Kotlin not nullable value can be null?

Question

I have backend that return me some json.

I parse it to my class:

class SomeData(
  @SerializedName("user_name") val name: String,
  @SerializedName("user_city") val city: String,
  var notNullableValue: String
)

Use gson converter factory:

Retrofit retrofit = new Retrofit.Builder()
  .baseUrl(ENDPOINT)
  .client(okHttpClient)
  .addConverterFactory(GsonConverterFactory.create(gson))
  .addCallAdapterFactory(RxJava2CallAdapterFactory.create())
  .build();

And in my interface:

interface MyAPI {
    @GET("get_data")
    Observable<List<SomeData>> getSomeData();
}

Then I retrieve data from the server (with rxJava) without any error. But I expected an error because I thought I should do something like this (to prevent GSON converter error, because notNullableValue is not present in my JSON response):

class SomeData @JvmOverloads constructor(
  @SerializedName("user_name") val name: String,
  @SerializedName("user_city") val city: String,
  var notNullableValue: String = ""
)

After the data is received from backend and parsed to my SomeData class with constructor without def value, the value of the notNullableValue == null.

As I understand not nullable value can be null in Kotlin?

Please see if this helps: https://stackoverflow.com/a/39963353/2196460 — hotkey, Aug 21 '17 at 09:29
Its explain situation with def value in constructor. But in my situation I don have def value. — bitvale, Aug 21 '17 at 09:51
the `notNullableValue` was changed by gson after created. because it is `var` not `val`. — holi-java, Aug 21 '17 at 09:57
Yes, I'll find this with val, than change val to var and get the same behavior. — bitvale, Aug 21 '17 at 10:06
Can you please provide a minimum verifiable example of the problem and post it somewhere on github? — Strelok, Aug 21 '17 at 12:10

Strelok · Accepted Answer · 2017-08-21T12:40:58.303

17

Yes, that is because you're giving it a default value. Ofcourse it will never be null. That's the whole point of a default value.

Remove ="" from constructor and you will get an error.

Edit: Found the issue. GSON uses the magic sun.misc.Unsafe class which has an allocateInstance method which is obviously considered very unsafe because what it does is skip initialization (constructors/field initializers and the like) and security checks. So there is your answer why a Kotlin non-nullable field can be null. Offending code is in com/google/gson/internal/ConstructorConstructor.java:223

Some interesting details about the Unsafe class: http://mishadoff.com/blog/java-magic-part-4-sun-dot-misc-dot-unsafe/

edited Aug 21 '17 at 12:40

answered Aug 21 '17 at 09:30

Strelok

50,229
9
102
115

No, as You read correct all works fine without ="". And notNullableValue == null – bitvale Aug 21 '17 at 09:48
I am sorry for misunderstanding, I am edit question. – bitvale Aug 21 '17 at 09:54
@jQuick I updated my answer. TLDR: Gson is using unsafe java magic classes. – Strelok Aug 21 '17 at 12:41
1

If you're unhappy with Gson behaviour of a non-nullable having a null value and prefer default value instead, you can use Jackson-kotlin-module. Refer: https://stackoverflow.com/questions/38426579/in-kotlin-jackson-deserialization-error-with-data-class – ChinLoong Apr 30 '18 at 09:33
This is not only affecting the non-nullable type, but also, all of the properties that are initiated in the class body instead of primary constructor will not be initialized, aka null. – katie Aug 15 '18 at 22:25

score 4 · Answer 2 · answered Nov 24 '17 at 15:40

4

Try to override constructor like this:

class SomeData(
        @SerializedName("user_name") val name: String,
        @SerializedName("user_city") val city: String,
        var notNullableValue: String = "") {
    constructor() : this("","","")
}

Now after server response you can check the notNullableValue is not null - its empty

answered Nov 24 '17 at 15:40

Yurii

552
3
14

This method not only solves the problem but also let you set the default value for the parameters. – Sanlok Lee Feb 18 '19 at 04:55

Kotlin not nullable value can be null?

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