I have a simple Kotlin interface:
#FunctionalInterface
interface ServiceMethod<T> {
fun doService(): T
}
This, in spite of the name, is essentially identical to Java's Supplier functional interface. The only difference is that I can implement the Supplier, and I can't implement my own.
val supplier = Supplier<String> {
"Hello"
}
val serviceMethod = ServiceMethod<String> {
"Hello"
}
The ServiceMethod implementation gives me a compiler error saying "Interface ServiceMethod does not have constructors." Huh? Of course it doesn't! It's a functional interface.
I know that I can write it as an anonymous inner class:
val serviceMethod = object : ServiceMethod<String> {
override fun doService(): String {
return "Hello"
}
}
But this is much more verbose. In this case I could just use the Supplier interface, but that won't work for other interfaces. I shouldn't have to write an interface in Java, just to be able to a lambda in Kotlin. I'd rather use a lambda for all my Kotlin interfaces, especially since I'll be writing a lot of these. Am I missing something obvious?
Use the fun interface modifier since Kotlin 1.4
In Kotlin 1.3 and earlier, SAM (single abstract method) conversions, where you can instantiate an interface like Supplier using a lambda function, were only supported for Java interfaces.
The language designers originally thought SAM conversions wouldn't be useful for Kotlin interfaces, because a Kotlin function already has a type. For example, the type of your doService function can be written as () -> T. Instead of creating an object that implements an interface, you could simply write:
val serviceMethod: () -> String = { "Hello" }
Kotlin 1.4 adds SAM conversions for Kotlin interfaces, but it doesn't work out of the box for every interface. Instead, you have to apply the special fun modifier to a Kotlin interface to make it eligible for SAM conversion.
In your example, it would simply look like this:
fun interface ServiceMethod<T> {
fun doService(): T
}
With the modifier added, you can create an instance using a lambda exactly as you were hoping in your question.
val serviceMethod = ServiceMethod<String> { "Hello" }
You can learn more in the Kotlin documentation for functional interfaces.
Related
In Kotlin, I want to add a "namespace" to a class that has a set of related functions. Clients of my class will use that namespace to help classify what type of operation they want to do. (I know you're thinking the functions should be in different classes, problem solved. But for other reasons, it's convenient to house all the functions in a single class).
So, I might have a class Uber that contains fooInsert fooOpen fooDispose along with barInsert barTerminate and barHop. As you can see there's no common interface. Just a bunch of functions that for some reason belong in the same class. Some have an affinity with others (i.e. the fooXXX functions "belong" together, as do the "barYYY" functions).
What I've come up with is:
class Uber {
inner class FooNamespace {
fun insert(): Unit {}
fun open(): Unit {}
fun dispose(): Unit {}
}
val foo = FooNamespace()
inner class BarNamespace {
fun insert(): Unit {}
fun terminate(): Unit {}
fun hop(): Unit {}
}
val bar = BarNamespace()
}
Users of the class can do something like this:
val uber = Uber()
uber.foo.insert()
uber.bar.hop()
What I'd like is something that combines the inner class ... and val xxx = XxxNamespace() into one expression. Something like:
// This doesn't actually compile
val foo = object: inner class {
fun insert(): Unit {}
fun open(): Unit {}
fun dispose(): Unit {}
}
The problem here is that you need a properly defined type if you to want to access these members publicly.
For private properties, the syntax val foo = object { ... } is sufficient, but for publicly exposed properties these are inferred as Any and it makes them unusable.
One option is obviously to define an interface for these types, but it's even more boilerplate than what you came up with already, so I am pretty sure this won't suit your needs:
interface FooNamespace {
fun insert()
fun open()
fun dispose()
}
class Uber {
val foo = object : FooNamespace {
override fun insert(): Unit {}
override fun open(): Unit {}
override fun dispose(): Unit {}
}
}
I know you're thinking the functions should be in different classes, problem solved. But for other reasons, it's convenient to house all of the functions in a single class
I'm indeed really thinking that, and would love to hear more about what makes it so convenient to put everything in the same class :) Since the classes are inner classes, I'm assuming this has to do with accessing private state from Uber, but that could also be done by wrapping this private state into another class that's passed to foo and bar.
I believe this is not possible, at least for now.
The main technical problem here is that uber.foo.insert() is really interpreted as chaining uber.foo and then .insert(). So for this to work, uber.foo needs to have an explicitly defined type. It can't be anonymous class/object, because then there is no way to describe what is the type of uber.foo.
That being said, I've always wondered why Kotlin does not support this syntax:
val foo = object Foo {}
This is consistent with the object declaration where the name of the singleton is at the same time the name of the class. And the compiler even understands this above syntax, because it throws the error: "An object expression cannot bind a name". So Kotlin authors seem to intentionally disallow such use.
I found an issue in the YouTrack, so we can at least upvote it: https://youtrack.jetbrains.com/issue/KT-21329
Consider the following interface
interface EntityConverter<in A, out B> {
fun A.convert(): B
fun List<A>.convert(): List<B> = this.map { it.convert() }
}
I want to use it in a spring boot application where specific implementations get injected so that the extension function becomes usable on the type.
However this doesn't work. The compiler does not resolve the extension function.
Note that you're defining extension functions that are also member functions of the EntityConverter type. You should take a look at this part of the doc for information about how this works.
Essentially, in order to use them, you need 2 instances in scope:
the dispatch receiver (an instance of EntityConverter<A, B>)
the extension receiver (an instance of A or List<A>, where A matches the first type parameter of the EntityConverter in scope)
You can use with() to bring the EntityConverter in scope so you can use convert on your other instances using the usual . syntax:
val converter = object : EntityConverter<Int, String> {
override fun Int.convert() = "#$this"
}
val list = listOf(1, 2, 3)
val convertedList = with(converter) {
list.convert()
}
println(convertedList) // prints [#1, #2, #3]
Now you have to decide whether this kind of usage pattern is what makes most sense for your use case. If you'd prefer more "classic" calls without extensions (converter.convert(a) returning a B), you can declare your functions as regular methods taking an argument instead of a receiver.
Bonus: functional interface
As a side note, if you add the fun keyword in front of your EntityConverter interface, you can create instances of it very easily like this:
val converter = EntityConverter<Int, String> { "#$this" }
This is because your converter interface only has a single abstract method, making it easy to implement with a single lambda. See the docs about functional interfaces.
I'm not sure if you can mention extension functions as a part of interface, because it's like static functions.
I'd recommend to put "common" function in interface with A typed parameter. Then just put extension method for list nearby.
interface EntityConverter<in A, out B> {
fun convert(a: A): B
}
fun <A, B> EntityConverter<A, B>.convert(list: List<A>): List<B> = list.map { convert(it) }
Update
I wasn't aware about possibility of inheritance of extension methods in Kotlin. And about its overriding as well. So my answer could be just an alternative of using extension methods.
With Kotlin 1.4 we now have Functional Interfaces
fun interface Todo {
fun run()
}
fun runBlock(todo: Todo){
if(condition)
todo.run()
}
fun runBlock{
println("Hello world")
}
Before i was always using (T) -> T
inline fun runBlock(block: ()-> Unit){
if(condition)
block()
}
fun runBlock{
println("Hello world")
}
So basically I can make the same task with both methods , there is any performance advantage by using Functional SAM() Interfaces over Function Type?.
It's a performance dis-advantage because the lambda is no longer inlined (unless the JIT decides to, but it won't be instant). Even if you mark runBlock as inline, the compiler will warn you that the argument won't be inlined.
There are only two reasons to use fun interfaces instead of function types:
Backwards compatibility when porting code using Java functional interfaces.
Not exposing Kotlin function types in API intended for use from Java.
To expand on point 1: before Kotlin 1.4 it was advised to keep functional interfaces as Java code, even if all your other code was Kotlin. This way you could use lambdas for parameters of those types both in Java and Kotlin code. But if you declared the interface in Kotlin, you could only use lambdas for them in Java.
https://kotlinlang.org/docs/reference/whatsnew14.html#sam-conversions-for-kotlin-interfaces
the compiler automatically converts the lambda to an instance of the class that implements the abstract member function.
So, no performance advantage, it’s the same thing as before. The compiler now does what you had to do before.
As other answers and comments have pointed out, in your case, using inlined lambda is faster, since there is no function call overhead when invoking it.
However, there is one specific use case where using SAM interface is faster, that is when you 1. do not inline the lambda and 2. the arguments/return value of the lambda is a primitive (or any other type that may cause boxing when used with generics).
For example, using SAM interface like so:
fun interface Foo() {
fun run(i: Int): Int
}
fun foo(fn: Foo) {
fn.run(42)
}
foo { it * 2 }
Will not cause any boxing when invoked, while:
fun foo(fn: (Int) -> Int) {
fn(42)
}
foo { it * 2 }
Will box the integer argument since (Int) -> Int is essentially Function1<Integer, Integer> in Java, which uses generic.
coming across a sample with a class and a function and trying to understand the koltin syntax there,
what does this IMeta by dataItem do? looked at https://kotlinlang.org/docs/reference/classes.html#classes and dont see how to use by in the derived class
why the reified is required in the inline fun <reified T> getDataItem()? If someone could give a sample to explain the reified?
class DerivedStreamItem(private val dataItem: IMeta, private val dataType: String?) :
IMeta by dataItem {
override fun getType(): String = dataType ?: dataItem.getType()
fun getData(): DerivedData? = getDataItem()
private inline fun <reified T> getDataItem(): T? = if (dataItem is T) dataItem else null
}
for the reference, copied the related defines here:
interface IMeta {
fun getType() : String
fun getUUIDId() : String
fun getDataId(): String?
}
class DerivedData : IMeta {
override fun getType(): String {
return "" // stub
}
override fun getUUIDId(): String {
return "" // stub
}
override fun getDataId(): String? {
return "" // stub
}
}
why the reified is required in the inline fun <reified T> getDataItem()? If someone could give a sample to explain the reified?
There is some good documentation on reified type parameters, but I'll try to boil it down a bit.
The reified keyword in Kotlin is used to get around the fact that the JVM uses type erasure for generic. That means at runtime whenever you refer to a generic type, the JVM has no idea what the actual type is. It is a compile-time thing only. So that T in your example... the JVM has no idea what it means (without reification, which I'll explain).
You'll notice in your example that you are also using the inline keyword. That tells Kotlin that rather than call a function when you reference it, to just insert the body of the function inline. This can be more efficient in certain situations. So, if Kotlin is already going to be copying the body of our function at compile time, why not just copy the class that T represents as well? This is where reified is used. This tells Kotlin to refer to the actual concrete type of T, and only works with inline functions.
If you were to remove the reified keyword from your example, you would get an error: "Cannot check for instance of erased type: T". By reifying this, Kotlin knows what actual type T is, letting us do this comparison (and the resulting smart cast) safely.
(Since you are asking two questions, I'm going to answer them separately)
The by keyword in Kolin is used for delegation. There are two kinds of delegation:
1) Implementation by Delegation (sometimes called Class Delegation)
This allows you to implement an interface and delegate calls to that interface to a concrete object. This is helpful if you want to extend an interface but not implement every single part of it. For example, we can extend List by delegating to it, and allowing our caller to give us an implementation of List
class ExtendedList(someList: List) : List by someList {
// Override anything from List that you need
// All other calls that would resolve to the List interface are
// delegated to someList
}
2) Property Delegation
This allows you to do similar work, but with properties. My favorite example is lazy, which lets you lazily define a property. Nothing is created until you reference the property, and the result is cached for quicker access in the future.
From the Kotlin documentation:
val lazyValue: String by lazy {
println("computed!")
"Hello"
}
I'm pretty new in Kotlin language, but I have just encountered some strange behavior that didn't have in other languages, so I wanted to ask why I can't do something like this:
fun <T> methodName()
{
// whatev~
}
fun <T, K> methodName()
{
// whatev~
}
This code throws an error of "Conflicting overloads".
In other languages, for example C# I can do this and it's a pretty neat trick to have only one method that work for one or multiple types at the same time.
The only workaround I've found it's adding in each new method that I do an optional parameter that I'll never use, like:
fun <T> methodName()
{
}
fun <T, K> methodName(crappyParam: String = "")
{
}
The two methods would have the same signature in JVM type system (which doesn't support generics), which isn't allowed.
A JVM language could "mangle" such methods, e.g. giving them different names in bytecode. A JVM implementation of C# would have to.
But Kotlin doesn't. And doing so would hurt interoperability with Java, which is one of Kotlin's major requirements.