kotlinlang #getting-started

In 1.0.x the type inference of that method is just broken, but in 1.1 we've intentionally decided to restrict the return type of lambda, so that it's the same as the type of map values.

opas90

01/17/2017, 2:44 AM

@ilya.gorbunov i am using kotlin 1.0.6

ilya.gorbunov

01/17/2017, 2:48 AM

You can use

compute

function instead:

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cache.compute(agentId, { id, value -> value ?: getUserFromDB(agentId) })

opas90

01/17/2017, 2:51 AM

wouldn't that execute the lambda everytime?

ilya.gorbunov

01/17/2017, 2:52 AM

Yes it would, but it returns almost immediately when value is not null

opas90

01/17/2017, 2:58 AM

@cedric making the value as String? will work, but it looks odd that value can be null for a given key even though it won't

ilya.gorbunov

01/17/2017, 3:15 AM

@opas90 Btw, do you know that some requests to the

cache

might be blocked while getUserFromDB is executing?

cedric

01/17/2017, 3:17 AM

"that" or "if"?

cedric

01/17/2017, 3:17 AM

Sounds like

cache

is @opas90's own cache implementation?

ilya.gorbunov

01/17/2017, 3:22 AM

It's a ConcurrentHashMap

opas90

01/17/2017, 3:26 AM

@cedric since it is a concurrentHashMap it locks only a certain portion for updates and the reason for using computeIfAbsent to do everything (check then act) in one atomic operation since i am using any kind of specific synchronization

opas90

01/17/2017, 3:26 AM

am not using*

cedric

01/17/2017, 3:26 AM

Yes,

computeIfAbsent

is the right method

opas90

01/17/2017, 3:32 AM

@cedric @ilya.gorbunov thank you guys for your feedback, i will proceed with String? as the value type for now, it will work for me since i am not exposing it outside, the only thing concerned me i am using String? when i know it is String (in Kotlin) 😞

cedric

01/17/2017, 3:32 AM

But you don't know it's

String

, since the look up can fail, correct?

opas90

01/17/2017, 3:33 AM

yes, in that case the value wont be added the the hashmap

ilya.gorbunov

01/17/2017, 3:33 AM

You can declare your map as

ConcurrentHashMap<Long, String>

and cast it to

ConcurrentHashMap<Long, String?>

before calling computeIfAbsent.

opas90

01/17/2017, 3:34 AM

so the cache will never have a null value even though the look up may return null when not found

opas90

01/17/2017, 3:35 AM

@ilya.gorbunov i tried that but then i have unchecked cast warning

opas90

01/17/2017, 3:36 AM

so it is better to use String?

ilya.gorbunov

01/17/2017, 3:37 AM

unchecked cast warning

Yes, but it's the case when it's safe to ignore or suppress it, as you know that computeIfAbsent would never put null into map.

miha-x64

01/17/2017, 8:21 AM

@okkero there are just ideas how that could be syntactically achieved, of course this is not valid. simple smile

okkero

01/17/2017, 8:22 AM

Oh, right. I misunderstood then. :)

inaction

01/17/2017, 9:24 AM

Hi, I’m a rookie about kotlin, and I have a question about null-safety. Here is an example:

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class People(n: String) {
    val name = n
}

class Test {
    var me: People? = null
    
    fun initial() {
    	me = People("Inaction")
	}
    
    fun hi(name: String) {
        println(name)
    }
    
    fun doSomething() {
        initial()
        me?.let {
            hi(me.name)
        }
    }
}

fun main(args: Array<String>) {
    Test().doSomething()
}

Check the codes in

doSomething

function, and I got the error:

Smart cast to 'People' is impossible, because 'me' is a mutable property that could have been changed by this time

So what should I do to solve this error?

uhe

01/17/2017, 9:26 AM

hi(it.name)

inaction

01/17/2017, 9:27 AM

wow, it works, thank you! @uhe

inaction

01/17/2017, 9:27 AM

so basically

kotlin

always put an

it

in the context, right? @uhe

uhe

01/17/2017, 9:28 AM

if you want to know more about these kinds of things: http://beust.com/weblog/2015/10/30/exploring-the-kotlin-standard-library/

inaction

01/17/2017, 9:28 AM

ok, thanks man

miha-x64

01/17/2017, 9:29 AM

The short answer is no,

it

is default name of lambda’s single argument.