Kotlin data class create dynamically json of its fields using GSON

Question

I have a data class like this:

data class TestModel(
     val id: Int, 
     val description: String, 
     val picture: String)

If I create JSON from this data class using GSON and it generates a result like this

{"id":1,"description":"Test", "picture": "picturePath"}

What to do if I need the following JSON from my data class:

{"id":1, "description":"Test"}

And other times:

`{"id":1, "picture": "picturePath"}

` Thanks in advance!

given your use of the GSON tag, I added GSON to the title and text to make it clear that this was a GSON specific question. — Jayson Minard
– Jayson Minard, Commented Nov 27, 2018 at 14:13

SBotirov · Accepted Answer · 2018-11-27 16:33:56Z

You can solve this problem with writing custom adapter and with optional types:

import com.google.gson.Gson
import com.google.gson.GsonBuilder
import com.google.gson.TypeAdapter
import com.google.gson.stream.JsonReader
import com.google.gson.stream.JsonToken
import com.google.gson.stream.JsonWriter

data class TestModel(
    val id: Int,
    val description: String? = "",
    val picture: String? = "")

class TesModelTypeAdapter : TypeAdapter<TestModel>() {
    override fun read(reader: JsonReader?): TestModel {
        var id: Int? = null
        var picture: String? = null
        var description: String? = null

        reader?.beginObject()
        while (reader?.hasNext() == true) {
            val name = reader.nextName()

            if (reader.peek() == JsonToken.NULL) {
                reader.nextNull()
                continue
            }

            when (name) {
                "id" -> id = reader.nextInt()
                "picture" -> picture = reader.nextString()
                "description" -> description = reader.nextString()
            }
        }
        reader?.endObject()

        return when {
            !picture.isNullOrBlank() && description.isNullOrBlank() -> TestModel(id = id ?: 0, picture = picture)
            !description.isNullOrBlank() && picture.isNullOrBlank() -> TestModel(id = id ?: 0, description = description)
            else -> TestModel(id ?: 0, picture, description)
        }
    }

    override fun write(out: JsonWriter?, value: TestModel?) {
        out?.apply {
            beginObject()

            value?.let {
                when {
                    !it.picture.isNullOrBlank() && it.description.isNullOrBlank() -> {
                        name("id").value(it.id)
                    name("picture").value(it.picture)
                    }
                    !it.description.isNullOrBlank() && it.picture.isNullOrBlank() -> {
                        name("id").value(it.id)
                    name("description").value(it.description)
                    }
                    else -> {
                        name("id").value(it.id)
                        name("picture").value(it.picture)
                    name("description").value(it.description)
                    }
                }
            }

            endObject()
        }
    }
}

class App {
    companion object {
        @JvmStatic fun main(args: Array<String>) {
            val tm = TestModel(12, description = "Hello desc")
            val tm2 = TestModel(23, picture = "https://www.pexels.com/photo/daylight-forest-glossy-lake-443446/")
            val tm3 = TestModel(12, "Hello desc", "https://www.pexels.com/photo/daylight-forest-glossy-lake-443446/")

            val gson = GsonBuilder().registerTypeAdapter(TestModel::class.java, TesModelTypeAdapter()).create()

            System.out.println(gson.toJson(tm))
            System.out.println(gson.toJson(tm2))
            System.out.println(gson.toJson(tm3))
        }
    }
}

nyarian · Accepted Answer · 2018-11-27 13:13:12Z

1

Here is actually a way to ignore fields, that are not marked via @Exposed annotation. In order for this to work, special configuration should be used when instantiating Gson. Here is how you can to this.

Easy way is to mark the field as @Transient. Then it would not be either serialized and deserialized.

answered Nov 27, 2018 at 13:13

nyarian

4,4151 gold badge24 silver badges54 bronze badges

7 Comments

nAkhmedov Over a year ago

I edited my question Expose and Transient are ignoring my field always, i need to create dynamically json

nyarian Over a year ago

Have you considered a variant of manual implementation of JsonSerializer then? static.javadoc.io/com.google.code.gson/gson/2.6.2/com/google/…

nAkhmedov Over a year ago

Thank you for yr quick reply. I think, It is a last chance, but i need easy way from developers who have faced already?

nAkhmedov Over a year ago

For example in Java, i create multiple constructor then generate gson with constructor fields which i wanted.

nyarian Over a year ago

It's the same bytecode received from Kotlin source code after compilation as Java's... Writing secondary constructors didn't help?

|

nAkhmedov · Accepted Answer · 2018-11-28 06:25:03Z

1

I want to give you alternative ways without manually serialization/deserialization.

data class TestModel(
    val id: Int,
    val description: String? = null,
    val picture: String? = null)

When you create json from data class

val params = TestModel(id = 1, description = "custom text")

or

val params = TestModel(id = 1, picture = "picture path")

If one of them field is null of data class GSON skips that field automatically.

answered Nov 28, 2018 at 6:25

nAkhmedov

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