kotlinlang #getting-started

class LazyProperty(val initializer: () -> Int) {
    private val lazyValue: Int? = null
        get() {
            if (field == null) field = initializer()
            return field
}
    val lazy: Int
        get() = lazyValue!!
}

elect

09/30/2016, 10:51 AM

what is the connection between the

initializer

and the Int?

elect

09/30/2016, 10:51 AM

I don't get that

elect

09/30/2016, 10:52 AM

In the test class there is the following:

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var initialized = false
        val lazyProperty = LazyProperty({ initialized = true; 42 })
        assertFalse("Property shouldn't be initialized before access", initialized)
        val result: Int = lazyProperty.lazy
        assertTrue("Property should be initialized after access", initialized)
        assertEquals(42, result)

elect

09/30/2016, 10:53 AM

I wonder

elect

09/30/2016, 10:54 AM

is the initializer the whole function

initialized = true; 42

yole

09/30/2016, 10:56 AM

the

initializer

parameter is a function that takes no parameters and returns an

Int

yole

09/30/2016, 10:56 AM

the lambda

{ initialized = true; 42 }

is passed as the argument for that parameter

👍 1

elect

09/30/2016, 11:08 AM

which are in general the reasons behind lazy initialization?

yole

09/30/2016, 11:10 AM

you use lazy initialization when something takes a long time to initialize and is only accessed for some instances of the class

voddan

09/30/2016, 11:51 AM

I don't see any

assertDoubleWithPressision

in stdlib. Is there any?

voddan

09/30/2016, 12:15 PM

I remember there was aYT ticket about that, but I can't find it now 😞

elect

09/30/2016, 1:07 PM

what's the

===

here?

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fun compareStrings(s1: String?, s2: String?) {
    s1 == s2
    // is compiled to
    s1?.equals(s2) ?: s2 === null
}

spand

09/30/2016, 1:08 PM

reference comparison: https://kotlinlang.org/docs/reference/equality.html

👍 1

elect

09/30/2016, 2:29 PM

in the last koans (here https://github.com/Kotlin/kotlin-koans/blob/resolutions/src/vi_generics/_41_GenericFunctions.kt), I don' get why it has to be:

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<T, C: MutableCollection<T>> Collection<T>.partitionTo

instead a simple

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MutableCollection<T>.partitionTo

yole

09/30/2016, 2:30 PM

it doesn't have to be, but the solution in the koans gives you more flexibility than the one using a single type parameter

elect

09/30/2016, 2:42 PM

fun <T> T.whatever

, does the part

<T>

have a specific technical name?

yole

09/30/2016, 2:42 PM

it's a type parameter

elect

09/30/2016, 2:44 PM

that's

, I really mean the whole leading part

<T>

pawegio

09/30/2016, 2:51 PM

diamond operator?

kevinmost

09/30/2016, 2:53 PM

no, that's

<>

in Java 7+

pawegio

09/30/2016, 3:00 PM

so maybe target type?

elect

09/30/2016, 3:01 PM

maybe it doesnt have a name, however isn't still possible even in Kotlin doing something like

(condition ? first : second).add(e)

elect

09/30/2016, 3:02 PM

where

first

and

second

are collections in this case for example

paulblessing

09/30/2016, 3:07 PM

Kotlin doesn’t have the ternary operator but

if/else

can be used as an expression:

(if (condition) first else second).add(e)

👍 1

ilya.gorbunov

09/30/2016, 3:09 PM

https://kotlinlang.slack.com/archives/getting-started/p1475246540001105 It's called "type parameter list".

👍 2

kqr

09/30/2016, 10:45 PM

hi all

kqr

09/30/2016, 10:46 PM

lets say I have a class which takes date in string as input but I want it to be represented as Date in class

kqr

09/30/2016, 10:46 PM

I cannot find right combination of init val get set

andrewoma

09/30/2016, 10:54 PM

If by input, you mean in the constructor, then something like this:

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class Foo(dateString: String) {
    val date = LocalDate.parse(dateString)
}