How to have a constant computed cached value in an interface

Question

How to have a constant computed cached value in an interface Well that s not my use case directly and I know that interface properties cannot have backing fields Also I remember seeing a hackish workaround for that My intention actually is to have this computed constant in an enum class inhering from that interface Currently I have this ``` interface CellMapping val sheetAddress String val cellAddress CellAddress get = CellAddress sheetAddress private enum class Cells override val sheetAddress String override val sheetType SheetType CellMapping``` This works but the CellAddress is constructed every time the `cellAddress` prop is accessed Is there any way to avoid that

Casey Brooks · Answer

Unfortunately no Interfaces cannot have any backing properties and the only way to cache that value would be for the interface itself to have a property to hold the cached value You ll need to manually override `cellAddress` Typically you would address this by using an abstract or open class instead of interface but enums cannot extend other classes so that doesn t work here

David Kubecka · Answer

gt enums cannot extend other classes Yeah forgot to add this known detail Btw what is the reason for that

dmitriy.novozhilov · Answer

It s because of JVM specification On JVM each enum class should be inherited from `java langEnum` and it s not allowed for class to have several super classes

Adam S · Answer

maybe use a sealed interface with regular classes as subtypes instead of an enum

ephemient · Answer

```enum class Cells override val sheetAddress String CellMapping override val cellAddress by lazy CellAddress sheetAddress ```

Casey Brooks · Answer

Depending on how often you need to implement the interface and what s in it it may be worth setting up a delegate It can add a fair amount of boilerplate but for many computed properties or where it s implemented by many different classes it can save you a lot more boilerplate and help make sure things are done consistently while also allowing individual classes to override if it needed ```interface CellMapping val sheetAddress String val cellAddress CellAddress class CellMappingDelegate override val sheetAddress String CellMapping override val cellAddress CellAddress = CellAddress sheetAddress private enum class Cells sheetAddress String CellMapping by CellMappingDelegate sheetAddress ```

David Kubecka · Answer

This is a nice workaround I will need to think about it more though whether the introduced boilerplate is worth it