Develop Reference

Custom Components Sorting based on required Ordering

Trying to sort the custom components on defined order, and other subjects should be after that desired order: Like if there is any other subject except the defined List ie "GK" then it should be on last position etc.
However I am getting Null Pointer Exception due to subject is not defined in the requireList if scheduleCommandList have the subject which is not in requiredList. How can I overcome this?
Desired Order List is Below:
private val requiredList: HashMap<String, Int> = hashMapOf(
"Maths" to 0,
"Physics" to 1,
"Science" to 2,
)
Sorting function to sort the List:
private fun sortCommandList(scheduleCommandList: ArrayList<BaseComponent>): ArrayList<BaseComponent> {
val comparator = Comparator { o1: BaseComponent, o2: BaseComponent ->
return#Comparator requiredList[o1.name]!! - requiredList[o2.name]!!
}
val copy = arrayListOf<BaseComponent>().apply { addAll(scheduleCommandList) }
copy.sortWith(comparator)
return copy
}

It seems you understand the problem correctly. If an item is not present in requiredList then you still try to compare their required positions and this causes NullPointerException. Remember that you should use !! only in cases when you are sure there can't be a null. In this case null is possible and we have to handle it somehow. The easiest is to replace it with Int.MAX_VALUE which places the item at the end. Also, this code can be really much simpler:
private fun sortCommandList(scheduleCommandList: List<BaseComponent>): List<BaseComponent> {
return scheduleCommandList.sortedBy { requiredList[it.name] ?: Int.MAX_VALUE }
}
It can be even better to create this utility as extension function:
private fun List<BaseComponent>.mySort(): List<BaseComponent> {
return sortedBy { requiredList[it.name] ?: Int.MAX_VALUE }
}
Then we can simplify the name of the function, because it is implicit that it is used to sort BaseComponent objects.

get the value of a parameter from a data class in kotlin

So, i'm pretty new to kotlin and still learning stuff, I have a data class named Country with 4 parameters
County(name:String, policePhone:String, ambulancePhone:String,
firebrigadePhone:String)
, a listOf Country with 27 objects in it and a var nameC1 taken from the MainActivity.
I've called the list method forEach and I want to confront every name in the list with the variable nameC and when a match is found execute some code.
data class Country(val name: String, val police:String, val ambulance:String,val firefighter:String) {
}
var nameC1 = (activity as MainActivity).nameC
val numberList= listOf<Country>(
Country("Austria","133","144","122"),
Country("Belgium","101","100","100"),
Country("Bulgaria","166","150","160"),
Country("Croatia","192","194","193"),
Country("Cyprus","199","199","199"),
Country("Czech Republic","158","155","150"),
Country("Denmark","112","112","112"),
Country("Estonia","112","112","112"),
Country("Finland","112","112","112"),
Country("France","17","15","18"),
Country("Germany","110","112","112"),
Country("Greece","100","166","199"),
Country("Hungary","107","104","105"),
Country("Ireland","112","112","112"),
Country("Italy","113","118","115"),
Country("Latvia","112","112","112"),
Country("Lithuania","02","03","01"),
Country("Luxembourg","113","112","112"),
Country("Malta","112","112","112"),
Country("Netherlands","112","112","112"),
Country("Poland","997","999","998"),
Country("Portugal","112","112","112"),
Country("Romania","112","112","112"),
Country("Slovakia","158","155","150"),
Country("Slovenia","113","112","112"),
Country("Spain","092","061","080"),
Country("Sweden","112","112","112")
)
numberList.forEach { if (Country.name==nameC1 ) }
// i'm expecting String1==String2 but i'm
//stuck here because it says name is an unresolved reference
}
I'd use a getName() but i know in kotlin getter/setter are automated ( I'm not used to it) and ihaven't found anything useful on the kotlin doc. site,
I've seen on this site that someone suggested to implement Kotlin-reflection but I don't understand how I'm not supposed to get a parameter from a class by default.

forEach creates a lambda for each of the element in the collection. The default name for the element inside the lambda is it. But you can rename it to something else too. Refer to the doc
Here is a working example of your code
data class Country(val name: String, val police:String, val ambulance:String,val firefighter:String)
fun doThis(nameC1: String) {
val numberList= listOf<Country>(
Country("Austria","133","144","122"),
Country("Belgium","101","100","100"),
Country("Bulgaria","166","150","160"),
Country("Croatia","192","194","193"),
Country("Cyprus","199","199","199"),
Country("Czech Republic","158","155","150"),
Country("Denmark","112","112","112"),
Country("Estonia","112","112","112"),
Country("Finland","112","112","112"),
Country("France","17","15","18"),
Country("Germany","110","112","112"),
Country("Greece","100","166","199"),
Country("Hungary","107","104","105"),
Country("Ireland","112","112","112"),
Country("Italy","113","118","115"),
Country("Latvia","112","112","112"),
Country("Lithuania","02","03","01"),
Country("Luxembourg","113","112","112"),
Country("Malta","112","112","112"),
Country("Netherlands","112","112","112"),
Country("Poland","997","999","998"),
Country("Portugal","112","112","112"),
Country("Romania","112","112","112"),
Country("Slovakia","158","155","150"),
Country("Slovenia","113","112","112"),
Country("Spain","092","061","080"),
Country("Sweden","112","112","112") )
numberList.forEach {
if (it.name == nameC1) {
println("Match")
}
}
}
fun main() {
doThis("Slovenia")
}
Try it for yourself on play.kotlinlang.org - Link
The above code will execute the println function when the condition is true.

In the forEach loop you have to use it to access the name parameter.
like this
numberList.forEach { if (it.name==nameC1 )}

Try with the following code. You can apply filter on list
//if you want iterate your list try with below code
numberList.forEach {
val name = it.name
val police = it.police
}
//If you want apply filter on list take reference from below code
private var countryList: ArrayList<Country> = arrayListOf(
Country("Austria", "133", "144", "122"),
Country("Belgium", "101", "100", "100")
)
val searchList = countryList.filter { country-> country.name == nameC1}

Kotlin: How can I reduce child arrays into a single array?

I have a pice of code in Swift that reduces a list of TVSchedule objects into an array of TVMatch pobjects. Each TVSchedule, has a property called events, that is a list of TVMatches.
The code in swift is the following:
var matches: [TVMatch] {
let slots = timeSlots.reduce(into: [TVMatch]()) { (result, schedule) in
result.append(contentsOf: schedule.events)
}
return slots
}
I'm trying to do the same reduce in Kotlin and the code I have is the following:
val matches: ArrayList<TVMatch>
get() {
val slots = timeSlots.fold(arrayListOf<TVMatch>()) { result, schedule ->
result.addAll(schedule.events)
}
return slots
}
However, the Kotlin code gives me a type error, and does not compile. What is the problem here?

addAll returns a boolean, but the return value of the fold-operation should be of same type as the given initial object (in this case ArrayList).
You can solve that one easily by just adding result after your addAll-statement, e.g.:
result.addAll(schedule.events)
result // this is now the actual return value of the fold-operation
Alternatively just use apply or similar instead:
result.apply {
addAll(schedule.events)
} // result is the return value then
Note that you can actually simplify altogether using flatMap to just (side-note: if you use this approach the matches are evaluated only once of course, but flatMap is the star here anyway ;-))):
val matches = timeSlots.flatMap { it.events } // this is a new list! (note, if you do several mappings in a row, you may want to use timeSlots.asSequence().flatMap { }.map { }.toList() / or .toMutableList() instead
Alternatively if you really require the matches to be of type ArrayList, use flatMapTo instead:
val matches = timeSlots.flatMapTo(ArrayList()) { it.events }
You can of course keep the get() if you must, or just move the getting of the matches to its own function, e.g.:
fun getMatches() = timeSlots.flatMapTo(ArrayList()) { it.events }

Am I crazy, or can't you just replace the code with
val matches: List<TVMatch>
get() = timeSlots.flatMap { schedule -> schedule.events }
?

Combine the latest values of a nested Observable on demand

I have some custom fields that each have a value that can be retrieved using a BehaviorSubject<Value>. What fields are shown are based on what I get from the API, so in the end I have n amount of BehaviorSubject<Value>s. I would like to group these values together into an Observable<List<Value>> where the list contains latest values from these fields (order is irrelevant). The problem however is that these fields are not all available at the same time because they're created while the UI loads so I cannot use Observable.combineLatest with the list of subjects.
What I currently have done is I have created the following variable:
private val values = BehaviorSubject.create<Pair<Int, Value>>()
I use this subject to subscribe to all of the field's subjects but map the subjects first with their position and make a pair of it.
fieldSubject.map {
Pair(position, value)
}.subscribe(values)
What I then want to do is group the values based on their position in the pair and get an Observable<List<Value>> where the list contains the latest values from each position. However I don't know how to proceed after grouping them using groupBy:
values.groupBy {
it.first
}
This results in an Observable<GroupedObservable<Pair, Value>>>. Finally this is how I think I should get to Observable<List<Value>>, but I don't know what to do from here.

groupBy doesn't seem helpful to me here.
Accumulating values is generally done using scan, here you can use a transformation like:
values.scanWith({ arrayOfNulls<Value>(N) }) { acc, (index, value) ->
acc.copyOf().apply {
set(index, value)
}
}
.map { it.filterNotNull() }
You could probably get away without copyOf(), but I think I encountered problems in the past when the accumulator function wasn't pure.
By the way, you can write position to value instead of Pair(position, value).
Also, you can use merge to get an Observable<Pair<Int, Value>> instead of creating a BehaviorSubject and subscribing it manually to all fields:
Observable.mergeArray(
fieldX.map { 0 to it },
fieldY.map { 1 to it },
fieldZ.map { 2 to it }
// ...
)
So overall, you could have a function that does it all for you:
inline fun <reified T : Any> accumulateLatest(vararg sources: Observable<out T>): Observable<List<T>> {
return Observable.merge(sources.mapIndexed { index, observable ->
observable.map { index to it }
})
.scanWith({ arrayOfNulls<T>(sources.size) }) { acc, (index, value) ->
acc.copyOf().apply {
set(index, value)
}
}
.map { it.filterNotNull() }
}
And then just call:
accumulateLatest(fieldX, fieldY, fieldZ)
.subscribe {
println("Latest list: $it")
}

How to return member values in a array of objects using lambda expressions

I have an array of "Dog" where i want to print the name of all dogs older then 5 years.
I tried something like
Dogs.filter{ it.age > 5 }.forEach { it.name }
This gives me the value i need, but how do I store and return it as a list of strings?
I tried things like adding .join(",") but since I don't get any array in return it wont work.

I think you're looking for the map operator:
val dogNames: List<String> = dogs.filter { it.age > 5 }.map { it.name }