Add optional parameter to Android Kotlin class

I've created an extension of `DialogFragment()`:


class AlertDialogFragment(context: Context,
val positiveButtonText: String,
val positiveButtonListener: DialogInterface.OnClickListener,
val negativeButtonText: String,
val negativeButtonListener: DialogInterface.OnClickListener,
neutralButtonText: String,
neutralButtonListener: DialogInterface.OnClickListener
) : DialogFragment() {

however I want the last 2 parameters to be optional.

How can I achieve this?

I can't set `neutralButtonListener: DialogInterface.OnClickListener = null` because `DialogInterface.OnClickListener` is a non null type.

In general you would provide no-op or null-value implementations like

neutralButtonText: String = ""
neutralButtonListener: OnClickListener = OnClickListener {}

But in your use-case you **must not** use constructor parameters at all!

Fragments can be recreated from the system and require a default constructor.

You need to [communicate with the `activity` or `parentFragment`][1] by letting these implement the interface you require.


[1]:

[To see links please register here]

We can add the optional paramater like below. z is optional here



fun main() {
add(5,6,7)
}

fun add(x:Int,y:Int,z:Int?=0){
if(z!=null && z>0)
println(x+y+z)
else
println(x+y)
}

If you want to call kotlin function from Java file which has default arguments

Then use this annotation for your kotlin function

@JvmOverloads

Default parameters to the rescue.

class AlertDialogFragment(
context: Context,
val positiveButtonText: String,
val positiveButtonListener: DialogInterface.OnClickListener,
val negativeButtonText: String,
val negativeButtonListener: DialogInterface.OnClickListener,
neutralButtonText: String = "",
neutralButtonListener: DialogInterface.OnClickListener = OnClickListener {}
) : DialogFragment()

Basically, Kotlin will generate multiple methods, for each combination of parameters possible.

If could also be better to include `@JvmOverload` annotation on the constructor to allow the same thing in Java.

Quite simple...

For class use
```
class MyClass(param1: Int, param2: Boolean) {
constructor(param1: Int) : this(param1, true)
constructor(param2: Boolean) : this(10, param2)
}
```

and for functions

@JvmOverloads
fun yourMethod(param1: Int, param2: Boolean = true) {
// ... define your method
}

Now you can call this method any of the below

`yourMethod(1)`

`yourMethod(1, true)`

`yourMethod(param1 = 1, param2 = false)` etc.,

Check this out

class AlertDialogFragment(
context: Context,
val positiveButtonText: String,
val positiveButtonListener: DialogInterface.OnClickListener,
val negativeButtonText: String,
val negativeButtonListener: DialogInterface.OnClickListener,
neutralButtonText: String = "",
neutralButtonListener: DialogInterface.OnClickListener ?= null
) : DialogFragment() { }

You may try this:

class AlertDialogFragment(
context: Context,
val positiveButtonText: String,
val positiveButtonListener: DialogInterface.OnClickListener,
val negativeButtonText: String,
val negativeButtonListener: DialogInterface.OnClickListener,
neutralButtonText: String = "",
neutralButtonListener: DialogInterface.OnClickListener = OnClickListener {}
) : DialogFragment() { }