Kotlin object deserialization with GSON

Question

I have a class

class ThreadComment(
    banned: Int,
    closed: Int,
    comment: String?,
    date: String?,
    email: String?,
    files: ArrayList<File>?,
    lasthit: Int,
    name: String?,
    num: String?,
    op: Int,
    parent: String?,
    postsCount: Int,
    sticky: Int,
    subject: String?,
    tags: String?,
    timestamp: Int,
    trip: String?) : Comment(banned, closed, comment, date, email, files, lasthit, name, num, op, parent, postsCount, sticky, subject, tags, timestamp, trip) {

val answers: ArrayList<String> = ArrayList()}

Parent class looks like

open class Comment(
    @com.google.gson.annotations.SerializedName("banned")
    val banned: Int = 0,

    @com.google.gson.annotations.SerializedName("closed")
    val closed: Int = 0,

    @com.google.gson.annotations.SerializedName("comment")
    val comment: String? = null,

    @com.google.gson.annotations.SerializedName("date")
    val date: String? = null,

    @com.google.gson.annotations.SerializedName("email")
    val email: String? = null,

    @com.google.gson.annotations.SerializedName("files")
    val files: ArrayList<File>? = null,

    @com.google.gson.annotations.SerializedName("lasthit")
    val lasthit: Int = 0,

    @com.google.gson.annotations.SerializedName("name")
    val name: String? = null,

    @com.google.gson.annotations.SerializedName("num")
    val num: String? = null,

    @com.google.gson.annotations.SerializedName("op")
    val op: Int = 0,

    @com.google.gson.annotations.SerializedName("parent")
    val parent: String? = null,

    @com.google.gson.annotations.SerializedName("posts_count")
    val postsCount: Int = 0,

    @com.google.gson.annotations.SerializedName("sticky")
    val sticky: Int = 0,

    @com.google.gson.annotations.SerializedName("subject")
    val subject: String? = null,

    @com.google.gson.annotations.SerializedName("tags")
    val tags: String? = null,

    @com.google.gson.annotations.SerializedName("timestamp")
    val timestamp: Int = 0,

    @com.google.gson.annotations.SerializedName("trip")
    val trip: String? = null) : Serializable

But after deserialization with GSON answers field is null.

I also tried to put initiation into init{}, but it still null, by lazy throws an exception about calling lazy.getValue on a null object reference

@miensol json objects don't contain field "answers". But I also triyed to use ExclusionStrategy for serialization and deserialization to exclude this field. — Ufkoku, Sep 29 '16 at 21:45
A side note: Jackson (JSON deser) has Kotlin support for dealing with Kotlin constructors and use cases via https://github.com/FasterXML/jackson-module-kotlin ... for other Java libraries you should check for similar because they do not understand all concepts Kotlin nor take advantage of them. You will end up doing unnatural Kotlin (i.e. the answer from @miensol) just to satisfy your deser library. unfortuneatly GSON is not extensible in a way that allows it to be friendlier to Kotlin. — Jayson Minard, Sep 30 '16 at 11:23

score 9 · Answer 1 · answered Feb 28 '17 at 18:05

9

That´s weird, I just used @SerializedName normally as in Java with a data class and it worked:

data class(@SerializedName("my_string_value") val myStringValue: String, @SerializedName("my_int_value") val myIntValue: Int)

answered Feb 28 '17 at 18:05

Jesus Almaral - Hackaprende

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score 8 · Accepted Answer · edited May 23 '17 at 12:18

As described in this answer Gson can unsafely handle no-args constructor scenarios. It does so by leveraging UnsafeAllocator that in turn uses allocateInstanсe(Class p). This means that when there is no no-args constructor and there is no custom instance creator registered the object created by Gson will have fields initialized with their declared types default values.

To resolve that either add a no-arg constructor i.e

private constructor() : this(-1, -1, "", "", "", null, -1, "", "", -1, null, -1, -1, null, null, -1, null)

Or add default parameter values to your primary constructor like so:

class ThreadComment(
        banned: Int = -1,
        closed: Int = -1,
        comment: String? = null,
        date: String? = null,
        email: String? = null,
        files: ArrayList<File>? = null,
        lasthit: Int = -1,
        name: String? = null,
        num: String? = null,
        op: Int = -1,
        parent: String? = null,
        postsCount: Int = -1,
        sticky: Int = -1,
        subject: String? = null,
        tags: String? = null,
        timestamp: Int = -1,
        trip: String? = null
) : Comment(banned, closed, comment, date, email, files, lasthit, name, num, op, parent, postsCount, sticky, subject, tags, timestamp, trip) {
    val answers: ArrayList<String> = ArrayList()
}

Kotlin object deserialization with GSON

2 Answers2