I have this kotlin function, and I undersatand that the expression : row.map(Char::digitToInt), converts a Char into an Int.
What I need, is more explanation regarding Char::digitToInt.
How can I relate the use of :: in the kotlin documentation?
Thank's in advance for your reply.
private fun parseInput(input: List<String>): Array<IntArray> =
input.map { row: String ->
row.map(Char::digitToInt).toIntArray()
}.toTypedArray()
As stated in the docs this is a class reference:
Class References: The most basic reflection feature is getting the
runtime reference to a Kotlin class. To obtain the reference to a
statically known Kotlin class, you can use the class literal syntax:
val c = TestClass::class //The reference is a value of type KClass.
Note that a Kotlin class reference is not the same as a Java class
reference. To obtain a Java class reference, use the .java property on
a KClass instance.
It’s also the syntax for method references as in this simple example:
myList.forEach(::println)
It refers to println defined in Kotlin Standard library.
Related
Why does Kotlin in one case infer type returned from Java to be nullable and in another case it is can be either, nullable or non-nullable?
I've checked both HashMap.get and JsonNode.get and I could not identify any #NotNull-like annotations neither in calsses nor anywhere in inheritance chain. What makes Kotlin treating those 2 calls differently?
I have read documentation https://kotlinlang.org/docs/java-interop.html#null-safety-and-platform-types but it explanation use "Platform Types" without explaining what those are and it does not explain differences in behavior anyway.
import com.fasterxml.jackson.databind.JsonNode
private fun docType(node: JsonNode, map: java.util.HashMap<String,String>) {
val x: JsonNode = node.get("doc_type") // DOES compile and can throw NPE at runtime
val y: JsonNode? = node.get("doc_type") // DOES compile and Kotlin's type system will force you to check for null
val z: String = map.get("a") // ERROR: Type mismatch: inferred type is String? but String was expected
}
Kotlin provides seamless interoperability with Java, without compromising its own null-safety... almost. One exception is that Kotlin assumes that all types that are defined in Java are not-null.
To understand, let's look at JsonNode.get()
Platform types
public JsonNode get(String fieldName) { return null; }
Note that JsonNode is defined in Java, and is a therefore 'platform type' - and Kotlin does not 'translate' it to JsonNode?, even though that would be technically correct (because in Java all types are nullable).
When calling Java from Kotlin, for convenience it's assumed that the platform type is non-nullable. If this wasn't the case, you would always have to check that any instance of any platform type is not null.
So, to answer your question about what a 'platform type' is, it's a term that means
some type that is defined in an external target language,
you can't mention it explicitly in Kotlin code (but there's probably a synonymous Kotlin equivalent),
and we're going to assume that it's non-nullable for convenience.
Also the notation is <type>!, for example String! - which we can take to mean String or String?
Nullability annotations
The closest Java equivalent of Kotlin's nullable ? symbol are nullability annotations, which the Kotlin compiler can parse and take into account. However, none are used on JsonNode methods. And so Kotlin will quite happily assume that node.get("") will return JsonNode, not JsonNode?.
As you noted, there are none defined for HashMap.get(...).
So how does Kotlin know that map.get("a") returns a nullable type?
Type inference
Type inference can't help. The (Java) method signature
public V get(Object key) {
//...
}
indicates that a HashMap<String, String> should return String, not String?. Something else must be going on...
Mapped types
For most Java types, Kotlin will just use the definition as provided. But for some, Kotlin decides to treat them specially, and completely replace the Java definition with its own version.
You can see the list of mapped types in the docs. And while HashMap isn't in there, Map is. And so, when we're writing Kotlin code, HashMap doesn't inherit from java.util.Map - because it's mapped to kotlin.collections.Map
Aside: in fact if you try and use java.util.Map you'll get a warning
So if we look at the code for the get function that kotlin.collections.Map defines, we can see that it returns a nullable value type
/**
* Returns the value corresponding to the given [key], or `null` if such a key is not present in the map.
*/
public operator fun get(key: K): V?
And so the Kotlin compiler can look at HashMap.get(...) and deduce that, because it's implementing kotlin.collections.Map.get(...), the returned value must be a nullable value, which in our case is String?.
Workaround: External annotations
For whatever reason, Jackson doesn't use the nullability annotations that would solve this problem. Fortunately IntelliJ provides a workaround that, while not as strict, will provide helpful warnings: external annotations.
Once I follow the instructions...
Alt+Enter → 'Annotate method...'
Select 'Nullable' annotation
Save annotations.xml
Now node.get("") will show an warning.
This annotation isn't visible to the Kotlin compiler, so it can only be a warning - not a compilation error.
java.util.HashMap.get implements the interface method java.util.Map.get. Kotlin maps some Java types to its own types internally. The full table of these mappings is available on the website. In our particular case, we see that java.util.Map gets mapped internally to kotlin.collections.Map, whose get function looks like
abstract operator fun get(key: K): V?
So as far as Kotlin is concerned, java.util.Map is just a funny name for kotlin.collections.Map, and all of the methods on java.util.Map actually have the signatures of the corresponding ones from kotlin.collections.Map (which are basically the same except with correct null annotations).
So while the first two node.get calls are Java calls and return platform types, the third one (as far as Kotlin is concerned) is actually calling a method Kotlin understands: namely, get from its own Map type. And that type has an explicit nullability annotation already available, so Kotlin can confidently say that that value can be null and needs to be checked.
I want a map between Int and any class. In Java it would be Map<Class<?>, Integer>. What's the Kotlin equivalent of that?
KClass is Kotlin's equivalent to java.lang.Class.
An instance of KClass can be obtained with ::class on either a type or a value (i.e. String::class, 3.8::class).
If you require a Java Class instance from a KClass you can use the java extension property:
val kotlinClass: KClass<String> = String::class
val javaClass: Class<String> = String::class.java
Keep in mind that if you want to use kotlin-reflect's full features you will need kotlin-reflect on the classpath.
So in your case, the equivalent would be Map<KClass<*>, Int>.
The equivalent declaration would be Map<Class<*>, Int>.
You're looking for KClass. You need to add the Kotlin reflection library in order to use it.
I am currently practicing in developing kotlin and as of now I seem to get confused with kotlin's class structure.
this is a code in java
//properties
private String var;
//constructor
public SampleClass(String var){
this.var = var;
}
public SampleClass(){
}
//getters and setters
public String getVar(){
return this.var;
}
public String setVar(String var){
this.var = var;
}
what's the kotlin equivalent of this ?
This is the equivalent Kotlin code for your Java code:
class SampleClass(var `var`: String? = null)
There are a few things to note:
Your Java snippet above omits the wrapping class SampleClass code
Your setVar() indicates that it returns a String, but it's actually void. I assume you intended for it to have a void return type.
Your property var is not ideal for Kotlin, because it's a reserved word. That's why we have to escape it with backticks. (It could also be kind of confusing in Java 10, since var is a reserved type name there now).
Here's why this one-liner is equivalent to the Java listing.
The constructor part - the part between the parentheses - can be used to accept constructor arguments, but by putting the Kotlin keyword var at the beginning, we tell Kotlin that we want this to also be a property. Kotlin will create a getter and setter for it.
The String? part makes this property of type nullable String.
Instead of creating two different constructors, we just give our var property argument a default value of null by using = null. When creating this class from Java, it'll still show up as two constructors.
If you're using IntelliJ or Android Studio, you can tell it to convert any Java class to Kotlin. Just open the class file, and go to the Code menu, and choose Convert Java file to Kotlin file. It won't necessarily generate very idiomatic code (e.g., it might create two constructors instead of using a default for the constructor argument), but it'll get you started.
For "what is Kotlin equivalent of some code in Java", there is an universal answer: copy the Java code and paste it into a Kotlin file in IDEA/Android Studio. Or convert the entire file.
On the web, you can use https://try.kotlinlang.org/#/Kotlin%20Koans/Introduction/Java%20to%20Kotlin%20conversion/Task.kt.
I have a generically typed class Builder<T> that takes a constructor argument Class<T> so I can keep the type around. This is a class that I use a lot in java code so I don't want to change the signature.
When I try to use the constructor like this:
Builder<List<Number>>(List<Number>::class)
I get an error: "Only classes are allowed on the left hand side of a class literal"
Any way to resolve this?
I can't change the constructor for Builder, too many java classes rely upon it.
I understand the whole type erasure issue, I really just want to make the compiler happy.
Due to generic type erasure List class has a single implementation for all its generic instantiations. You can only get a class corresponding to List<*> type, and thus create only Builder<List<*>>.
That builder instance is suitable for building a list of something. And again due to type erasure what that something is you can decide by yourself with the help of unchecked casts:
Builder(List::class.java) as Builder<List<Number>>
Builder(List::class.java as Class<List<Number>>)
Another approach is to create inline reified helper function:
inline fun <reified T : Any> Builder() = Builder(T::class.java)
and use it the following way:
Builder<List<Number>>()
The solution is to use reified generics in couple with super class tokens.
Please refer to this question for the method explained. Constructors in Kotlin don't support reified generics, but you can use TypeReference described there to write a builder factory function which will retain actual generic parameters at runtime:
inline <reified T: Any> fun builder(): Builder<T> {
val type = object : TypeReference<T>() {}.type
return Builder(type)
}
Then inside Builder you can check if type is ParameterizedType, and if it is, type.actualTypeArguments will contain the actual generic parameters.
For example, builder<List<Number>>() will retain the information about Number at runtime.
The limitation of this approach is that you cannot use non-reified generic as a reified type parameter because the type must be known at compile-time.
I am running some experiments on Kotlin's reflection.
I am trying to get a reflection object of a generic class with its argument.
In Java, that would be a ParameterizedType.
The way to get such a thing using Java's reflection API is a bit convoluted: create an anonymous subclass of a generic class, then get its super-type first parameter.
Here's an example:
#Suppress("unused") #PublishedApi
internal abstract class TypeReference<T> {}
inline fun <reified T> jGeneric() =
((object : TypeReference<T>() {}).javaClass.genericSuperclass as ParameterizedType).actualTypeArguments[0]
When I println(jGeneric<List<String?>>()), it prints java.util.List<? extends java.lang.String>, which is logical considering that Kotlin's List uses declaration-site out variance and that Java types have no notion of nullability.
Now, I would like to achieve the same kind of result, but with the Kotlin reflection API (that would, of course, contain nullability information).
Of course, List<String>::class cannot work since it yields a KClass. and I am looking for a KType.
However, when I try this:
inline fun <reified T> kGeneric() =
(object : TypeReference<T>() {})::class.supertypes[0].arguments[0].type
When I println(kGeneric<List<String?>>()), it prints [ERROR : Unknown type parameter 0], which is quite... well, anticlimactic ;)
How can I get, in Kotlin, a KType reflecting List<String> ?
To create a KType instance in Kotlin 1.1, you have two options:
To create a simple non-nullable type out of a KClass, where the class is either not generic or you can substitute all its type parameters with star projections (*), use the starProjectedType property. For example, the following creates a KType representing a non-nullable type String:
val nonNullStringType = String::class.starProjectedType
Or, the following creates a KType representing a non-nullable type List<*>:
val nonNullListOfSmth = List::class.starProjectedType
For more complex cases, use the createType function. It takes the class, type arguments and whether or not the type should be nullable. Type arguments are a list of KTypeProjection which is simply a type + variance (in/out/none). For example, the following code creates a KType instance representing List<String>:
val nonNullStringType = String::class.starProjectedType
val projection = KTypeProjection.invariant(nonNullStringType)
val listOfStrings = listClass.createType(listOf(projection))
Or, the following creates the type List<String>?:
val listOfStrings = listClass.createType(listOf(projection), nullable = true)
Both starProjectedType and createType are defined in package kotlin.reflect.full.
We're planning to introduce the possibility of getting a KType instance simply from a reified type parameter of an inline function which would help in some cases where the needed type is known statically, however currently it's not entirely clear if that's possible without major overhead. So, until that's implemented, please use the declarations explained above.