Below is a example of a pattern from Android (Just an example, not interested in android specifics):
/*Im a kotlin file*/
class ListItemViewHolder(itemView: View) : RecyclerView.ViewHolder(itemView) {
val text: = itemView.my_view
}
Then the pattern is that you access the text field as such:
/*Im a Java file*/
holder.text.setText("Metasyntactic variable");
It's unfortunate to have a large file with a set structure doing the above and then have:
/*Im a Java file, but this particular holder is a kotlin file*/
holder.getText().setText("Metasyntactic variable");
Is it possible to solve this? Maybe with some #Jvm annotation
It's #JvmField:
If you need to expose a Kotlin property as a field in Java, you need to annotate it with the #JvmField annotation. The field will have the same visibility as the underlying property. You can annotate a property with #JvmField if it has a backing field, is not private, does not have open, override or const modifiers, and is not a delegated property.
Related
I have an interface which has annotation:
#Target(AnnotationTarget.PROPERTY)
annotation class Foo()
interface Bah {
#Foo val prop: String
}
I'm implementing a jackson contextual deserializer, and I need to pick up this annotation from the methods in the interface.
override fun createContextual(ctxt: DeserializationContext, property: BeanProperty?): JsonDeserializer<*> {
val clzz = ctxt.contextualType.rawClass as Class<T>
for (method in clzz.methods) {
val anns = method.getAnnotationsByType(Foo::class.java)
ctxt.contextualType is a JavaType. I obtain clzz from it, which yields me a class of type Bah (i.e. the interface). I can iterate the methods, which include "prop"; however, prop has no annotations.
It DOES work if I modify the annotation site to look like this:
interface Bah {
val prop: String
#Foo() get
However, that's ugly. How can I modify things so that I can retrieve from the interface property directly?
Thanks
You can't. As the documentation says, annotations targeting a property are not visible from Java (because Java does not have the concept of properties).
I would like to semantically constrain a map to only accept "data" class object types as the value in kotlin like so:
class Test(
val test : Int
)
data class Test2 (
val test : Int
)
fun test(map : Map<String, /* compile error on Test, but accept Test2 or any other data class */>) {
}
I'm mainly trying to do this so that I can keep everything in the map cloneable, but when I do this:
fun <T: Cloneable> test(map : Map<String, T>) {
// test clone
map.map { it.key.uuid to it.value.clone() } .toMap() // error on .clone() Cannot access 'clone': it is protected in 'Cloneable'
}
but I thought implementing the Cloneable interface made your clone method public? Essentially I'm looking for a compile time guarantee that all data is copyable in that method invocation, (is a primitive type, a data class that I can call .copy() on, or any object that has implemented Cloneable). Is my only option reflection and runtime assertions?
I thought implementing the Cloneable interface made your clone method public?
No, it's simply a marker interface, which tells the protected Object.clone() method not to throw a CloneNotSupportedException. In practice, classes that implement Cloneable will usually override clone() and make it public, but that's not necessary. And of course that's no help when you don't know the exact type!
The cloning mechanism was an early part of Java, and not very well-designed. (Effective Java calls it “a highly atypical use of interfaces and not one to be emulated”.) But it's still used, so we're stuck with it…
(See also these related answers.)
I don't know whether this is the best way or not, but how about you to use property like below.
SomeClass::class.isData
Kdoc says
true if this class is a data class.
How is #Named used with #BindsInstance? I have the this component
interface AppComponent : AndroidInjector<MyApplication>{
#Component.Builder
abstract class Builder : AndroidInjector.Builder<MyApplication>() {
#BindsInstance
abstract fun preferenceName( #Named("PreferenceName") name : String ) : Builder
}
}
and trying to inject in MyApplication
#Inject
#Named("PreferenceName")
lateinit var prefName : String
But it fails with MissingBinding for String. I could resolve this with a module provider but trying to avoid provider for constants.
Update: Dagger 2.25.2 has eliminated the need for workaround:
Kotlin support
ii. Qualifier annotations on fields can now be understood without
The need for #field:MyQualifier (646e033)
iii. #Module object classes no longer need #JvmStatic on the
provides methods. (0da2180)
This doesn't have anything to do with #BindsInstance, but rather the #Named annotations on fields. You can tell from the "MissingBinding for String", which would otherwise give you an error about a Named string.
As in Svetlozar Kostadinov's article Correct usage of Dagger 2 #Named annotation in Kotlin, you'll need to clarify to Kotlin that you'd like the annotations to apply to the field.
#field:[Inject Named("PreferenceName")]
lateinit var prefName : String;
As Svetlozar puts it:
The reason is because in Kotlin annotations need to be slightly more complicated in order to work as expected from Java perspective. That’s coming from the fact that one Kotlin element may be a facade of multiple Java elements emitted in the bytecode. For example a Kotlin property is a facade of an underlying Java member variable, a getter and a setter. You annotate the property but what Dagger expects to be annotated is the underlying field.
Related: Dagger 2 constructor injection in kotlin with Named arguments
I'm trying to get annotations from Kotlin data class
package some.meaningless.package.name
import kotlin.reflect.full.memberProperties
annotation class MyAnnotation()
#MyAnnotation
data class TestDto(#MyAnnotation val answer: Int = 42)
fun main(args: Array<String>) {
TestDto::class.memberProperties.forEach { p -> println(p.annotations) }
println(TestDto::class.annotations)
}
I need to process class annotation to make a custom name serialization of GSON however no matter how I declare annotation class it never gets detected
The program always outputs
[]
[#some.meaningless.package.name.MyAnnotation()]
which means only class level annotations are present
Ok,
it seems that the culprit was, that Kotlin annotations have default #Target(AnnotationTarget.CLASS) which is not stressed enough in documentation.
After I added #Target to the annotation class it now works properly
#Target(AnnotationTarget.CLASS, AnnotationTarget.PROPERTY)
annotation class MyAnnotation()
Now it prints out
[#some.meaningless.package.name.MyAnnotation()]
[#some.meaningless.package.name.MyAnnotation()]
As a side affect it will force the compiler to check that the annotation is applied as required, in current version of Kotlin, if explicit #Targetis not present only class level annotations are kept but no validity checks performed.
As Kotlin reference said as below:
If you don't specify a use-site target, the target is chosen according to the #Target annotation of the annotation being used. If there are multiple applicable targets, the first applicable target from the following: param > property > field.
To make the annotation annotated on a property, you should use site target, for example:
#MyAnnotation
data class TestDto(#property:MyAnnotation val answer: Int = 42)
However, annotations with property target in Kotlin are not visible to Java, so you should double the annotation, for example:
#MyAnnotation // v--- used for property v--- used for params in Java
data class TestDto(#property:MyAnnotation #MyAnnotation val answer: Int = 42)
Is there a way to use Parceler with Kotlin data classes and constructor for serialization without using #ParcelProperty annotation for each field?
If I try and use library like this:
#Parcel
data class Valve #ParcelConstructor constructor(val size: Int)
I get Error:Parceler: No corresponding property found for constructor parameter arg0. But if I add #ParcelProperty("size") it works just fine.
Why is that?
Update:
There are other another way to use this library.
I could just remove #ParcelConstructor annotation, but then I will get error
Error:Parceler: No #ParcelConstructor annotated constructor and no default empty bean constructor found.
I think (haven't tested it) I also could make all constructor parameters optional and add #JvmOverloads but that has a side effect that I have to check all properties of the class if they are null or not.
Update 2:
This is what worked for me:
#Parcel
data class Valve(val size: Int? = null)
In short generated Java class must have default empty constructor. One way to achieve that is to do as above - all variables should have default values.
According to the docs, Parceler by default works with public fields. But a usual Kotlin data class (as in your example) is rather a "traditional getter/setter bean", since every Kotlin property is represented by a private field and a getter/[setter].
TL; DR: I think this will work:
#Parcel(Serialization.BEAN)
data class Valve(val size: Int = 10)
Note the default value, it allows Kotlin to automatically generate an additional empty constructor, which is required by the Java Been specification.
Another way would be to mark the constructor that we already have:
#Parcel(Serialization.BEAN)
data class Driver #ParcelConstructor constructor(val name: String)
The specific document: https://github.com/johncarl81/parceler#gettersetter-serialization
I know this question already has an answer, but for future viewers who are also struggling to get Parceler to work with kotlin data objects, I wrote a new annotation processor to generate the Parcelable boilerplate for Kotlin data classes. It's designed to massively reduce the boilerplate code in making your data classes Parcelable:
https://github.com/grandstaish/paperparcel
Usage:
Annotate your data class with #PaperParcel, implement PaperParcelable, and add a JVM static instance of the generated CREATOR e.g.:
#PaperParcel
data class Example(
val test: Int,
...
) : PaperParcelable {
companion object {
#JvmField val CREATOR = PaperParcelExample.CREATOR
}
}
Now your data class is Parcelable and can be passed directly to a Bundle or Intent
Edit: Update with latest API
Just add the default constructor:
#Parcel
data class Valve(val size: Int) {
constructor() : this(0)
}
if you use Kotlin 1.1.4 or above it's easier to use #Parcelize annotation
For doing this first add this to build.gradle
android {
//other codes
//for using latest experimental build of Android Extensions
androidExtensions {
experimental = true
}
}
Then change your class like this
#Parcelize
data class Valve(val size: Int? = null) : Parcelable