is it possible to get a "reference" to a HashMap e...

# announcements

[Karlo Lozovina]

is it possible to get a "reference" to a HashMap entry? something so that I can write something like this:

var f = fooMap["key"]
f = f+1

[Michael de Kaste]

no, properties of a map are immutable and need to be reinserted after modification.

What you could do is call the helper functions like:

map.computeIfPresent(3.0) { key: Double, value: Int ->
        value + 1
    }

[Karlo Lozovina]

Thanks

[Nikky]

you can get the reference to a MutableMap.MutableEntry<K,V>

val hashmap = mutableMapOf("key" to 1)
val entry = hashmap.entries.find { it.key == "key" }!!
val oldValue = entry.setValue(2)
println("oldValue: $oldValue") // oldValue: 1
println("map: $hashmap") // map: {key=2}

but i don't think you should

if the key is known you can also do

var key by hashmap
println("map: $hashmap") // map: {key=2}
key = 3
println("map: $hashmap") // map: {key=3}

the last bit works on all mutable maps via extensions in MapAccesorsKt

[Karlo Lozovina]

oh, this MutableEntry is interesting, might simplify some code... btw, is it possible to get somethin similar to a MutableEntry but for a class property thats Int or Double?

[Matteo Mirk]

if your class has a mutable property you just use standard syntax:

class Mutable {
  var i:Int = 0
}

val m = Mutable()
m.i = 1
println(m.i) // 1

If you need to introspect on a class like you wanted to do on maps, you have to use reflection, for example:

class C(val a: Int, var b: Double)

fun main() {
    val c = C(0, 0.0)
    val propB = C::class.declaredMemberProperties
            .find { it.name == "b" } as KProperty1<C, Double>?
    if (propB is KMutableProperty1) {
        propB.set(c, 3.14)
        println(c.b)
    }
}

but I don’t know honestly what you’re getting into… what is that you’re trying to achieve?

[Karlo Lozovina]

I was thinking more like (from your example)

var x = m.i
x = 1000
// m.i is also 1000 now

[Karlo Lozovina]

I know that doesn't work, but I was wondering if there was some syntax to get a "reference" to an Int field...

[Karlo Lozovina]

(I'm mostly porting something from Rust, so obviously lot's of patterns don't translate directly, I'm just messing around)

[Nikky]

you can do this

var x by m::i

and modifying x will modify m.i

[Karlo Lozovina]

oh, cool! didn't know about that feature... I did come across delegation, that's it?

[Karlo Lozovina]

how does that excatly work?

[Nikky]

::property gives you a KProperty or KMutablePRoperty and these extension functions exist in the stdlib: https://github.com/JetBrains/kotlin/blob/master/libraries/stdlib/src/kotlin/properties/PropertyReferenceDelegates.kt

at least i think that is how this works