Does Kotlin '?:' works when data class field/property is not null but Gson set it null?

The code bellow shows gson.fromJson() will ignore kotlin data class not null.

data class Test(val txt:String)

val gson=Gson()
val testClass=gson.fromJson("{\"txt\":null}",Test::class.java)

The testClass in the code will get a null txt property. It can be understood, because the null check is happened in compile.

But what if val txtNotNull=testClass.txt?:"not null"?

Is it a bad code style(undefined behavior), or is it unwise to use gson in Kotlin?

Gson uses reflection to set properties to the data class. That's why Kotlin checks aren't run during object construction. And then, you can get null into not-null type in Kotlin.

Your code will work, but you will get a warning from the compiler:
> Elvis operator (?:) always returns the left operand of non-nullable type String

There is a cool official library kotlinx.serialization which is written for use in Kotlin.

Gson doesn't know about kotlin's non-null types.

You can create you own TypeAdapterFactory to throw an exception if any of the values in json (which should be not null in Kotlin object) is null.

See this post :

https://medium.com/swlh/using-gson-with-kotlins-non-null-types-468b1c66bd8b

The fact that Gson is sneaking a null value in there is not the problem. The problem is that you’re marking a property as non-nullable that should be nullable because it can be omitted in the JSON that you’re importing. You should make the property nullable, and then it will actually make sense if you use something like val txtNotNull=testClass.txt?:"not null".