# Property with different public signature than private

**URL:** <https://discuss.kotlinlang.org/t/property-with-different-public-signature-than-private/10590>\
**Category:** Language Design\
**Created:** [November 30, 2018, 5:32pm UTC](https://discuss.kotlinlang.org/t/property-with-different-public-signature-than-private/10590 "2018-11-30T17:32:15Z")\
**Posts on this page:** 13\
**Page:** 1

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**Author:** ![nbilyk](https://sea1.discourse-cdn.com/flex019/user_avatar/discuss.kotlinlang.org/nbilyk/32/1483_2.png) [@nbilyk](https://discuss.kotlinlang.org/u/nbilyk)\
**Post date:** [November 30, 2018, 5:32pm UTC](https://discuss.kotlinlang.org/t/property-with-different-public-signature-than-private/10590/1 "2018-11-30T17:32:15Z")

</div>

A situation I hit quite frequently that I think has the potential to be prettier is that I often have a property that I want to expose as public with a different type than how I use it as private or protected.

How I do this now:

```
private val _foo: MutableList<Float> = ArrayList()
val foo: List<Float>
  get() = _foo

```

It’s not that that code is all that ugly, it’s that this is such a common case where I want to expose a property in an immutable and more general way than how it’s used internally that I have examples of that absolutely everywhere in my codebase and if that were two lines instead of 3 it would make my life 33% happier 🙂

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**Author:** ![Wasabi375](https://sea1.discourse-cdn.com/flex019/user_avatar/discuss.kotlinlang.org/wasabi375/32/4741_2.png) [@Wasabi375](https://discuss.kotlinlang.org/u/Wasabi375)\
**Post date:** [November 30, 2018, 5:36pm UTC](https://discuss.kotlinlang.org/t/property-with-different-public-signature-than-private/10590/2 "2018-11-30T17:36:58Z")

</div>

There is a KEEP about this:

> <https://github.com/matejdro/KEEP/blob/private_public_property_types/proposals/private_public_property_types.md>

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**Author:** ![nbilyk](https://sea1.discourse-cdn.com/flex019/user_avatar/discuss.kotlinlang.org/nbilyk/32/1483_2.png) [@nbilyk](https://discuss.kotlinlang.org/u/nbilyk)\
**Post date:** [November 30, 2018, 7:15pm UTC](https://discuss.kotlinlang.org/t/property-with-different-public-signature-than-private/10590/3 "2018-11-30T19:15:39Z")

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Beautiful, thanks!  
Reading the KEEP, that’s exactly what I wanted 🙂

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<div class="post-metadata">

**Author:** ![Wasabi375](https://sea1.discourse-cdn.com/flex019/user_avatar/discuss.kotlinlang.org/wasabi375/32/4741_2.png) [@Wasabi375](https://discuss.kotlinlang.org/u/Wasabi375)\
**Post date:** [November 30, 2018, 7:29pm UTC](https://discuss.kotlinlang.org/t/property-with-different-public-signature-than-private/10590/4 "2018-11-30T19:29:34Z")

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Let’s hope it will be part of Kotlin at some time.

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**Author:** ![jacobz\_20](https://sea1.discourse-cdn.com/flex019/user_avatar/discuss.kotlinlang.org/jacobz_20/32/1468_2.png) [@jacobz\_20](https://discuss.kotlinlang.org/u/jacobz_20)\
**Post date:** [December 5, 2018, 7:26pm UTC](https://discuss.kotlinlang.org/t/property-with-different-public-signature-than-private/10590/5 "2018-12-05T19:26:15Z")

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Have you looked into Delegated Properties? I think you could create either a universal one to cover all similar cases, or create one specifically around lists. I’ll try to remember to try it out when I get time and update this.

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<div class="post-metadata">

**Author:** ![nbilyk](https://sea1.discourse-cdn.com/flex019/user_avatar/discuss.kotlinlang.org/nbilyk/32/1483_2.png) [@nbilyk](https://discuss.kotlinlang.org/u/nbilyk)\
**Post date:** [December 5, 2018, 7:28pm UTC](https://discuss.kotlinlang.org/t/property-with-different-public-signature-than-private/10590/6 "2018-12-05T19:28:51Z")

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Yeah, but that doesn’t change the member type. You could access the delegate and grab the mutable property it wraps, but that wouldn’t really be any cleaner. As far as I know a property delegate can’t split visibility.

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<div class="post-metadata">

**Author:** ![jacobz\_20](https://sea1.discourse-cdn.com/flex019/user_avatar/discuss.kotlinlang.org/jacobz_20/32/1468_2.png) [@jacobz\_20](https://discuss.kotlinlang.org/u/jacobz_20)\
**Post date:** [December 5, 2018, 7:41pm UTC](https://discuss.kotlinlang.org/t/property-with-different-public-signature-than-private/10590/7 "2018-12-05T19:41:20Z")

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Right, you’d still have the private one. The delegated property is on the public one and works with the private one.  
That’s the part I have to check whether it works.

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<div class="post-metadata">

**Author:** ![jacobz\_20](https://sea1.discourse-cdn.com/flex019/user_avatar/discuss.kotlinlang.org/jacobz_20/32/1468_2.png) [@jacobz\_20](https://discuss.kotlinlang.org/u/jacobz_20)\
**Post date:** [December 6, 2018, 2:53pm UTC](https://discuss.kotlinlang.org/t/property-with-different-public-signature-than-private/10590/8 "2018-12-06T14:53:30Z")

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like this:

```
class Redirect<T>(val source: KMutableProperty<out List<T>>){
    operator fun getValue(thisRef: Any?, property: KProperty<*>): List<T> {
        return source.getter.call(thisRef)
    }

    operator fun setValue(thisRef: Any?, property: KProperty<*>, value: List<T>) {
        source.setter.call(thisRef, ArrayList<T>(value))
    }
}

class User (list: MutableList<Int>) {
    private var _list = list
    public val outList: List<Int> by Redirect(User::_list)
}

```

I tried this on [try.kotlinlang.org](http://try.kotlinlang.org) and it compiled fine, but it failed with an exception:

> Exception in thread “main” java.lang.IllegalStateException: No BuiltInsLoader implementation was found. Please ensure that the META-INF/services/ is not stripped from your application and that the Java virtual machine is not running under a security manager

which makes it sound like maybe the site doesn’t have the full reflection access. Try it out and let me know. I won’t likely get a chance to try it out properly on IntelliJ until Monday

Here’s a “universal” one, btw:

```
class Redirect <Private, Public> (
    val source: KMutableProperty<Private>,
    val transUp: (Private) -> Public,
    val transDown: (Public) -> Private) {

    operator fun getValue(thisRef: Any?, property: KProperty<*>): Public {
        return transUp(source.getter.call(thisRef))
    }

    operator fun setValue(thisRef: Any?, property: KProperty<*>, value: Public) {
        source.setter.call(thisRef, transDown(value))
    }
}

```

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<div class="post-metadata">

**Author:** ![jacobz\_20](https://sea1.discourse-cdn.com/flex019/user_avatar/discuss.kotlinlang.org/jacobz_20/32/1468_2.png) [@jacobz\_20](https://discuss.kotlinlang.org/u/jacobz_20)\
**Post date:** [December 6, 2018, 2:57pm UTC](https://discuss.kotlinlang.org/t/property-with-different-public-signature-than-private/10590/9 "2018-12-06T14:57:12Z")

</div>

And obviously, you can make a read-only version so that the `transDown` argument isn’t needed. Or you can make it nullable with a default to `null` and hope people aren’t dummies.

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<div class="post-metadata">

**Author:** ![nbilyk](https://sea1.discourse-cdn.com/flex019/user_avatar/discuss.kotlinlang.org/nbilyk/32/1483_2.png) [@nbilyk](https://discuss.kotlinlang.org/u/nbilyk)\
**Post date:** [December 7, 2018, 3:10am UTC](https://discuss.kotlinlang.org/t/property-with-different-public-signature-than-private/10590/10 "2018-12-07T03:10:13Z")

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Sorry, I’m not really following you.  
Why would you go through all that trouble of the delegate over just doing this:

```
class User (private val mutableList: MutableList<Int>) {
    public val list: List<Int> = mutableList
}

```

The KEEP that wasabi linked was a very succinct discussion of the problem, without such a feature however the next best thing is simply two properties pointing to the same value but with different access.

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<div class="post-metadata">

**Author:** ![jacobz\_20](https://sea1.discourse-cdn.com/flex019/user_avatar/discuss.kotlinlang.org/jacobz_20/32/1468_2.png) [@jacobz\_20](https://discuss.kotlinlang.org/u/jacobz_20)\
**Post date:** [December 7, 2018, 3:21am UTC](https://discuss.kotlinlang.org/t/property-with-different-public-signature-than-private/10590/11 "2018-12-07T03:21:16Z")

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Except your `User` class here doesn’t make it possible to make the private property be a `var`. If you used a var instead of a val and reassigned the private one, it wouldn’t change what the public one returns.

What you’re asking for is a way to define the public property without having to rewrite the getter (and possibly setter) every time. Well, that’s exactly what the Delegated Property does.

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<div class="post-metadata">

**Author:** ![jacobz\_20](https://sea1.discourse-cdn.com/flex019/user_avatar/discuss.kotlinlang.org/jacobz_20/32/1468_2.png) [@jacobz\_20](https://discuss.kotlinlang.org/u/jacobz_20)\
**Post date:** [December 7, 2018, 3:26am UTC](https://discuss.kotlinlang.org/t/property-with-different-public-signature-than-private/10590/12 "2018-12-07T03:26:31Z")

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Alas, it doesn’t work because reflection didn’t let me get around it being private.

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<div class="post-metadata">

**Author:** ![HughG](https://avatars.discourse-cdn.com/v4/letter/h/35a633/32.png) [@HughG](https://discuss.kotlinlang.org/u/HughG)\
**Post date:** [September 15, 2019, 4:13pm UTC](https://discuss.kotlinlang.org/t/property-with-different-public-signature-than-private/10590/13 "2019-09-15T16:13:03Z")

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> [@jacobz\_20](#):
>
> If you used a var instead of a val and reassigned the private one, it wouldn’t change what the public one returns.

That’s true for the example you’re replying to, but not for the OP’s, because that used `get() =`, so will always return the current value of `_foo`. Hmm … I wonder if such getters can be made inline. I’m too lazy / busy to try just now 😉