kotlin why multiple elvis operator didn't return default value but null

Question

I have a data class:

data class Example(
    @SerializedName("id")
    val id: String,
    @SerializedName("url")
    val url: String,
    @SerializedName("more")
    val more: String = "",
)

When I use Gson to instantiate Example:

val example1 = Gson().fromJson("{id: 1}", Example::class.java)
val example2 = Gson().fromJson("{id: 2}", Example::class.java)

I got two examples whose url is null, because there is no value for url.

Look at this expression:

val url: String = example1?.url ?: example2?.url ?: "aaaaaaaaaa"
println("url:$url")

I got url:null

But the expression below prints the default value url:aaaaaaaaaa:

val url: String = example1.url ?: example2.url ?: "aaaaaaaaaa"
println("url:$url")

Any body knows why? It is supposed to return the default value.

gson+kotlin tags combined make me cringe every time. Consider kotlinx.serialization or other solution that has actual kotlin support and does not rely on brutal reflection to spawn the objects. — Pawel
– Pawel, Commented Feb 1 at 13:04

Sweeper · Accepted Answer · 2025-02-03 09:22:20Z

2

This appears to be a compiler bug. By inspecting the bytecode, both there are null checks on example1, example1.url, and example2, but not example2.url.

A change as simple as:

val x = example1?.url
val url: String = x ?: example2?.url ?: "aaaaaaaaaa"

produces the expected output.

This is likely due to your declaring url as non-nullable, but Gson setting it to null anyway. If you change url to a nullable type, the code produces the expected output too.

data class Example(
    // these SerializedNames are also redundant
    @SerializedName("id")
    val id: String,
    @SerializedName("url")
    val url: String?,
    @SerializedName("more")
    val more: String = "",
)

The same thing applies to the more property. Gson will not see the = "" default parameter value, and sets more to null if the key does not exist in the JSON. Generally, Gson is not suitable for use with Kotlin. See these related posts: 1, 2.

From this answer, it seems like jackson-module-kotlin can recognise default values like this, so consider using Jackson instead.

edited Feb 3 at 9:22

answered Feb 1 at 12:52

Sweeper

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4 Comments

Garfieldmao Feb 3 at 6:16

Thanks for your answer! But I am still puzzled, According to what you said, expression "val url: String = example1?.url ?: "aaaaaaaaaa"" or "val url: String = example1?.url ?: example2.url ?: "aaaaaaaaaa""should return null, but both of them return default value "aaaaaaaaaa". It seems that there are still some unknown mechanisms

Sweeper Feb 3 at 9:22

@Garfieldmao Right. I think this is likely a compiler bug. Bottom line is, don't use Gson with Kotlin.

Sweeper Feb 3 at 11:13

@Garfieldmao I have reported this as KT-74895.

Garfieldmao Feb 4 at 14:21

Thanks a lot!!! I will also continue to pay attention to this issue!

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