I have ``` Serializable data class Outer val innerName Inner

Question

I have ``` Serializable data class Outer val innerName Inner ``` and then I have an instance of this Outer class How can I do something like this `outer innerName getSerializedName ` Basically I need to save the name of the field onto disk I can do `outer innerName class java simpleName` but that gives me Inner instead of what I want which is innerName

Casey Brooks · Answer

`innerName` is a property of the `Outer` class not `Inner` So you need to use reflection on `Outer class` instead `outer innerName` is a reference to an instance of `Inner` so `outer innerName class java simpleName` gives you the class name for `Inner` For example ```Outer class java declaredFields map it name innerName ``` Note that this gives you the Java field name for that property not necessarily the serial name If you re not using any obfuscation or ` SerialName` annotations they should be the same but it s not really guaranteed If you do need the actual serial name you ll probably need to write a <https github com Kotlin kotlinx serialization blob master docs formats md custom formats experimental|custom format> for persisting that data to disk which also has the benefit of working on other targets besides JVM

ephemient · Answer

```Outer serializer descriptor getElementName 0 ```

Colton Idle · Answer

Using descriptor means I m using reflection right

ephemient · Answer

it s not reflection serialization descriptors are generated at compile time

Colton Idle · Answer

gotcha okay GetElementName 0 made me think that its reflection based or something Wonder if there s anyway to do it without needing the magic number of 0

Colton Idle · Answer

this example is slimmed down from my actual example where I have ``` Serializable data class Outer val innerName Inner val fooName Foo val barName Bar ``` and I m just trying to write a function to get the left side of the ` `

ephemient · Answer

```for i in 0 until descriptor getElementCount if descriptor getElementDescriptor i == Inner serializer descriptor return descriptor getElementName i ``` but it doesn t provide a mapping between serial name and property name if that s what you re looking for

ephemient · Answer

any reason why

Casey Brooks · Answer

Reflection is specifically the ability of a program to introspect itself at runtime especially with the ability to change itself at runtime It s a form of metaprogramming which is a program that understands its own structure Kotlinx serialization is a form of metaprogramming but since it does that by generating code at compile time it s not reflection

ephemient · Answer

yeah the line is a little fuzzy but in general on the JVM reflection refers to what is provided by the platform plus what is built on top of it such as kotlin reflect while kotlinx serialization doesn t require any runtime support except for extensions like `fun lt T gt KClass lt T gt serializer `

ephemient · Answer

going a bit off topic the line can be a little fuzzy when it comes to things like <https doc qt io qt 5 metaobjects html> where it s the reflection runtime that Qt uses isn t part of C++ itself but comes from Qt but it s pretty well defined in Java