I'm trying to handle CompletableFuture exceptions in Kotlin, but I'm not able to figure out how to supply the appropriate parameters. So, for example, I have:
CompletableFuture.runAsync { "sr" }
.exceptionally{e -> {}}
but then the compiler complains Cannot infer type parameter T.
How do I fix this?
Quite a tricky case which becomes tricky because of some Kotlin magic :)
The direct solution to your problem would be the following code:
CompletableFuture.runAsync {"sr"}
.exceptionally({e -> null})
The detailed explanation goes here:
The runAsync method accepts a Runnable which means after execution it will return Void. The function passed to exceptionally method must match the generic parameter of the CompletableFuture so in this particular case, you need to help a compiler by returning null explicitly.
So the following will compile without problems:
CompletableFuture.runAsync {"sr"}
.exceptionally({null})
CompletableFuture.runAsync {}
.exceptionally({null})
In the first case, the "sr" String will simply be ignored and not returned since the runAsync accepts a Runnable.
You probably wanted to do something like:
CompletableFuture.supplyAsync {"sr"}
.exceptionally({"sr_exceptional"})
or:
CompletableFuture.supplyAsync {"sr"}
.exceptionally({e -> "sr_exceptional"})
Related
When would we ever need with in Kotlin if we can already use apply, run, also and let?
Can anyone give me a clear example?
In most situations, a with call can be transformed to a run like this:
with(foo) {
// some code ...
}
// is the same as:
foo.run {
// the same code ...
}
run and with will both return the lambda result, and will use foo as the lambda receiver.
However, I can think of one case where this wouldn't work - when foo declares its own run method that takes a lambda, e.g.
// having something like this isn't too uncommon, right?
fun run(x: () -> Unit) {}
The lambda type doesn't have to be exactly the same as the scope function run. Any function type should work. Then overload resolution wouldn't resolve to the built-in run.
You can force the resolution by doing some casts, but using with in this case is much better. Don't you agree?
I don’t think there’s any better example than with(context). Maybe it’s not clear if English isn’t one of your primary languages, but it semantically is translated into English much clearer than context.run when the object is being used to produce a result but isn’t the primary actor, so it makes code a little easier to read.
This of course raises the question of why run exists. Well, it semantically makes more sense in English when the object is the thing doing the action. In English, the context of an action is what you’re doing something with. But if the object is what is directly producing the result, then it is running the action.
Also, you can’t do ?.with.
I know there was documented in main kotlin page, but there is no clear explanation about when to use it, why this function need a receiver as a function. What would be the correct way to create a correct definition of inline function.
This is inline function
inline fun String?.toDateString(rawDateFormat: String = MMMM_DD_YYYY, outputDate: String = MM_DD_YYYY, block: (date: String) -> String): String {
return try {
var sdf = SimpleDateFormat(rawDateFormat, Locale.US)
val date = sdf.parse(this.orEmpty())
sdf = SimpleDateFormat(outputDate, Locale.US)
block(sdf.format(date ?: Date()).orEmpty())
} catch (ex: Exception) {
block("")
}
}
The same way we also can do
inline fun String?.toDateString(rawDateFormat: String = MMMM_DD_YYYY, outputDate: String = MM_DD_YYYY): String {
return try {
var sdf = SimpleDateFormat(rawDateFormat, Locale.US)
val date = sdf.parse(this.orEmpty())
sdf = SimpleDateFormat(outputDate, Locale.US)
sdf.format(date ?: Date()).orEmpty()
} catch (ex: Exception) {
""
}
}
If anyone could have a detail explanation about this?
Edit:
I understand that the inline function will insert the code whenever it called by the compiler. But this come to my attention, when I want to use inline function without functional parameter receiver type the warning show as this in which should have a better explain. I also want to understand why this is such recommendation.
There are few things here.
First, you ask about using a function with a receiver. In both cases here, the receiver is the String? part of String?.toDateString(). It means that you can call the function as if it were a method of String, e.g. "2021-01-15 12:00:00".toDateString(…).
The original String? is accessible as this within the function; you can see it in the sdf.parse(this.orEmpty()) call. (It's not always as obvious as this; you could simply call sdf.parse(orEmpty()), where the this. is implied.)
Then you ask about inline functions. All you have to do is to mark the function as inline, and the compiler will automatically insert its code wherever it's called, instead of defining a function in the usual way. But you don't need to worry about how it's implemented; there are just a few visible effects in the code. In particular, if a function is inline and accepts a function parameter, then its lambda can do a few things (such as calling return) that it couldn't otherwise do.
Which leads us to what I think is your real question: about the block function parameter. Your first example has this parameter, with the type (date: String) -> String — i.e. a function taking a single String parameter and returning another String. (The technical term for this is that toDateString() is a higher-order function.)
The toDateString() function calls this block function before returning, applying it to the date string it has formatted before returning it to the caller.
As to why it does this, it's hard to tell. That's why we put documentation comments before functions: to explain anything that's not obvious from the code! Ideally, there would be a comment explaining why you're required to supply a block lamdba (or function reference), when it's not vital to what the function does.
There are times when blocks passed this way are very useful. For example, the joinToString() function accepts an optional transform parameter, which it applies to each item before joining it to the list. If it didn't, the effect would be a lot more awkward to obtain. (You'd probably have to apply a map() to the collection before calling joinToString(), which would be less efficient.)
But this isn't one of those times. As your second example shows, toDateString() would work perfectly well without the block parameter — and then if you needed to pass the result through another function, you could just call it on toDateString()'s result.
Perhaps if you included a link to the ‘main kotlin page’ where you saw this, it might give some more context?
The edited question also asks about the IDE warning. This is shown when it thinks inlining a function won't give a significant improvement.
When no lambdas are involved, the only potential benefit from inlining a function is performance, and that's a trade-off. It might avoid the overhead of a function call wherever it's called — but the Java runtime will often inline small functions anyway, all on its own. And having the compiler do the inlining comes at the cost of duplicating the function's code everywhere it's called; the increased code size is less likely to fit into memory caches, and less likely to be optimised by the Java runtime — so that can end up reducing the performance overall. Because this isn't always obvious, the IDE gives a warning.
It's different when lambdas are involved, though. In that case, inlining affects functionality: for example, it allows non-local returns and reified type parameters. So in that case there are good reasons for using inline regardless of any performance implications, and the IDE doesn't give the warning.
(In fact, if a function calls a lambda it's passed, inlining can have a more significant performance benefit: not only does the function itself get inlined, but the lambda itself usually does as well, removing two levels of function call — and the lambda is often called repeatedly, so there can be a real saving.)
Example:
data class T(val flag: Boolean) {
constructor(n: Int) : this(run {
// Some computation here...
<Boolean result>
})
}
In this example, the custom constructor needs to run some computation in order to determine which value to pass to the primary constructor, but the compiler does not accept the run, citing Cannot access 'run' before superclass constructor has been called, which, if I understand correctly, means instead of interpreting it as the non-extension run (the variant with no object reference in https://kotlinlang.org/docs/reference/scope-functions.html#function-selection), it construes it as a call to this.run (the variant with an object reference in the above table) - which is invalid as the object has not completely instantiated yet.
What can I do in order to let the compiler know I mean the run function which is not an extension method and doesn't take a scope?
Clarification: I am interested in an answer to the question as asked, not in a workaround.
I can think of several workarounds - ways to rewrite this code in a way that works as intended without calling run: extracting the code to a function; rewriting it as a (possibly highly nested) let expression; removing the run and invoking the lambda (with () after it) instead (funnily enough, IntelliJ IDEA tags that as Redundant lambda creation and suggests to Inline the body, which reinstates the non-compiling run). But the question is not how to rewrite this without using run - it's how to make run work in this context.
A good answer should do one of the following things:
Explain how to instruct the compiler to call a function rather than an extension method when a name is overloaded, in general; or
Explain how to do that specifically for run; or
Explain that (and ideally also why) it is not possible to do (ideally with supporting references); or
Explain what I got wrong, in case I got something wrong and the whole question is irrelevant (e.g. if my analysis is incorrect, and the problem is something other than the compiler construing the call to run as this.run).
If someone has a neat workaround not mentioned above they're welcome to post it in a comment - not as an answer.
In case it matters: I'm using multi-platform Kotlin 1.4.20.
Kotlin favors the receiver overload if it is in scope. The solution is to use the fully qualified name of the non-receiver function:
kotlin.run { //...
The specification is explained here.
Another option when the overloads are not in the same package is to use import renaming, but that won't work in this case since both run functions are in the same package.
There are some ways to fulfill a null-checking in Kotlin:
1.
if(myVar != null) {
foo(myVar)
}
2.
myVar?.let {
foo(it)
}
3.
myVar?.run {
foo(this)
}
What are the difference between these ways?
Are there any reasons (performance, best practice, code style etc.) why I should prefer on way over the other?
!! is to tell the compiler that I am sure the value of the variable is not null, and if it is null throw a null pointer exception (NPE) where as ?. is to tell the compiler that I am not sure if the value of the variable is null or not, if it is null do not throw any null pointer.
Another way of using a nullable property is safe call operator ?.
This calls the method if the property is not null or returns null if that property is null without throwing an NPE (null pointer exception).
nullableVariable?.someMethodCall()
All three code are behave same null check in operation-wise.
?. is used for chain operations.
bob?.department?.head?.name // if any of the properties in it is null it returns null
To perform a chain operation only for non-null values, you can use the safe call operator together with let
myVar?.let {
foo(it)
}
the above code is good for code style and performance
more details refer Null Safety
The ways 2 and 3 are more idiomatic for Kotlin. Both functions are quite similar. There is little difference with argument passing.
For example, we have a nullable variable:
var canBeNull: String? = null
When you working with T.run you work with extension function calling and you pass this in the closure.
canBeNull?.run {
println(length) // `this` could be omitted
}
When you call T.let you can use it like lambda argument it.
canBeNull?.let {
myString -> println(myString.length) // You could convert `it` to some other name
}
A good article about Kotlin standard functions.
All three are roughly equivalent.
The if case is more like most other languages, and so many developers may find it easier to read.
However, one difference is that the if case will read the value of myVar twice: once for the check, and again when passing it to foo(). That makes a difference, because if myVar is a property (i.e. something that could potentially be changed by another thread), then the compiler will warn that it could have been set to null after the check. If that's a problem (e.g. because foo() expects a non-null parameter), then you'll need to use one of the other cases.
For that reason, the let case has become fairly common practice in Kotlin. (The run case does just about the same thing, but for some reason isn't as popular for this sort of thing. I don't know why.)
Another way around it is to assign myVar to a temporary value, test that, and then use that. That's also more like other languages, but it's more verbose; many people prefer the conciseness of the let case — especially when myVar is actually a complicated expression.
The examples in your question don't show the true reason to decide.
First of all, since you're not using the return value of foo, you should use neither let nor run. Your choice is between also and apply.
Second, since you already have the result you want to null-check in a variable, the difference fades. This is a better motivating example:
complexCall(calculateArg1(), calculateArg2())?.also {
results.add(it)
}
as opposed to
val result = complexCall(calculateArg1(), calculateArg2())
if (result != null) {
results.add(result)
}
The second example declares an identifier, result, which is now available to the rest of the lexical scope, even though you're done with it in just one line.
The first example, on the other hand, keeps everything self-contained and when you go on reading the rest of the code, you are 100% confident that you don't have to keep in mind the meaning of result.
Kotlin have new features with NullPoint-Exception as Compare to Java.
Basically When we do Coding in Java , then we have to Check with !! in every Flied.
But in Kotlin, it is Easy way to Implement First
as Like,
Suppose, in Kotlin
var response:Json?=Null
response:Json?.let {
this part will handle automatic if response is Not Null....then this Block start Executing }?.run {
This is Nullable But, where we Can put Warring } So, I am Suggest you Guys to Start Work in Kotlin with this Features Provided by Kotlin.
(Flied)?.let { Not Null Value Comes Under }?.run{ Null Value Code }
This will Handle to NullPoint Exception or Protect You App for Crash
What you want to achieve
What you want to achieve is that the Kotlin compiler does a smart cast on the variable you are working with.
In all of your three examples, the compiler can do that.
Example:
if(myVar != null) {
foo(myVar) // smart cast: the compiler knows, that myVar can never be null here
}
The choice
Which one of the options to use, is really a matter of style. What you should not do is mix it up to often. Use one and stick to it.
You don't need to worry about performance since let and run are inlined (see inline function). This means that their code (body) is copied to the call site at compile time so there is no runtime overhead.
How can i make an arraylist of functions, and call each function easily? I have already tried making an ArrayList<Function<Unit>>, but when i tried to do this:
functionList.forEach { it }
and this:
for(i in 0 until functionList.size) functionList[i]
When i tried doing this: it() and this: functionList[i](), but it wouldn't even compile in intellij. How can i do this in kotlin? Also, does the "Unit" in ArrayList<Function<Unit>> mean return value or parameters?
Just like this:
val funs:List<() -> Unit> = listOf({}, { println("fun")})
funs.forEach { it() }
The compiler can successfully infer the type of funs here which is List<() -> Unit>. Note that () -> Unit is a function type in Kotlin which represents a function that does not take any argument and returns Unit.
I think there are two problems with the use of the Function interface here.
The first problem is that it doesn't mean what you might think. As I understand it, it's a very general interface, implemented by all functions, however many parameters they take (or none). So it doesn't have any invoke() method. That's what the compiler is complaining about.
Function has several sub-interfaces, one for each 'arity' (i.e. one for each number of parameters): Function0 for functions that take no parameters, Function1 for functions taking one parameter, and so on. These have the appropriate invoke() methods. So you could probably fix this by replacing Function by Function0.
But that leads me on to the second problem, which is that the Function interfaces aren't supposed to be used this way. I think they're mainly for Java compatibility and/or for internal use by the compiler.
It's usually much better to use the Kotlin syntax for function types: (P1, P2...) -> R. This is much easier to read, and avoids these sorts of problems.
So the real answer is probably to replace Function<Unit> by () -> Unit.
Also, in case it's not clear, Kotlin doesn't have a void type. Instead, it has a type called Unit, which has exactly one value. This might seem strange, but makes better sense in the type system, as it lets the compiler distinguish functions that return without an explicit value, from those which don't return. (The latter might always throw an exception or exit the process. They can be defined to return Nothing -- a type with no values at all.)