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How does differentiate between Serialized data class and normal Serialized class?

While writing code for RecyclerView to get data I figured out there's a data class in Kotlin.
Following codes are taken from two different projects which are linked above.
#Serializable
data class MarsPhoto(
val id: String,
#SerialName(value = "img_src")
val imgSrc: String
)
class Contacts {
#SerializedName("country")
private val country:String? = null
fun getCountry():String?{
return country
}
}
I know that both classes are doing same job. So what does differentiate them? I also wonder in the MarsPhoto data class how they can get the id without declaring SerialName just the way they did for imgSrc. (I am just on the way to learning kotlin now, so I'm absolute beginner).

Basically for "data" class the compiler automatically derives the following members from all properties declared in the primary constructor:
equals()/hashCode() pair
toString() of the form "MarsPhoto(id=1, imgSrc=asdf)"
componentN() functions corresponding to the properties in their order of declaration.
copy()
You can read a lot more at enter link description here
On the SerializedName part of your question. if you are dealing with Gson lib by default it is using fields name as "SerializedName". And only if you want to use something different then field name, you can use SerializedName annotation and pass your custom value there. But usually, everybody just writes #SerializedName() with duplication of field names as value for every field.
It's a good idea if you are receiving and Serializing data from server from Json. Because Backend developers can use a bad keys in response, which you don't want to use in your code, so #SerializedName will be the only place where you will have to see this key, and you can name your fields however you like.

#Serializable used to mark class as serializable to disk or like into a file( alternative is Parcel able in android) special useful in case of process death or configuration changes and #SerializedName("country") used for json parsing when u receive the response from server

You get the id without #SerializedName because the JSON property field is the same as your variable name, but imgSrc and img_src is not. Still, even if they are the same, you should always use #SerializedName, because your variable names could be converted to random letters during code optimization, and obfuscation.

Moshi - Not enough information to infer type variable T

I have an app which is mixed Java and Kotlin.
In the Kotlin code I use Moshi to convert an object to Json in a convertor for a Room database table.
I have one case that works perfectly but another one produces the error:
Not enough information to infer type variable T
This is what my code looks like:
val type: Type = Types.newParameterizedType(
MutableMap::class.java,
LayerTwoConn::class.java,
TWeFiState::class.java,
WfMeasureFileMgr::class.java,
Traffic::class.java,
ThroughputCalculator::class.java,
CellSubTechThroughput::class.java,
LongValuesAverageCalculator::class.java,
LayerTwoConn.SenselessTraffic::class.java
)
val json = Moshi.Builder().build().adapter(type).toJson(layerTwoConn)
I have included all the classes that are used in the objects.
What have I missed?
This case works perfectly:
val type: Type = Types.newParameterizedType(
MutableList::class.java,
CnrScan::class.java,
)
val jsonAdapter: JsonAdapter<List<CnrScan>> = Moshi.Builder().build().adapter(type)
val json = jsonAdapter.toJson(list)
In this object, all the internally used classes are standard Java class and not my own.
Have I missed something simple?
I don't know if this is important but the class LayerTwoConn's constructor is private.

I think you are trying to convert too many classes into one type, try to convert MutableMap class and LayerTwoConn class.
Do note that Room uses SQL architecture, so try to predict what you want your table to contain

android Room with kotlin value class?

I'm trying to use a room entity with a value class:
#JvmInline
value class UserToken(val token: String)
and the entity:
#Entity(tableName = TABLE_AUTH_TOKEN)
data class TokenEntity(
#PrimaryKey val id: Int = 0,
val token: UserToken
)
I get the following error:
error: Entities and POJOs must have a usable public constructor. You can have an empty constructor or a constructor whose parameters match the fields (by name and type).
public final class TokenEntity {
^
is it even possible to use room with value class? I couldn't find anything about this. thanks

See the comment from #CommonsWare. Android does not yet support value classes for Room.
The same holds true for the value classes introduced in kotlin 1.5. The type is not supported.
— Support Inline class in Room entity
Here is a possible explanation according to Kotlin Inline Classes in an Android World.
Looking to solve this you could try and add a TypeConverter for your Inline class, but since your Inline class is just the value it wraps when it’s compiled, this doesn’t make much sense and it doesn’t work as you’d expect even if you tried...
I’m just guessing it’s because this is a TypeConverter converting UserId to Int which is basically the same as Int to Int 😭. Someone will probably solve this problem, but if you have to create a TypeConverter for your Inline class then you are still plus one class for the count (multidex). 👎

I think yes if you can provide a type converter for it to change it to some sort of primitive data type (int , string, long ...etc) when it needs to be stored, and to change it back to its class type when it's fetched from database.
You can read about Type Converters from here
Referencing complex data using Room
other than that, your other class should be an entity and bind both your entities together using a Relation.
at least that's what I know about how to use Room.

UserToken always will have only one attribute? In this case, you don't need two classes, just use token: String directly on your entity class;
If you really need keep this class, you have two options:
TypeConverter, where you basically will convert the object into a json, and save as string in the database;
Relation, where you will transform the UserToken in a entity, and on TokenEntity save the tokenId.

Best way to integrate MoShi and Retrofit 2 in an Android Studio project

I would like to know what is the best way to integrate Retrofit with MoShi on my Android Studio project.
First of all, I use the moshi converter of retrofit :
implementation 'com.squareup.retrofit2:converter-moshi:2.9.0'
I use the popular plugin "Json to Kotlin class" for the generation of my POJO :
https://plugins.jetbrains.com/plugin/9960-json-to-kotlin-class-jsontokotlinclass-
When I create a new POJO, I use the annotation "MoShi (Reflect)" :
It generate me a Pojo with fields like this one :
#Json(name = "image_url")
val imageUrl: String?
The problem is Retrofit2 seem don't like underscore in my fields name because I get null results with the underscored names.
Searching in the Github issues, I found a solution. It work if I use this annotation :
#field:Json(name = "image_url")
val imageUrl: String?
So my questions are :
Using the #field:Json annotation is the best way to use MoShi with Retrofit ?
If yes, how to generate easily a Kotlin class with this annotation (using the same plugin or an other) ?

As apparent from this part of Moshi's documentation: "There’s no field naming strategy, versioning, instance creators, or long serialization policy. Instead of naming a field visibleCards and using a policy class to convert that to visible_cards, Moshi wants you to just name the field visible_cards as it appears in the JSON."
The preferred way is for you to not use the annotation, but instead name the fields the same way as in the JSON. With Kotlin you can use backticks for names that wouldn't otherwise be valid, e.g.val `image-url`: String?
Of course you wouldn't want to be working with such names in other parts of your code, and for that you should have separate objects for describing the data as it appears on the back-end, and another object for how it should appear in your application. We call those DTOs (Data-Transfer Objects.)

Unexpected behavior in Kotlin Parcelize

I am having trouble in understanding the way #Parcelize working in Kotlin. According to documentations
only the primary constructor properties will be serialized.
But when I serialize and deserialize classes with empty primary contractors, it is still serializing and deserializing all the fields. For example, below class
#Parcelize
class Node(): Parcelable {
var field: String? = null
}
As primary contractor doesn't have any field, according to documentations I should have field = null always after ser/des. But whenever I run below codes
val before = Node()
before.field = "someField"
val bundle = Bundle().apply{ putParcelable("someKey", before) }
val after = bundle.getParcelable<Node>("someKey")
field is successfully serialized and deserialized and will have value of someField.
Am I missing something or did Parcelize got updated but they didn't update documentation?
By the way if I leave Node declaration as above, Android Studio gives me warning that field will not be serialized into Parcel. But it is.

You should define it in the constructor itself and it will work just fine.
#Parcelize
class Node( var field: String? = null) : Parcelable
And to use empty constructors in kotlin you can add this in app gradle file.
apply plugin: 'kotlin-noarg'
With this you can use classes with empty constructors.
I hope this helps.