, as the compiler will just see an unknown method being called on an object and error out.

Answer 3

I have a Kotlin file called NumberFormatting.kt that has the following function

fun Double.formattedFuelAmountString(): String? {
    val format = NumberFormat.getNumberInstance()
    format.minimumFractionDigits = 2
    format.maximumFractionDigits = 2
    val string = format.format(this)
    return string
}

In java I simple access it over the file NumberFormattingKt in the following way after the required import import ....extensions.NumberFormattingKt;

String literString = NumberFormattingKt.formattedFuelAmountString(item.getAmount());