Lets assume the following when-statement:
when(a)
{
x -> doNothing()
y -> doSomething()
else -> doSomethingElse()
}
Now i'm looking to eliminate the boilerplate-function "doNothing()", e.g.:
x -> //doesn't compile
x -> null //Android Studio warning: Expression is unused
x -> {} //does work, but my corporate codestyle places each '{‘ in a new line, looking terrible
//also, what is this actually doing?
Any better ideas?
I can't just eliminate x -> completely, as that would lead to else -> doSthElse()
Edit: directly after writing this Question, i figured out a possible answer x -> Unit. Any shortcomings with that?
Kotlin has two existing possibilities to express a "do nothing" construct in when statements. Either Unit or an empty pair of braces. An empty block will just execute nothing.
There's nothing else planned in that regard (see here).
To answer your question regarding "also, what is this actually doing?" for the empty block, looking at the bytecode and translating it into Java helps:
val x = 33
when(x)
{
1 -> {}
2 -> Int
3 -> Unit
else -> Double
}
Translates to
int x = 33;
switch(x) {
case 1:
case 3:
break;
case 2:
IntCompanionObject var10000 = IntCompanionObject.INSTANCE;
break;
default:
DoubleCompanionObject var1 = DoubleCompanionObject.INSTANCE;
}
Related
For example, Char has a member function isLetter(). Is there any way to call it in a case? The code below does not work.
var ch:Char = null;
when(ch)
{
'*' -> print("You typed an asterisk.");
isLetter() -> print("You typed a letter.");
else -> print("You typed something.");
}
The uses of the parameter to when are quite restrictive: equality, order with < &c, inclusion with in, type checking with is, and that's about it. So for anything more complex, it's usually easier not to specify a parameter. In this case, the following isn't much less concise or clear:
when {
ch == '*' -> print("You typed an asterisk.")
ch.isLetter() -> print("You typed a letter.")
else -> print("You typed something.")
}
Some points worth noting:
Semicolons are not needed in Kotlin (except for a few rare ambiguous cases such as putting multiple statements on a line — which is rarely a good idea).
print() does not add a trailing newline, which can cause problems; println() is more common.
var ch: Char = null won't compile, because the type Char is not nullable. Either give a non-null default value, or make it nullable by specifying the type as Char?. However, in the latter case, you won't be able to call ch.isLetter(), because that would risk a NullPointerException. So you'd either need to add a null case before that, or handle the null in that check, either with ch != null && ch.isLetter(), or ch?.isLetter() == true — both of which are ugly.
There is, but there's also the possibility of it not being applicable in your situation.
when can be used without an argument, like this:
fun main() {
val ch: Char = 'd'
when {
ch.isLetter() -> println("It's a letter")
ch.isDigit() -> println("It's a number")
else -> println("It's neither a letter, nor a number")
}
}
But if you make ch nullable like var ch: Char? = null, you won't be able to call member functions in that when clause.
You can check whether ch is in a specific CharCategory. For isLetter() those are the following five:
when(ch)
{
'*' -> print("You typed an asterisk.")
in CharCategory.UPPERCASE_LETTER,
in CharCategory.LOWERCASE_LETTER,
in CharCategory.TITLECASE_LETTER,
in CharCategory.MODIFIER_LETTER,
in CharCategory.OTHER_LETTER -> print("You typed a letter.")
else -> print("You typed something.")
}
first in your code var ch:Char = null
it must be Char?
the solution is simple don't put argument:
var ch:Char = 'c'
when {
ch == '*' -> println("*")
ch.isLetter() -> println("letter")
ch.isDigit() -> println("digit")
}
hope my answer helped you
In Java, it is possible to do ->
someList.stream().map(x -> {
y = doSomeOperation(x);
z = doSomeOtherOperation(y);
return z;
}).collect(Collectors.toList());
I need to convert above code to Kotlin. But in all the online tutorials, I am learning that it is only possible to have simple lambdas, like x -> x*x or x->doSomethingThenReturnValue(x).
Is it not possible to write a complex lambda (which does some complex inline operation) like above in kotlin? I tried writing ->
someList.map{ x -> {
y = doSomeOperation(x);
z = doSomeOtherOperation(y);
return z;
}}
But it threw error. Can you please tell what would be the correct way to do it?
I think the problem here is return z. When placed inside lambda it returns from the enclosing function, unlike in Java where it only returns from the lambda itself. So you should write either
someList.map { x ->
y = doSomeOperation(x)
z = doSomeOtherOperation(y)
z
}
or
someList.map { x ->
y = doSomeOperation(x)
z = doSomeOtherOperation(y)
return#map z
}
More details on "return" issue can be found here - https://kotlinlang.org/docs/reference/returns.html
You have written too many brackets. In kotlin, parameter definition for lambdas are set inside the bracket (see reference documentation).
EDIT: Also, return statement in lambdas is not always allowed, and when it is, its behavior is really specific. More information in official documentation.
So, your example needs to be rewritten as following:
someList.map { x ->
val y = doSomeOperation(x)
val z = doSomeOtherOperation(y)
// z implicitely returned as lambda result
z
}
I have some code that roughly looks like this:
val myObject = myObjectRepository.findById(myObjectId);
when {
matchesSomething(myObject) -> doSomethingWithMyObject(myObject)
matchesSomethingElse(myObject) -> doSomethingElseWithMyObject(myObject)
else -> log.warn("No match, aborting");
}
While this works I would think that the following (which doesn't work) would be an improvement if I only need access to myObject inside the scope of when:
when(myObjectRepository.findById(myObjectId)) { myObject ->
matchesSomething(myObject) -> doSomethingWithMyObject(myObject)
matchesSomethingElse(myObject) -> doSomethingElseWithMyObject(myObject)
else -> log.warn("No match, aborting");
}
The error I get here is:
Unresolved reference: myObject
Can you do something like this in Kotlin and if so how? If not, is there a particular reason for why this shouldn't be allowed?
As shown in the documentation, the proper syntax would be
val myObject = myObjectRepository.findById(myObjectId);
when {
matchesSomething(myObject) -> doSomethingWithMyObject(myObject)
matchesSomethingElse(myObject) -> doSomethingElseWithMyObject(myObject)
else -> log.warn("myObject not found, aborting")
}
Or, to actually match what your first snippet does:
val myObject = myObjectRepository.findById(myObjectId);
when(myObject) {
null -> log.warn("myObject not found, aborting");
matchesSomething(myObject) -> doSomethingWithMyObject(myObject)
matchesSomethingElse(myObject) -> doSomethingElseWithMyObject(myObject)
}
You have to be careful about the syntax. In a while we use an arrow -> which has nothing to do with lambdas. I think this is what you were trying in your example.
The only valid syntax for when is this:
when (x) {
1 -> print("x == 1")
2 -> print("x == 2")
else -> { // Note the block
print("x is neither 1 nor 2")
}
On the left side of the arrow -> you declare what the object (x) is being matched against, whereas on the right side you tell what will be executed in that case. Read about this here.
In your example you tried to chain multiple -> which does not work.
This is supported as of Kotlin 1.3. It's referred to as "Capturing when subject in a variable" and looks like this (taken from their documentation):
fun Request.getBody() =
when (val response = executeRequest()) {
is Success -> response.body
is HttpError -> throw HttpException(response.status)
}
I'm trying to use the WHEN clause with a > or < comparison.
This doesn't compile. Is there a way of using the normal set of boolean operators (< <=, >= >) in a comparison to enable this?
val foo = 2
// doesn't compile
when (foo) {
> 0 -> doSomethingWhenPositive()
0 -> doSomethingWhenZero()
< 0 -> doSomethingWhenNegative()
}
I tried to find an unbounded range comparison, but couldn't make this work either? Is it possible to write this as an unbounded range?
// trying to get an unbounded range - doesn't compile
when (foo) {
in 1.. -> doSomethingWhenPositive()
else -> doSomethingElse()
}
You can put the whole expression in the second part, which is OK but seems like unnecessary duplication. At least it compiles and works.
when {
foo > 0 -> doSomethingWhenPositive()
foo < 0 -> doSomethingWhenNegative()
else -> doSomethingWhenZero()
}
But I'm not sure that is any simpler than the if-else alternative we have been doing for years. Something like:
if ( foo > 0 ) {
doSomethingWhenPositive()
}
else if (foo < 0) {
doSomethingWhenNegative()
}
else {
doSomethingWhenZero()
}
Of course, real world problems are more complex than the above, and the WHEN clause is attractive but doesn't work as I expect for this type of comparison.
Even a flexible language such as Kotlin doesn't have a "elegant" / DRY solution for each and every case.
You can write something like:
when (foo) {
in 0 .. Int.MAX_VALUE -> doSomethingWhenPositive()
0 -> doSomethingWhenZero()
else -> doSomethingWhenNegative()
}
But then you depend on the variable type.
I believe the following form is the most idiomatic in Kotlin:
when {
foo > 0 -> doSomethingWhenPositive()
foo == 0 -> doSomethingWhenZero()
else -> doSomethingWhenNegative()
}
Yeah... there is some (minimal) code duplication.
Some languages (Ruby?!) tried to provide an uber-elegant form for any case - but there is a tradeoff: the language becomes more complex and more difficult for a programmer to know end-to-end.
My 2 cents...
We can use let to achieve this behaviour.
response.code().let {
when {
it == 200 -> handleSuccess()
it == 401 -> handleUnauthorisedError()
it >= 500 -> handleInternalServerError()
else -> handleOtherErrors()
}
}
Hope this helps
The grammar for a when condition is as follows:
whenCondition (used by whenEntry)
: expression
: ("in" | "!in") expression
: ("is" | "!is") type
;
This means that you can only use is or in as special cases that do not have to be a full expression; everything else must be a normal expression. Since > 0 is not a valid expression this will not compile.
Furthermore, ranges are closed in Kotlin, so you cannot get away with trying to use an unbounded range.
Instead you should use the when statement with a full expression, as in your example:
when {
foo > 0 -> doSomethingWhenPositive()
foo < 0 -> doSomethingWhenNegative()
else -> doSomethingWhenZero()
}
Or alternatively:
when {
foo < 0 -> doSomethingWhenNegative()
foo == 0 -> doSomethingWhenZero()
foo > 0 -> doSomethingWhenPositive()
}
which may be more readable.
You want your code to be elegant, so why stay on the when expression. Kotlin is flexible enough to build a new one using extension.
First we should claim that we can only pass a Comparable<T> here because you have to compare the value.
Then, we have our framework:
fun <T: Comparable<T>> case(target: T, tester: Tester<T>.() -> Unit) {
val test = Tester(target)
test.tester()
test.funFiltered?.invoke() ?: return
}
class Tester<T : Comparable<T>>(val it: T) {
var funFiltered: (() -> Unit)? = null
infix operator fun Boolean.minus(block: () -> Unit) {
if (this && funFiltered == null) funFiltered = block
}
fun lt(arg: T) = it < arg
fun gt(arg: T) = it > arg
fun ge(arg: T) = it >= arg
fun le(arg: T) = it <= arg
fun eq(arg: T) = it == arg
fun ne(arg: T) = it != arg
fun inside(arg: Collection<T>) = it in arg
fun inside(arg: String) = it as String in arg
fun outside(arg: Collection<T>) = it !in arg
fun outside(arg: String) = it as String !in arg
}
After that we can have elegant code like:
case("g") {
(it is String) - { println("hello") } // normal comparison, like `is`
outside("gg") - { println("gg again") } // invoking the contains method
}
case(233) {
lt(500) - { println("less than 500!") }
// etc.
}
If you're happy, you can rename the minus function to compareTo and return 0. In such way, you can replace the - with =>, which looks like scala.
Mo code that works:
val fishMan = "trouttrout"
when (fishMan.length){
0 -> println("Error")
in (3..12) -> println("Good fish name")
else -> println ("OK fish name")
}
Result:
Good fish name
I use this:
val foo = 2
when (min(1, max(-1, foo))) {
+1 -> doSomethingWhenPositive()
0 -> doSomethingWhenZero()
-1 -> doSomethingWhenNegative()
}
The imports needed for this case are:
import java.lang.Integer.min
import java.lang.Integer.max
but they can be generalized to other types.
You're welcome!
I found a bit hacky way that can help you in mixing greater than, less than, or any other expression with other in expressions.
Simply, a when statement in Kotlin looks at the "case", and if it is a range, it sees if the variable is in that range, but if it isn't, it looks to see if the case is of the same type of the variable, and if it isn't, you get a syntax error. So, to get around this, you could do something like this:
when (foo) {
if(foo > 0) foo else 5 /*or any other out-of-range value*/ -> doSomethingWhenPositive()
in -10000..0 -> doSomethingWhenBetweenNegativeTenKAndZero()
if(foo < -10000) foo else -11000 -> doSomethingWhenNegative()
}
As you can see, this takes advantage of the fact that everything in Kotlin is an expression. So, IMO, this is a pretty good solution for now until this feature gets added to the language.
Probably a little bit broad question, but the official documentation doesn't even mentioning the arrow operator (or language construct, I don't know which phrase is more accurate) as an independent entity.
The most obvious use is the when conditional statement, where it is used to assign an expression to a specific condition:
val greet = when(args[0]) {
"Appul" -> "howdy!"
"Orang" -> "wazzup?"
"Banan" -> "bonjur!"
else -> "hi!"
}
println(args[0] +" greets you: \""+ greet +"\"")
What are the other uses, and what are they do?
Is there a general meaning of the arrow operator in Kotlin?
The -> is part of Kotlin's syntax (similar to Java's lambda expressions syntax) and can be used in 3 contexts:
when expressions where it separates "matching/condition" part from "result/execution" block
val greet = when(args[0]) {
"Apple", "Orange" -> "fruit"
is Number -> "How many?"
else -> "hi!"
}
lambda expressions where it separates parameters from function body
val lambda = { a:String -> "hi!" }
items.filter { element -> element == "search" }
function types where it separates parameters types from result type e.g. comparator
fun <T> sort(comparator:(T,T) -> Int){
}
Details about Kotlin grammar are in the documentation in particular:
functionType
functionLiteral
whenEntry
The -> is a separator. It is special symbol used to separate code with different purposes. It can be used to:
Separate the parameters and body of a lambda expression
val sum = { x: Int, y: Int -> x + y }
Separate the parameters and return type declaration in a function type
(R, T) -> R
Separate the condition and body of a when expression branch
when (x) {
0, 1 -> print("x == 0 or x == 1")
else -> print("otherwise")
}
Here it is in the documentation.
From the Kotlin docs:
->
separates the parameters and body of a lambda expression
separates the parameters and return type declaration in a function
type
separates the condition and body of a when expression branch