Since we are talking about Gson here I have a different ques

Question

Since we are talking about Gson here I have a different question My data class has member properties that has default values e g ```data class Person val name String val age Int val desc String = ``` When cloud JSON via Retrofit and Gson comes back w null value for `desc` field the resultant `Person` instance will have null value for `desc` field instead of empty string So the non null field `desc` can have null value I searched online for some solutions but they all seem hacky anyone has any good suggestion

maxmello · Answer

Gson is currently Java only so it has no knowledge about Kotlin default values and uses Java s reflection to set the values Kotlin on the other hand won t do any checking on the actual value of desc until you call desc assuming it will be non nullable and get a NPE There are workarounds with custom getters in Gson but I would recommend using a library written in Kotlin so Moshi would be a good choice

Hong · Answer

Thank you Max Will give it a try

gildor · Answer

Moshi is not written in Kotlin

gildor · Answer

Moshi just has additional library adapter for Kotlin

gildor · Answer

You can use Gson with default arguments but you must have default constructor to do that you can add default arguments to all constructor params

gildor · Answer

gt uses Java s reflection to set the values Moshi and Jackson everyone use Java reflections by default moshi kotlin also uses reflections but Kotlin reflections but also provides an option to generate adapters using kapt

Hong · Answer

Thanks Andrey Will give that a try first It s just the class I have has a lot member properties lol

gildor · Answer

In you case if you do this `data class Person val name String = val age Int = 1 val desc String = ` Kotlin will generate default constuctor and Gson will use this constructor instead of sun misc Unsafe to create an instance of the object and then set values from Json using reflection

gildor · Answer

But be careful with nullable values if Json has ` desc null` Gson will overwrite your default empty string with null and your program will crash on access to this property

Hong · Answer

Yes that s exactly the problem I m trying to solve here `desc` can have null value which will lead to runtime NPE

gildor · Answer

So you cannot use default string in this case

gildor · Answer

if you Json doesn t contain desc field everything will be fine

Hong · Answer

slightly smiling face yes this is the corner case I m trying to resolve

gildor · Answer

so the only safe option in this casse `val desc String = `

Hong · Answer

okay got it thanks

maxmello · Answer

gt Moshi just has additional library adapter for Kotlin Yeah sorry thats what I meant even though its not what I wrote I didn t actually know that the default values work when every property has a default but the part about passing in null is pretty risk in my opinion unless you are in full control of the API you call and can make sure nulls are never included in a JSON

gildor · Answer

gt make sure nulls are never included in a JSON Yes if you not sure about API you always can use nullable values Kotlin provides a lot of tools for that

gildor · Answer

But again I agree Gson and other libraries without direct Kotlin support are not so safe and pleasant to use but still useful For example Moshi has support of Kotlin but I don t want to include huge kotlin reflect because of this or even generate 1 additional adapter class for each of my data classes It s just a trade off So for now pure Moshi or Gson is fine for my and I suppose not only my case but I would like to use kotlinx serialization at some point sounds like a best approach in global perspective

zone · Answer

gildor · Answer

This is a set of DSLs to make Geon usage more idiomatic but as I understand it changes nothing about nullability or default arguments