Can the parameter in data class be var in Kotlin? - android

I'm a beginner of Kotlin, I have read some sample code about data class, it seems that the parameter are all val type just like Code A
I need to change some values of data class MSetting, so I design the Code B, could you tell me whether the Code B is good way?
Code A
data class MSetting (
val _id: Long,
val name: String,
val createdDate: Long,
val description: String
)
Code B
data class MSetting (
var _id: Long,
var name: String,
var createdDate: Long,
var description: String
)


it seems that the parameter are all val type...
NO
could you tell me whether the Code B is good way?
The difference between val and var: Properties declared with val can't be updated over time; its just like constants in java. Properties declared with var can be changed overtime.
It totally depends on your requirement. If you need to change properties over time then go for var; val otherwise. You can mix both in a object without any issue.
Read more about properties in Kotlin documentation here https://kotlinlang.org/docs/reference/properties.html


I like to use val as a rule of thumb because it makes data class immutable. Of course I am using var but only if it is necessary. Why it is better to make data class immutable? read this.


The accepted answer is misleading. Yes, you can use var arg in data class constructor. But you should avoid it at all costs because it defeats the purpose of data class.
Consider this example:
data class Text(
val x: String,
var y: Int,
)
fun main() {
val t = Text("4124", 1)
val a = t.hashCode()
t.y = 125
val b = t.hashCode()
println(a)
println(b)
}
The output will be something like:
49532514
49532638
This means, object t cannot be used in Map/Set, because it's hashCode isn't constant which will lead to ghost-objects in these collections.
The proper way to have modifiable field in data class is:
data class Text(
val x: String,
) {
var y: Int
}
This way y isn't included in generated equals, hashCode, toString and problem with Map/Set is avoided.
See kotlin doc for this: https://kotlinlang.org/docs/data-classes.html#properties-declared-in-the-class-body

Related

How do I refer to data class property names for Firestore queries?

I'm using a simple data class Track, then saving it as an object so that fields and values are automatically saved. This is working as intended. However later when I want to query on 1 of those property/field names, I need to provide the String value.
How do I refer to the name of that property in the data class, so that I maintain the "single source of truth" for that value, without hard coding it a second time in the query? Example uses "spotifyId":
data class Track(
val spotifyId: String,
val name: String,
val artist: List<String>,
val duration: String
)
Save a track:
set(trackDocRef, track)
Query for a track:
db.collection("tracks").whereEqualTo("spotifyId", "sdfgsdfswer4543w5yer345").get()
Thank you!

I figured out one way of doing it, not sure how "correct" it is. I used reflection in the TracksContract object to refer to the Track model:
data class Track(
val spotifyId: String,
val name: String,
val artist: List<String>,
val duration: String
)
Single reference to Track model via TracksContract:
object TracksContract {
internal const val COLLECTION_NAME = "Tracks"
object Fields {
val SPOTIFY_ID = Track::spotifyId.name
val NAME = Track::name.name
val ARTIST = Track::artist.name
val DURATION = Track::duration.name
}
}
now if I need to run the query based on that spotifyId, I refer to through the TracksContract:
db.collection(TracksContract.COLLECTION_NAME)
.whereEqualTo(TracksContract.Fields.SPOTIFY_ID, "ID_VALUE")
.get()
Finally if I decide to change the property names in the Track model then the TracksContract will show a compiler error, and I can change the name and refactor references from there if I want to.

Kotlin apply function with data class

I have a data class like this -
data class User(val id: String, val name: String)
I want to use apply scope function to modify the values and return the modified object.
User("1", "Name").apply {
this.id = "2" // Gives me error since val cannot be assigned
}
I do not want to make my data class immutable and also don't want to use copy function. How do I fix this?

You can use copy method with data classes:
val user1 = User("1", "Name")
val user2 = user1.copy(id = "2")

change like this data class User(var id: String, var name: String)

why are variables initialized in data class

According to the official doc, this is the way to declare a data class
data class User(val name: String, val age: Int)
but most cases i see where data class is declared thus,
data class User(var username: String? = "", var email: String? = "", var age: Int = 0)
explanation from the docs: On the JVM, if the generated class needs to have a parameterless constructor, default values for all properties have to be specified
i don't understand, and what are the implications of the different methods
2ndly, what is the best way of writing a data class containing complex object variables such as, ArrayList, Bitmap, Uri

A parameterless constructor would probably only be necessary if you were using the class with some library that generates instances of your class through reflection, such as a serialization library or dependency injection library.
If you put = and some value after a constructor parameter, it allows you to omit that parameter when calling the constructor. The value you put there will be the default value used when the parameter is omitted. This is usually used as a convenience. But if you are using one of the aforementioned libraries, you would also need an empty constructor. By providing defaults for every parameter and annotating the primary constructor with #JvmOverloads, you can satisfy this requirement. It would look like this:
data class User #JvmOverloads constructor(var username: String? = "", var email: String? = "", var age: Int = 0)
If your data class has a lot of properties, you may not want to use #JvmOverloads because the number of constructors it generates is exponentially proportional to the number of properties with defaults. Instead, you can define a secondary constructor like this:
data class User(var username: String?, var email: String?, var age: Int) {
constructor(): this("", "", 0)
}
Your second question is very open-ended and opinion-based, so near-impossible to answer in a helpful way.

Merging lists of different types into List<Any>, but keeping their property

I only started learning Kotlin and I'm not a strong programmer anyway :) So I have this data class to parse a JSON with that looks like this:
data class DataClass(
val crazy: List<Crazy>,
val hardcore: List<Hardcore>,
val jackpot: List<Jackpot>,
val kinky: List<Kinky>,
val light: List<Light>
)
data class Crazy(
val text: String
)
...blah blah rest of data classes that have the same text property
data class Light(
val text: String
)
I need to merge them all in a single var, so I created a list of type Any.
challengesObject = gson.fromJson(fileStream, DataClass::class.java)
var allChallenges: List<Any> = challengesObject.light + challengesObject.crazy + challengesObject.hardcore + challengesObject.kinky
All good so far, it works, but obviously I can't access the text var because they lost their type. How could I go about implementing a solution to merge them into one list and still keep access to the text var?
Thanks in advance!

Cannot find setter for field - using Kotlin with Room database

I'm integrating with the Room persistence library. I have a data class in Kotlin like:
#Entity(tableName = "story")
data class Story (
#PrimaryKey val id: Long,
val by: String,
val descendants: Int,
val score: Int,
val time: Long,
val title: String,
val type: String,
val url: String
)
The #Entity and #PrimaryKey annotations are for the Room library. When I try to build, it is failing with error:
Error:Cannot find setter for field.
Error:Execution failed for task ':app:compileDebugJavaWithJavac'.
> Compilation failed; see the compiler error output for details.
I also tried providing a default constructor:
#Entity(tableName = "story")
data class Story (
#PrimaryKey val id: Long,
val by: String,
val descendants: Int,
val score: Int,
val time: Long,
val title: String,
val type: String,
val url: String
) {
constructor() : this(0, "", 0, 0, 0, "", "", "")
}
But this doesn't work as well. A thing to note is that it works if I convert this Kotlin class into a Java class with getters and setters. Any help is appreciated!

Since your fields are marked with val, they are effectively final and don't have setter fields.
Try switching out the val with var.
You might also need to initialize the fields.
#Entity(tableName = "story")
data class Story (
#PrimaryKey var id: Long? = null,
var by: String = "",
var descendants: Int = 0,
var score: Int = 0,
var time: Long = 0L,
var title: String = "",
var type: String = "",
var url: String = ""
)
EDIT
The above solution is a general fix for this error in Kotlin when using Kotlin with other Java libraries like Hibernate where i've seen this as well. If you want to keep immutability with Room, see some of the other answers which may be more specific to your case.
In some cases immutability with Java libraries is simply not working at all and while making sad developer noises, you have to switch that val for a var unfortunately.

Hey I don't know if everyone know or not, but you can not have column which is starting from is into Room.
For example you can't have like this
#Entity(tableName = "user")
data class User (
#PrimaryKey var id: Long? = null,
var userName: String = "",
var isConnectedToFB: Boolean = false,
)

If you have #Ignore field in the data class constructor you need to move it to class body like this:
#Entity(primaryKeys = ["id"])
data class User(
#field:SerializedName("id")
val id: Int,
#field:SerializedName("name")
val name: String,
#field:SerializedName("age")
val age: Int
) {
#Ignore
val testme: String?
}
All kudos go to marianperca on GitHub: https://github.com/android/architecture-components-samples/issues/421#issuecomment-442763610

There is an issue in room db library java code generation.
I was using optional field isFavorite. It gives me same error then I change my field name to favorite then compiled.
before
var isFavorite: Int? = 0,
after changing working fine
var favorite: Int? = 0,
Thanks

According to https://stackoverflow.com/a/46753804/2914140 if you have an autogenerated primary key, you should write so:
#Entity(tableName = "story")
data class Story (
val by: String,
val descendants: Int,
val score: Int,
val time: Long,
val title: String,
val type: String,
val url: String
) {
#PrimaryKey(autoGenerate = true)
var id: Int = 0
}
Note that #PrimaryKey is written inside the class body and contains modifier var.
If you later want to update a row in a database with different parameters, use these lines:
val newStory = story.copy(by = "new author", title = "new title") // Cannot use "id" in object cloning
newStory.id = story.id
dao.update(newStory)
UPDATE
I still don't use AndroidX, and Room is 'android.arch.persistence.room:runtime:1.1.1'.
You can extend this class from Serializable. But if you want to extend it from Parcelable, you will get a warning (over id variable): Property would not be serialized inro a 'Parcel'. Add '#IgnoredOnParcel' annotation to remove this warning:
Then I moved an id from the body to the constructor. In Kotlin I use #Parcelize to create Parcelable classes:
#Parcelize
#Entity(tableName = "story")
data class Story (
#PrimaryKey(autoGenerate = true)
var id: Int = 0,
val by: String,
val descendants: Int,
val score: Int,
val time: Long,
val title: String,
val type: String,
val url: String
) : Parcelable

Had this error in Java.
You cannot have a column starting with is or is_ in Java.
Try renaming the column.
Another solution:
You either have to pass the field in the constructor and initialize it with the constructor argument, or create a setter for it.
Example:
public MyEntity(String name, ...) {
this.name = name;
...
}
public void setName(String name) {
this.name = name;
}

This error will be thrown if your column starts with Is:
#ColumnInfo(name = "IsHandicapLeague")
#NonNull
var isHandicapLeague: String = "Y"
Add a default set() function to eliminate
fun setIsHandicapLeague(flag:String) {
isHandicapLeague = flag
}

Just make the variables mutable, change val into var for Kotlin, Or private into public for Java

This is a bug and is fixed in Room 2.1.0-alpha01
https://developer.android.com/jetpack/docs/release-notes#october_8_2018
Bug Fixes
Room will now properly use Kotlin’s primary constructor in
data classes avoiding the need to declare the fields as vars.
b/105769985

I've found that another cause of this compilation error can be due to the use of the Room's #Ignore annotation on fields of your entity data class:
#Entity(tableName = "foo")
data class Foo(
// Okay
#PrimaryKey
val id: String,
// Okay
val bar: String,
// Annotation causes compilation error, all fields of data class report
// the "Cannot find setter for field" error when Ignore is present
#Ignore
val causeserror: String
)
The same error also seems to happens when using the #Transient annotation.
I've noticed this issue using version 2.2.2 of Room:
// build.gradle file
dependencies {
...
kapt "androidx.room:room-compiler:2.2.2"
...
}
Hope that helps someone!

You can try to rename id variable to another name. It worked for me ;
var id: Long? = null
to
var workerId: Long? = null
If you have to name as id and you are using retrofit, then you may need to add SerializedName("id")

Another cause of this may be the naming of the field. If you use any of the pre-defined keywords, you will get the same error.
For instance, you can not name your column "is_active".
Reference: http://www.sqlite.org/lang_keywords.html

It seems like Room and Kotlin versions need to be matched. I have same issue with Room 2.3.0 and Kotlin 1.6.10 but it's ok with Kotlin 1.5.20. It looks ok after I updated Room to 2.4.2.
https://youtrack.jetbrains.com/issue/KT-45883
Also there is a possible solution to use #JvmOverloads constructor for better Java compability.

Updating Room library to the latest version 2.4.2 solve the issue

The correct way to fix this issue would be simply updating to Room v2.4.3 or higher.
Workaround
If you're running on an older version of Room, one that uses an old version of the kotlinx-metadata-jvm library which doesn't understand 1.5.x metadata, a simple workaround would be adding the following line to your build.gradle:
kapt "org.jetbrains.kotlinx:kotlinx-metadata-jvm:0.5.0"
Source: https://youtrack.jetbrains.com/issue/KT-45883/KAPT-Cannot-find-setter-for-field-compiling-projects-with-Room-db-breaks-using-150-M2

Just an update if somebody comes across this thread in 2019, after spending hours digging online on why this should work, but it doesn't.
Using val works as expected if you are using the AndroidX version ( androidx.room:room-<any>:2.*) but it doesn't when using the old android.arch.persistence.room:<any>:1.1.1 and it seems that version 2.* wasn't released on this latter repo.
Edit: typos

If you want the val immutability available for your entity, it is possible.
You should update to AndroidX room current version.
Check for the related issue here it is marked as Won't Fix
Now they have release a fix related to the issue with version 2.0.0-beta01
Now you can use immutable val with default value e.g:
#Entity("tbl_abc")
data class Abc(
#PrimaryKey
val id: Int = 0,
val isFavourite: Boolean = false
)
Previously, the above snippet will throw an error of Cannot find setter for field. Changing into var is a great workaround, but I prefer for the entity class to be immutable from outside invocation

You can now start your field with is but you can't have a number next to the is like : is2FooSelected, you have to rename to isTwoFooSelected.

I think that the variable we wrote as id is getting mixed up with the id in the system. Therefore, when I define it as uuid, my error is resolved. I think it will be solved too. Also, try using var instead of val.
#PrimaryKey(autoGenerate = true)
var uuid:Int=0

Just use var instead of val and if you are using private keyword, make it public.
#Entity(tableName = "story")
data class Story (
#PrimaryKey val id: Long,
var by: String,
var descendants: Int,
var score: Int,
var time: Long,
var title: String,
var type: String,
var url: String
)

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