I'm new to Kotlin and lambdas and I'm trying to understand it. I'm trying to generate a list of 100 random numbers.
This works:
private val maxRandomValues = (1..100).toList()
But I want to do something like that:
private val maxRandomValues = (1..100).forEach { RandomGenerator().nextDouble() }.toList()
But this is not working. I'm trying to figure out how to use the values generated into forEach are used in the toList()
It's way better to use kotlin.collections function to do this:
List(100) {
Random.nextInt()
}
According to Collections.kt
inline fun <T> List(size: Int, init: (index: Int) -> T): List<T> = MutableList(size, init)
It's also possible to generate using range like in your case:
(1..100).map { Random.nextInt() }
The reason you can't use forEach is that it return Unit (which is sort of like void in Java, C#, etc.). map operates Iterable (in this case the range of numbers from 1 to 100) to map them to a different value. It then returns a list with all those values. In this case, it makes more sense to use the List constructor above because you're not really "mapping" the values, but creating a list
Beside using constructor: List(10, { Random.nextInt() }),
Kotlin also has a built-in function for this purpose: buildList { repeat(10) { add(Random.nextInt()) } }
See: https://kotlinlang.org/docs/constructing-collections.html
Related
I'm trying to wrap my head around map and reduce operations in Kotlin. At least, I guess it's reduce what I'm trying to do.
Let's say that I have a class called Car that takes any number (varargs constructor) of CarPart. Then, I have a list of CarPart which I'll do a map operation and from the result of the operation I need to build one Car using each subelement, something along these lines:
class CarPart(val description: String)
class Car(vararg val carPart: CarPart)
val carParts = listOf(CarPart("Engine"), CarPart("Steering Wheel")))
carParts.map { it.description.toUpperCase() }
.map { CarPart(it) }
.reduce { acc, carPart -> Car(carPart) } <--- I'm struggling here, is reduce what I should be doing
to construct one car from all the subelement?
PS.1: I know that the class design could be better and not take a varargs, this is just an example of a legacy application I'm refactoring and originally that's a Java class taking varargs which I can't change now.
PS.2: The example of mapping to a String and then creating an object out of that String is just for the sake of the example. The actual code grabs an object within the list.
You can simply use a the spread operator (*) over an array:
val mappedCarParts = carParts
.map { it.description.toUpperCase() }
.map { CarPart(it) }
.toTypedArray()
val car = Car(*mappedCarParts)
// Or even:
val car = carParts
.map { it.description.toUpperCase() }
.map { CarPart(it) }
.toTypedArray()
.let{ Car(*it) }
You could just extract the constructor of the Car outside of the creation of the list. I don't see any reason as to why you'd want it inside.
val car = Car(
*carParts
.map { CarPart(it.description.uppercase(Locale.getDefault())) } //keep the .toUpperCase() if you are using an old version of Kotlin
.toTypedArray()
)
We need the spread operator there in order for the vararg to know that we are passing it the elements of the list and not the list itself.
I'd like to understand Kotlin extension functions more and am trying to implement an extension function for a List, to get the index of an element by passing the value of the position (if that makes sense).
What I have:
fun List<String>.getItemPositionByName(item: String): Int {
this.forEachIndexed { index, it ->
if (it == item)
return index
}
return 0
}
Although this works fine, I would need the same thing for Int too.
To my question, is there a way of combining this into one extension function instead of two seperate ones? I acknowledge that this isn't a lot of code and wouldn't hurt to be duplicated but out of interest and for future references.
I'm aware of this question Extension functions for generic classes in Kotlin where the response is - as I understand it at least - "doesn't quite work like this, but I don't really need it for type but "just" for String and Int.
Kotlin supports what C++ people would refer to as specialization to a certain degree. It works just fine for very basic types like you're using so what you're asking of is definitely possible.
We can declare the following declarations. Of course you could just duplicate the code and you'd be on your way.
public fun List<String>.getItemPositionByName(item: String) = ...
public fun List<Int>.getItemPositionByName(item: String) = ...
If you're not a fan of repeating the code, the idiomatic way would be to make use of file-private functions and simply delegating to the private function.
private fun <T> getItemImpl(list: List<T>, item: T): Int {
list.forEachIndexed { index, it ->
if (it == item)
return index
}
return -1
}
public fun List<String>.getItemPositionByName(item: String) = getItemImpl(this, item)
public fun List<Int>.getItemPositionByName(item: Int) = getItemImpl(this, item)
This limits the getItemImpl which is fully generic to the current file you're in while the Int and String specializations are publicly available anywhere else.
Attempting to call getItemPositionByName on any list which is not of type List<Int> or List<String> will fail with a type error.
Kotlin Playground Link: https://pl.kotl.in/NvIRXwmpU
And just in case you weren't aware, the method you're implementing already exists in the standard library (https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.collections/index-of.html)
The Kotlin standard library already has a function that does this: indexOf().
val one = listOf("a", "b", "c").indexOf("b")
check(one == 1)
One option is to look at the implementation of that function.
There is also the first() function, which you could use if you wanted write your own generic version:
fun <T> List<T>.getItemPositionByName(item: T) = withIndex()
.first { (_, value) -> item == value }
.index
fun main(args: Array<String>) {
val one = listOf("a", "b", "c").getItemPositionByName("b")
check(one == 1)
}
Or, rewriting your original version to use generics:
fun <T> List<T>.getItemPositionByName(item: T): Int {
this.forEachIndexed { index, it ->
if (it == item)
return index
}
return 0
}
I am trying to use the public interface Function (as I learned it in Java) in Kotlin.
For this I created my method
fun foo(input: List<String>, modifier1: Function<List<String>>? = null){
}
as far I remember here I should be able to do modifier1.apply(input)
but seems like it is not possible (it is possible to do modifier1.apply{input} though)
Reading more about it I found this:
Kotlin: how to pass a function as parameter to another?
So I changed my method signature to this:
fun foo(input:String, modifier2: (List<String>) -> (List<String>){
}
Here I am able to do modifier2(input)
and I can call foo this way
service.foo(input, ::myModifierFunction)
where
fun myModifierFunction(input:List<String>):List<String>{
//do something
return input
}
So far this seems possible but it is not acceptable to have the function reference as nullable, is there any way I can do that? or use Function ?
You were using kotlin.Function instead of java.util.function.Function in your first example. Note that the latter takes 2 generic types: 1 for the incoming parameter and 1 for the resulting one.
The apply method you saw is the default Kotlin one: apply, not the one of Java's Function-interface.
If you really want to have the Java-function as nullable type the following should work:
fun foo(input: List<String>, modifier1: java.util.function.Function<List<String>, List<String>>? = null) {
modifier1?.apply(input) ?: TODO("what should be done if there wasn't passed any function?")
}
Kotlin variant for the same:
fun foo(input: List<String>, modifier1: ((List<String>) -> List<String>)? = null) {
modifier1?.invoke(input) ?: TODO("what should be done if there wasn't passed any function?")
}
Maybe also a default function, such as { it } instead of null might better suite your needs? (Java variant would be Function.identity()):
// java modifier1 : Function<List<String>, List<String>> = Function.identity()
// kotlin modifier1 : (List<String>) -> List<String> = { it }
You can make the reference nullable simply with ? — the only wrinkle is that the whole function type needs to be in parens first:
fun foo(input: String, modifier2: ((List<String>) -> List<String>)? = null) {
}
As required, modifier2 is optional; if specified, it may contain null, or it may contain a function taking and returning a list of strings.
As mentioned in another answer, kotlin.Function is not the same as java.util.function.Function — though in practice you shouldn't need to refer to either directly, as the -> notation is simpler.
If you want to pass in a function that takes List<String> as its parameter and returns nothing meaningful, the type for you is Function1<List<String>, Unit>. The method name for invoking a function is invoke(), which you could also do with just regular parentheses, if it wasn't nullable. All in all, your code could look something like this:
fun foo(input: List<String>, modifier1: Function1<List<String>, Unit>? = null) {
modifier1?.invoke(input)
}
The 1 in the typename of Function1 means that it's a one parameter function, there's also Function0, Function2, etc.
The Function type on its own is not something you can use to call that function, as it's an empty marker interface. All functions implement this regardless of how many parameters they have.
Let's say I have an object which helps me to deserialize other objects from storage:
val books: MutableList<Book> = deserializer.getBookList()
val persons: MutableList<Person> = deserializer.getPersonList()
The methods getBookList and getPersonList are extension functions I have written. Their logic is allmost the same so I thought I may can combine them into one method. My problem is the generic return type. The methods look like this:
fun DataInput.getBookList(): MutableList<Book> {
val list = mutableListOf<Book>()
val size = this.readInt()
for(i in 0 .. size) {
val item = Book()
item.readExternal(this)
list.add(item)
}
return list
}
Is there some Kotlin magic (maybe with inline functions) which I can use to detect the List type and generify this methods? I think the problem would be val item = T() which will not work for generic types, right? Or is this possible with inline functions?
You cannot call the constructor of a generic type, because the compiler can't guarantee that it has a constructor (the type could be from an interface). What you can do to get around this though, is to pass a "creator"-function as a parameter to your function. Like this:
fun <T> DataInput.getList(createT: () -> T): MutableList<T> {
val list = mutableListOf<T>()
val size = this.readInt()
for(i in 0 .. size) {
val item = createT()
/* Unless readExternal is an extension on Any, this function
* either needs to be passed as a parameter as well,
* or you need add an upper bound to your type parameter
* with <T : SomeInterfaceWithReadExternal>
*/
item.readExternal(this)
list.add(item)
}
return list
}
Now you can call the function like this:
val books: MutableList<Book> = deserializer.getList(::Book)
val persons: MutableList<Person> = deserializer.getList(::Person)
Note:
As marstran mentioned in a comment, this requires the class to have a zero-arg constructor to work, or it will throw an exception at runtime. The compiler will not warn you if the constructor doesn't exist, so if you pick this way, make sure you actually pass a class with a zero-arg constructor.
You can't initialize generic types, in Kotlin or Java. At least not in the "traditional" way. You can't do this:
val item = T()
In Java, you'd pass a Class<T> and get the constructor. Very basic example of that:
public <T> void x(Class<T> cls){
cls.getConstructor().newInstance(); // Obviously you'd do something with the return value, but this is just a dummy example
}
You could do the same in Kotlin, but Kotlin has a reified keyword that makes it slightly easier. This requires an inline function, which means you'd change your function to:
inline fun <reified T> DataInput.getBookList(): MutableList<T> { // Notice the `<reified T>`
val list = mutableListOf<T>() // Use T here
val size = this.readInt()
for(i in 0 .. size) {
// This is where the initialization happens; you get the constructor, and create a new instance.
// Also works with arguments, if you have any, but you used an empty one so I assume yours is empty
val item = T::class.java.getConstructor().newInstance()!!
item.readExternal(this) // However, this is tricky. See my notes below this code block
list.add(item)
}
return list
}
However, readExternal isn't present in Any, which will present problems. The only exception is if you have an extension function for either Any or a generic type with that name and input.
If it's specific to some classes, then you can't do it like this, unless you have a shared parent. For an instance:
class Book(){
fun readExternal(input: DataInput) { /*Foo bar */}
}
class Person(){
fun readExternal(input: DataInput) { /*Foo bar */}
}
Would not work. There's no shared parent except Any, and Any doesn't have readExternal. The method is manually defined in each of them.
You could create a shared parent, as an interface or abstract class (assuming there isn't one already), and use <reified T : TheSharedParent>, and you would have access to it.
You could of course use reflection, but it's slightly harder, and adds some exceptions you need to handle. I don't recommend doing this; I'd personally use a superclass.
inline fun <reified T> DataInput.getBookList(): MutableList<T> {
val list = mutableListOf<T>()
val size = this.readInt()
val method = try {
T::class.java.getMethod("readExternal", DataInput::class.java)
}catch(e: NoSuchMethodException){
throw RuntimeException()
}catch(e: SecurityException){
throw RuntimeException()// This could be done better; but error handling is up to you, so I'm just making a basic example
// The catch clauses are pretty self-explanatory; if something happens when trying to get the method itself,
// These two catch them
}
for(i in 0 .. size) {
val item: T = T::class.java.getConstructor().newInstance()!!
method.invoke(item, this)
list.add(item)
}
return list
}
I wish to have a good example for each function run, let, apply, also, with
I have read this article but still lack of an example
All these functions are used for switching the scope of the current function / the variable. They are used to keep things that belong together in one place (mostly initializations).
Here are some examples:
run - returns anything you want and re-scopes the variable it's used on to this
val password: Password = PasswordGenerator().run {
seed = "someString"
hash = {s -> someHash(s)}
hashRepetitions = 1000
generate()
}
The password generator is now rescoped as this and we can therefore set seed, hash and hashRepetitions without using a variable.
generate() will return an instance of Password.
apply is similar, but it will return this:
val generator = PasswordGenerator().apply {
seed = "someString"
hash = {s -> someHash(s)}
hashRepetitions = 1000
}
val pasword = generator.generate()
That's particularly useful as a replacement for the Builder pattern, and if you want to re-use certain configurations.
let - mostly used to avoid null checks, but can also be used as a replacement for run. The difference is, that this will still be the same as before and you access the re-scoped variable using it:
val fruitBasket = ...
apple?.let {
println("adding a ${it.color} apple!")
fruitBasket.add(it)
}
The code above will add the apple to the basket only if it's not null. Also notice that it is now not optional anymore so you won't run into a NullPointerException here (aka. you don't need to use ?. to access its attributes)
also - use it when you want to use apply, but don't want to shadow this
class FruitBasket {
private var weight = 0
fun addFrom(appleTree: AppleTree) {
val apple = appleTree.pick().also { apple ->
this.weight += apple.weight
add(apple)
}
...
}
...
fun add(fruit: Fruit) = ...
}
Using apply here would shadow this, so that this.weight would refer to the apple, and not to the fruit basket.
Note: I shamelessly took the examples from my blog
There are a few more articles like here, and here that are worth to take a look.
I think it is down to when you need a shorter, more concise within a few lines, and to avoid branching or conditional statement checking (such as if not null, then do this).
I love this simple chart, so I linked it here. You can see it from this as written by Sebastiano Gottardo.
Please also look at the chart accompanying my explanation below.
Concept
I think it as a role playing way inside your code block when you call those functions + whether you want yourself back (to chain call functions, or set to result variable, etc).
Above is what I think.
Concept Example
Let's see examples for all of them here
1.) myComputer.apply { } means you want to act as a main actor (you want to think that you're computer), and you want yourself back (computer) so you can do
var crashedComputer = myComputer.apply {
// you're the computer, you yourself install the apps
// note: installFancyApps is one of methods of computer
installFancyApps()
}.crash()
Yup, you yourself just install the apps, crash yourself, and saved yourself as reference to allow others to see and do something with it.
2.) myComputer.also {} means you're completely sure you aren't computer, you're outsider that wants to do something with it, and also wants it computer as a returned result.
var crashedComputer = myComputer.also {
// now your grandpa does something with it
myGrandpa.installVirusOn(it)
}.crash()
3.) with(myComputer) { } means you're main actor (computer), and you don't want yourself as a result back.
with(myComputer) {
// you're the computer, you yourself install the apps
installFancyApps()
}
4.) myComputer.run { } means you're main actor (computer), and you don't want yourself as a result back.
myComputer.run {
// you're the computer, you yourself install the apps
installFancyApps()
}
but it's different from with { } in a very subtle sense that you can chain call run { } like the following
myComputer.run {
installFancyApps()
}.run {
// computer object isn't passed through here. So you cannot call installFancyApps() here again.
println("woop!")
}
This is due to run {} is extension function, but with { } is not. So you call run { } and this inside the code block will be reflected to the caller type of object. You can see this for an excellent explanation for the difference between run {} and with {}.
5.) myComputer.let { } means you're outsider that looks at the computer, and want to do something about it without any care for computer instance to be returned back to you again.
myComputer.let {
myGrandpa.installVirusOn(it)
}
The Way to Look At It
I tend to look at also and let as something which is external, outside. Whenever you say these two words, it's like you try to act up on something. let install virus on this computer, and also crash it. So this nails down the part of whether you're an actor or not.
For the result part, it's clearly there. also expresses that it's also another thing, so you still retain the availability of object itself. Thus it returns it as a result.
Everything else associates with this. Additionally run/with clearly doesn't interest in return object-self back. Now you can differentiate all of them.
I think sometimes when we step away from 100% programming/logic-based of examples, then we are in better position to conceptualize things. But that depends right :)
There are 6 different scoping functions:
T.run
T.let
T.apply
T.also
with
run
I prepared a visual note as the below to show the differences :
data class Citizen(var name: String, var age: Int, var residence: String)
Decision depends on your needs. The use cases of different functions overlap, so that you can choose the functions based on the specific conventions used in your project or team.
Although the scope functions are a way of making the code more concise, avoid overusing them: it can decrease your code readability and lead to errors. Avoid nesting scope functions and be careful when chaining them: it's easy to get confused about the current context object and the value of this or it.
Here is another diagram for deciding which one to use from https://medium.com/#elye.project/mastering-kotlin-standard-functions-run-with-let-also-and-apply-9cd334b0ef84
Some conventions are as the following :
Use also for additional actions that don't alter the object, such as logging or printing debug information.
val numbers = mutableListOf("one", "two", "three")
numbers
.also { println("The list elements before adding new one: $it") }
.add("four")
The common case for apply is the object configuration.
val adam = Person("Adam").apply {
age = 32
city = "London"
}
println(adam)
If you need shadowing, use run
fun test() {
var mood = "I am sad"
run {
val mood = "I am happy"
println(mood) // I am happy
}
println(mood) // I am sad
}
If you need to return receiver object itself, use apply or also
let, also, apply, takeIf, takeUnless are extension functions in Kotlin.
To understand these function you have to understand Extension functions and Lambda functions in Kotlin.
Extension Function:
By the use of extension function, we can create a function for a class without inheriting a class.
Kotlin, similar to C# and Gosu, provides the ability to extend a class
with new functionality without having to inherit from the class or use
any type of design pattern such as Decorator. This is done via special
declarations called extensions. Kotlin supports extension functions
and extension properties.
So, to find if only numbers in the String, you can create a method like below without inheriting String class.
fun String.isNumber(): Boolean = this.matches("[0-9]+".toRegex())
you can use the above extension function like this,
val phoneNumber = "8899665544"
println(phoneNumber.isNumber)
which is prints true.
Lambda Functions:
Lambda functions are just like Interface in Java. But in Kotlin, lambda functions can be passed as a parameter in functions.
Example:
fun String.isNumber(block: () -> Unit): Boolean {
return if (this.matches("[0-9]+".toRegex())) {
block()
true
} else false
}
You can see, the block is a lambda function and it is passed as a parameter. You can use the above function like this,
val phoneNumber = "8899665544"
println(phoneNumber.isNumber {
println("Block executed")
})
The above function will print like this,
Block executed
true
I hope, now you got an idea about Extension functions and Lambda functions. Now we can go to Extension functions one by one.
let
public inline fun <T, R> T.let(block: (T) -> R): R = block(this)
Two Types T and R used in the above function.
T.let
T could be any object like String class. so you can invoke this function with any objects.
block: (T) -> R
In parameter of let, you can see the above lambda function. Also, the invoking object is passed as a parameter of the function. So you can use the invoking class object inside the function. then it returns the R (another object).
Example:
val phoneNumber = "8899665544"
val numberAndCount: Pair<Int, Int> = phoneNumber.let { it.toInt() to it.count() }
In above example let takes String as a parameter of its lambda function and it returns Pair in return.
In the same way, other extension function works.
also
public inline fun <T> T.also(block: (T) -> Unit): T { block(this); return this }
extension function also takes the invoking class as a lambda function parameter and returns nothing.
Example:
val phoneNumber = "8899665544"
phoneNumber.also { number ->
println(number.contains("8"))
println(number.length)
}
apply
public inline fun <T> T.apply(block: T.() -> Unit): T { block(); return this }
Same as also but the same invoking object passed as the function so you can use the functions and other properties without calling it or parameter name.
Example:
val phoneNumber = "8899665544"
phoneNumber.apply {
println(contains("8"))
println(length)
}
You can see in the above example the functions of String class directly invoked inside the lambda funtion.
takeIf
public inline fun <T> T.takeIf(predicate: (T) -> Boolean): T? = if (predicate(this)) this else null
Example:
val phoneNumber = "8899665544"
val number = phoneNumber.takeIf { it.matches("[0-9]+".toRegex()) }
In above example number will have a string of phoneNumber only it matches the regex. Otherwise, it will be null.
takeUnless
public inline fun <T> T.takeUnless(predicate: (T) -> Boolean): T? = if (!predicate(this)) this else null
It is the reverse of takeIf.
Example:
val phoneNumber = "8899665544"
val number = phoneNumber.takeUnless { it.matches("[0-9]+".toRegex()) }
number will have a string of phoneNumber only if not matches the regex. Otherwise, it will be null.
You can view similar answers which is usefull here difference between kotlin also, apply, let, use, takeIf and takeUnless in Kotlin
According to my experience, since such functions are inline syntactic sugar with no performance difference, you should always choose the one that requires writing the least amount of code in the lamda.
To do this, first determine whether you want the lambda to return its result (choose run/let) or the object itself (choose apply/also); then in most cases when the lambda is a single expression, choose the ones with the same block function type as that expression, because when it's a receiver expression, this can be omitted, when it's a parameter expression, it is shorter than this:
val a: Type = ...
fun Type.receiverFunction(...): ReturnType { ... }
a.run/*apply*/ { receiverFunction(...) } // shorter because "this" can be omitted
a.let/*also*/ { it.receiverFunction(...) } // longer
fun parameterFunction(parameter: Type, ...): ReturnType { ... }
a.run/*apply*/ { parameterFunction(this, ...) } // longer
a.let/*also*/ { parameterFunction(it, ...) } // shorter because "it" is shorter than "this"
However, when the lambda consists of a mix of them, it's up to you then to choose the one that fits better into the context or you feel more comfortable with.
Also, use the ones with parameter block function when deconstruction is needed:
val pair: Pair<TypeA, TypeB> = ...
pair.run/*apply*/ {
val (first, second) = this
...
} // longer
pair.let/*also*/ { (first, second) -> ... } // shorter
Here is a brief comparison among all these functions from JetBrains's official Kotlin course on Coursera Kotlin for Java Developers:
I must admit that the difference is not so obvious at first glance, among other things because these 5 functions are often interchangeable. Here is my understanding :
APPLY -> Initialize an object with theses properties and wait for the object
val paint = Paint().apply {
this.style = Paint.Style.FILL
this.color = Color.WHITE
}
LET -> Isolate a piece of code and wait for the result
val result = let {
val b = 3
val c = 2
b + c
}
or
val a = 1
val result = a.let {
val b = 3
val c = 2
it + b + c
}
or
val paint: Paint? = Paint()
paint?.let {
// here, paint is always NOT NULL
// paint is "Paint", not "Paint?"
}
ALSO -> Execute 2 operations at the same time and wait for the result
var a = 1
var b = 3
a = b.also { b = a }
WITH -> Do something with this variable/object and don't wait for a result (chaining NOT allowed )
with(canvas) {
this.draw(x)
this.draw(y)
}
RUN -> Do something with this variable/object and don't wait for a result (chaining allowed)
canvas.run {
this.draw(x)
this.draw(y)
}
or
canvas.run {this.draw(x)}.run {this.draw(x)}