Converting java.util.function.Function to kotlin

Question

The java code :

public class Neuron implements Comparable<Neuron>, Serializable {

public interface Activation extends Function<Float, Float>, Serializable {
    Activation SIGMOID = z -> 1 / (1 + (float) Math.exp(-z));
    Activation LINEAR = z -> z;
    Activation TANH = x -> (float) Math.tanh(x);
}
...

somehow, i managed to translate it to this kotlin code (with the help of various answer i found on stackoverflow) :

class Neuron(
val id: Int,
val inputs: Array<Neuron?>,
val weights: FloatArray,
var type: Type?,
private var activation: (Float) -> Float,
val label: String?
) : Comparable<Neuron>, Serializable {

interface Activation : Function<Float?, Float?>, Serializable {
    companion object {
        val SIGMOID = fun(z: Float): Float { return 1 / (1 + exp(-z)) }
        val LINEAR = fun(z: Float): Float { return z }
        val TANH = fun(x: Float): Float  { return tanh(x) }
    }
}

I'm probably doing it wrong and i still have some error when i try to use it. The very good news is that, while my code is still a mix of java and kotlin, this code is only called by kotlin code. So there could be a way to solve it all in a proper way.

I fixed a lot of stuff here and there to fix argument type in various method/function but i'm stuck with this one :

fun tick() {
    nextState = 0.0f
    for (i in inputs.indices) nextState += inputs[i]!!.state * weights[i]
    nextState = activation!!.apply(nextState)!!
}

the errors, on the same line :

Type mismatch: inferred type is (Float) -> Float but Float? was expected
Type mismatch: inferred type is Float but TypeVariable(T).() -> Unit was expected

The first error is probably related to the type of "nextState" (which is indeed a Float), but apply is supposed to return a Float so i assume that solving the 2nd error will fix the 1st one. I don't understand the 2nd error.

Considering that SIGMOID, LINEAR, TANH, are (as far as i know) only called from Kotlin code, is there a way to fix it all in a graceful way ?

A temporary fix i just found :

nextState = activation.invoke(nextState)

is this the proper way to solve it all ? is there perhaps a nicer solution without interface and companion ? should i leave it as is and call it a day ?

mjcoder · Accepted Answer · 2023-01-29 11:53:44Z

1

If you are working in android studio then a good way is to just copy the java code into a Kotlin file. You will be prompted to turn the code into kotlin. This worked for me when I had to copy code from online. I hope this works for you.

answered Jan 29, 2023 at 11:53

mjcoder

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2 Comments

ker2x Over a year ago

i'm in intellij idea. i am indeed using this conversion feature, but the conversion failed for this part and generated erroneous code.

mjcoder Over a year ago

Maybe you can try with one of the online editors?

voismager · Accepted Answer · 2023-01-29 12:03:07Z

1

You can drop this Activation interface in Kotlin because you never use it.

Instead you said in code you want your var activation to be of type (Float) -> Float. So you can do it like this

val SIGMOID: (Float) -> (Float) = { 1 / (1 + exp(-it)) }
...

Oh, and then you can use activation(...) syntax.

edited Jan 29, 2023 at 12:03

answered Jan 29, 2023 at 11:44

voismager

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5 Comments

ker2x Over a year ago

thiank you, i like this syntax. if i remove the interface, how do i make a reference to it ? exemple of existing code : var defaultActivation = Neuron.Activation.LINEAR

voismager Over a year ago

@ker2x if it's located inside companion of Neuron, then just Neuron.LINEAR. If you're referring to it inside Neuron itself, then just LINEAR

ker2x Over a year ago

i get some error about conflicting declaration if i keep the companion object and just remove the interface. i don't really understand what's happening. i'll leave it as is for now. finish my conversion to full kotlin (i still have a lot of java code here and there) and the proceed to cleanup and make it proper kotlin code instead of semi-automated conversion from java. thank you for your help.

voismager Over a year ago

@ker2x If you remove Activation. from every reference then it should work without interface just fine

ker2x Over a year ago

i found the problem. there was a 1 line companion object already declared (at the very bottom) in this class. Hence the conflicting declaration error. i moved the 3 lines inside the already declared companion object and that solved the error. I also removed the reference to Activation as well, of course :)

Nikola Despotoski · Accepted Answer · 2023-01-29 11:21:54Z

0

Probably change to

interface Activation : (Float?) -> Float?, Serializable {
  
}

Then the instances

 companion object {
        val SIGMOID = (Float) -> Float = { return 1 / (1 + exp(-z)) }
        val LINEAR = (Float) -> Float =  { return z }
        val TANH = (Float) -> Float =   { return tanh(x) }
    }

and usage to

activation(nextState)

which is equvalent to .invoke(nextState)

answered Jan 29, 2023 at 11:21

Nikola Despotoski

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2 Comments

ker2x Over a year ago

thank you, now i don't have to import java.util.function.Function anymore. calling activation directly also worked. about the function itself, is there a difference with the conversion i provided or is it purely syntaxic sugar ? i find my val = fun easier to understand (probably because of my C/C++ background, this function pointer-ish style look more intuitive to me (i can't believe i'm writting this))

Louis Wasserman Over a year ago

The problem with both of these versions is that they produce function objects, not instances of the Activation interface. (Note also that you probably shouldn't have return.)

ker2x · Accepted Answer · 2023-01-29 12:52:09Z

I am answering my own question since the full, clean, solution is a mixture of multiple answers and comments.

I replaced nextState = activation!!.apply(nextState)!!

with nextState = activation(nextState)

using nextState = activation.invoke(nextState) also worked but wasn't necessary.

I replaced

interface Activation : Function<Float?, Float?>, Serializable {
    companion object {
        val SIGMOID = fun(z: Float): Float { return 1 / (1 + exp(-z)) }
        val LINEAR = fun(z: Float): Float { return z }
        val TANH = fun(x: Float): Float  { return tanh(x) }
    }
}

with

interface Activation : (Float) -> Float, Serializable {
    companion object {
        val SIGMOID: (Float) -> (Float) = { 1 / (1 + exp(-it)) }
        val LINEAR : (Float) -> (Float) = { it }
        val TANH   : (Float) -> (Float) = { tanh(it) }
    } 
}

And got rid of the java import.

Then i realized that i didn't need this interface since it was only called from Kotlin code.

I changed all reference to the Activation type to (Float) -> Float and removed the interface. I already had a companion object declared in this class so i moved it all in this companion object.

companion object {
    private const val serialVersionUID = 1L  // this one was already there
    val SIGMOID: (Float) -> (Float) = { 1 / (1 + exp(-it)) }
    val LINEAR : (Float) -> (Float) = { it }
    val TANH   : (Float) -> (Float) = { tanh(it) }
}

I changed all my reference to theses function from Neuron.Activation.LINEAR to Neuron.LINEAR

eg :

var defaultActivation = Neuron.Activation.LINEAR // with interface
var defaultActivation = Neuron.LINEAR // without interface

Thank you everyone. I hope this will help, it seems to be a common problem because intellij failed to convert automatically this kind code from java to kotlin using the builtin functionality.

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