The data class of Mcqs look like this:
data class Mcqss(
var answer: String,
val mcqs: String,
val option1: String,
val option2: String,
val option3: String,
val option4: String,
var topicId: String,
var sequence: String,
)
True false data class:
data class tf(
val answer: String,
val question: String,
val topicId: String,
val sequence: String,
)
Quiz data class:
data class quiz(
var topicId: String,
var sequence: String,
var mcq_question:String,
var trf_question:String
)
Function to combine two lists:
fun <T, U> combine(first: ArrayList<Mcqss>, second: ArrayList<tf>): MutableList<Any> {
val list: MutableList<Any> = first.map { i -> i }.toMutableList()
list.addAll(second.map { i -> i })
return list
}
But when I execute this line it gives me a class cast exception:
val joined: ArrayList<quiz> = combine<Any,Any>(mcqlist, tfs) as ArrayList<quiz>
for (item in joined) {
item.sequence
}
Any suggestions please.
Please try next code:
fun combine(first: ArrayList<Mcqss>, second: ArrayList<Tf>): ArrayList<Quiz> {
// I assume the sizes of `first` and `second` lists are the same
require(first.size == second.size)
val result = mutableListOf<Quiz>()
for (i in 0 until first.size) {
val quiz = Quiz(first[i].topicId, first[i].sequence, ...)
result.add(quiz)
}
return ArrayList(result)
}
val joined: ArrayList<Quiz> = combine(mcqlist, tfs)
I would recommend to name classes starting with a capital letter, e.g. quiz->Quiz.
val mcqlist: List<Mcqss> = ...
val tfs: List<TrueFalse> = ...
val joined = mcqlist + tfs
for (item in joined) {
if (item is Mcqss) {
println(item.option1)
} else if (item is TrueFalse) {
println(item.question)
}
}
I want to sort a list based on their latitude and longitude...
Here is my code:
import java.util.*
import com.google.gson.GsonBuilder
import java.io.File
import java.io.InputStream
import java.util.Comparator
data class Property(val Pcode: Int, val Locality: String, val State: String, val Comments: String, val Category: String, val Longitude: Double, val Latitude: Double)
class SortPlaces(currentLatitude: Double, currentLongitude: Double) : Comparator<Property> {
var currentLat: Double
var currentLng: Double
override fun compare(property1: Property, property2: Property): Int {
val lat1: Double = property1.Latitude
val lon1: Double = property1.Longitude
val lat2: Double = property2.Latitude
val lon2: Double = property2.Longitude
val distanceToPlace1 = distance(currentLat, currentLng, lat1, lon1)
val distanceToPlace2 = distance(currentLat, currentLng, lat2, lon2)
return (distanceToPlace1 - distanceToPlace2).toInt()
}
fun distance(fromLat: Double, fromLon: Double, toLat: Double, toLon: Double): Double {
val radius = 6378137.0 // approximate Earth radius, *in meters*
val deltaLat = toLat - fromLat
val deltaLon = toLon - fromLon
val angle = 2 * Math.asin(
Math.sqrt(
Math.pow(Math.sin(deltaLat / 2), 2.0) +
Math.cos(fromLat) * Math.cos(toLat) *
Math.pow(Math.sin(deltaLon / 2), 2.0)
)
)
return radius * angle
}
init {
currentLat = currentLatitude
currentLng = currentLongitude
}
}
fun main(args: Array<String>) {
val command = Scanner(System.`in`)
val running = true
while (running) {
val inputStream: InputStream = File("./src/main/kotlin/suburbs.json").inputStream()
val inputString = inputStream.bufferedReader().use { it.readText() }
val gson = GsonBuilder().create()
val packagesArray = gson.fromJson(inputString , Array<Property>::class.java).toList()
println("Please enter a suburb name: ")
val suburbName = command.nextLine()
println("Please enter the postcode: ")
val postcode = command.nextLine()
val userProperty: Property? = packagesArray.find{ it.Locality.toLowerCase().equals(suburbName.toLowerCase()) && it.Pcode == postcode.toInt()}
//sort the list, give the Comparator the current location
Collections.sort(packagesArray, new SortPlaces(userProperty.Latitude, userProperty.Longitude));
}
command.close()
}
I got error: Too many arguments for public open fun <T : Comparable<T!>!> sort(list: (Mutable)List<T!>!): Unit defined in java.util.Collections
at my sort{} function
my userProperty has to be Property? because the find{} method return Property?
Then Collections.sort() can not sort Property? type because the SortPLaces only accept Comparator not Comparator<Property?>
What should I do?
There are multiple errors in your code. To create a new object in Kotlin, you don't write the word new like you do in Java. Also, as you have noticed, find returns a nullable type - Property?. You need to check for nulls when using userProperty. A Property matching the criteria you want may not necessarily be found, after all.
if (userProperty != null) {
Collections.sort(packagesArray, SortPlaces(userProperty.Latitude, userProperty.Longitude))
} else {
// no property is found! Think about what you should do in such a case
}
Since you are sorting the list in line, you should not make an immutable list with toList when you are deserialising the JSON, but rather a MutableList:
val packagesArray = gson.fromJson(inputString, Array<Property>::class.java).toMutableList()
Also, you seem to be using a lot of Java APIs. In Kotlin, a lot of the Java APIs that you are using have more idiomatic Kotlin counterparts. To sort the list, you don't need the SortPlaces class at all. Simply use sortBy on the array, and call your distance function in the lambda.
data class Property(
val pcode: Int,
val locality: String,
val state: String,
val comments: String,
val category: String,
val longitude: Double,
val latitude: Double,
)
fun distance(fromLat: Double, fromLon: Double, toLat: Double, toLon: Double): Double {
val radius = 6378137.0 // approximate Earth radius, *in meters*
val deltaLat = toLat - fromLat
val deltaLon = toLon - fromLon
val angle = 2 * asin(
sqrt(
sin(deltaLat / 2).pow(2.0) +
cos(fromLat) * cos(toLat) *
sin(deltaLon / 2).pow(2.0)
)
)
return radius * angle
}
fun main(args: Array<String>) {
val running = true
while (running) {
val inputStream = File("./src/main/kotlin/suburbs.json").inputStream()
val inputString = inputStream.bufferedReader().use { it.readText() }
val gson = GsonBuilder().create()
val packagesArray = gson.fromJson(inputString , Array<Property>::class.java).toMutableList()
println("Please enter a suburb name: ")
val suburbName = readLine()
println("Please enter the postcode: ")
val postcode = readLine()
val userProperty = packagesArray.find {
it.locality.lowercase() == suburbName?.lowercase() && it.pcode == postcode?.toInt()
}
//sort the list, give the Comparator the current location
if (userProperty != null) {
packagesArray.sortBy {
distance(userProperty.latitude, userProperty.longitude, it.latitude, it.longitude)
}
} else {
// did not find such a property!
}
}
}
I have never worked before with Kotlin so I have a newbie question. I am working with an existing codebase, so I am wondering about a few things. I see that there is a function getDepartmentById which looks like this:
fun getDepartmentById(ctx: Ctx, params: JsonObject): Either<Failure, FlatResp> =
getOneByIdFlattened(ctx, params.right(), getDepartmentByIdSql(ctx), flattenOne = flattenerToType(MainAccessType.DEPARTMENT))
fun getDepartmentById(ctx: Ctx, id: Long): Either<Failure, FlatResp> =
getDepartmentById(ctx, jsonObject("id" to id))
Calling that function returns either Failure or FlatResp. From what I can see in the code FlatResponse is typealias for Map<MainAccessType, Entities>.
The function getOneByIdFlattened looks like this:
fun getOneByIdFlattened(ctx: Ctx,
params: Either<Long, JsonObject>,
statement: String,
rowConverter: (Row) -> Map<String, Any?> = ::mapFromDbNames,
grouper: (List<Map<String, Any?>>) -> List<Map<String, Any?>> = ::identity,
flattenOne: (List<Map<String, Any?>>) -> FlatResp
): Either<Failure, FlatResp> =
either.eager {
val id = when (params) {
is Either.Left -> Either.Right(params.value)
is Either.Right -> params.value.idL?.right()
?: Failure.JsonError(SErr(GlowErrs.MISSING_ID, "You must provide id")).left()
}.bind()
val dbDataList: List<Map<String, Any?>> = doQuery(ctx, statement, mapOf(
"courierIds" to ctx.user.courierIds,
"id" to id,
"count" to 1,
"offset" to 0,
"departmentIds" to ctx.userDepartments,
"customerIds" to ctx.user.customerIds
),
rowConverter, false
).bind()
val result = flattenOne(grouper(dbDataList))
addUpdatedAtEpoch(result)
}
I wonder how can I get from a FlatResp a property of an object, that looks like this:
So, for example if I want to get just name from this object what would be the best way to do this?
Also, I wonder why is this function returning a collection, and not just a single object when it should get a single row by id from DB?
This is the sql function:
private fun getDepartmentByIdSql(ctx: Ctx) =
"""select ${createSelectFields(departmentKeys)}
from department dept
where dept.id = :id
${
when (ctx.user.role) {
UserRoles.ADMIN -> ""
else -> "and dept.id = any (:departmentIds) "
}
}"""
So there are a lot of things wrong in the snippet provided. Given Snippets:
1
fun getDepartmentById(ctx: Ctx, params: JsonObject): Either<Failure, FlatResp> =
getOneByIdFlattened(ctx, params.right(), getDepartmentByIdSql(ctx), flattenOne = flattenerToType(MainAccessType.DEPARTMENT))
2
fun getDepartmentById(ctx: Ctx, id: Long): Either<Failure, FlatResp> =
getDepartmentById(ctx, jsonObject("id" to id))
3
private fun getDepartmentByIdSql(ctx: Ctx) =
"""select ${createSelectFields(departmentKeys)}
from department dept
where dept.id = :id
${
when (ctx.user.role) {
UserRoles.ADMIN -> ""
else -> "and dept.id = any (:departmentIds) "
}
}"""
4
fun getOneByIdFlattened(ctx: Ctx,
params: Either<Long, JsonObject>,
statement: String,
rowConverter: (Row) -> Map<String, Any?> = ::mapFromDbNames,
grouper: (List<Map<String, Any?>>) -> List<Map<String, Any?>> = ::identity,
flattenOne: (List<Map<String, Any?>>) -> FlatResp
): Either<Failure, FlatResp> =
either.eager {
val id = when (params) {
is Either.Left -> Either.Right(params.value)
is Either.Right -> params.value.idL?.right()
?: Failure.JsonError(SErr(GlowErrs.MISSING_ID, "You must provide id")).left()
}.bind()
val dbDataList: List<Map<String, Any?>> = doQuery(ctx, statement, mapOf(
"courierIds" to ctx.user.courierIds,
"id" to id,
"count" to 1,
"offset" to 0,
"departmentIds" to ctx.userDepartments,
"customerIds" to ctx.user.customerIds
),
rowConverter, false
).bind()
val result = flattenOne(grouper(dbDataList))
addUpdatedAtEpoch(result)
}
Issues:
in snippet 2, jsonObject should be JsonObject(..)
I have no idea what the following lines do :
//snippet1:
flattenOne = flattenerToType(MainAccessType.DEPARTMENT))
//snipper 4
either.eager {...block...}
addUpdatedAtEpoch(result)
doQuery(ctx, statement, mapOf(..)
//snippet3
UserRoles.ADMIN -> ""
"""select ${createSelectFields(departmentKeys)}
They are all probably some extension functions or util files made by your company or from some famous libraries like anko orsplitties . plus these are mixes with function calls of your own class, like createSelectFields or ctx.user.courierIds. also if i have to guess, then this seems like an unusual way of performing some operation on an sql dB
based on just code completion by android studio, i have been able to figure out the classes as following:
class Entities
typealias FlatResp = Map<MainAccessType, Entities>
class Ctx
sealed class Either<A,B>(val a:A?, val b:B?){
val value:A? = null
class Left<A>(val aa:A):Either<A,A>(aa,aa)
class Right<B>(val bv:B):Either<B,B>(bb,bb)
}
class Failure
class Row
class JsonObject(val pair:Pair<String,Long>):JSONObject(){
fun right():Either<Long,JsonObject>{
}
}
class jsonObject()
enum class MainAccessType{DEPARTMENT}
fun getDepartmentById(ctx: Ctx, params: JsonObject): Either<Failure, FlatResp> {
return getOneByIdFlattened(
ctx,
params.right(),
getDepartmentByIdSql(ctx),
flattenOne = flattenerToType(MainAccessType.DEPARTMENT))
}
fun getDepartmentById(ctx: Ctx, id: Long): Either<Failure, FlatResp> {
return getDepartmentById(ctx, JsonObject("id" to id))
}
fun getOneByIdFlattened(ctx: Ctx,
params: Either<Long, JsonObject>,
statement: String,
rowConverter: (Row) -> Map<String, Any?> = ::mapFromDbNames,
grouper: (List<Map<String, Any?>>) -> List<Map<String, Any?>> = ::identity,
flattenOne: (List<Map<String, Any?>>) -> FlatResp
): Either<Failure, FlatResp> {
return either.eager {
val id = when (params) {
is Either.Left -> Either.Right(params.value)
is Either.Right -> params.value.idL?.right()
?: Failure.JsonError(SErr(GlowErrs.MISSING_ID, "You must provide id")).left()
}.bind()
val dbDataList: List<Map<String, Any?>> = doQuery(ctx, statement, mapOf(
"courierIds" to ctx.user.courierIds,
"id" to id,
"count" to 1,
"offset" to 0,
"departmentIds" to ctx.userDepartments,
"customerIds" to ctx.user.customerIds
),
rowConverter, false
).bind()
val result = flattenOne(grouper(dbDataList))
addUpdatedAtEpoch(result)
}
}
private fun getDepartmentByIdSql(ctx: Ctx) =
"""select ${createSelectFields(departmentKeys)}
from department dept
where dept.id = :id
${
when (ctx.user.role) {
UserRoles.ADMIN -> ""
else -> "and dept.id = any (:departmentIds) "
}
}"""
fun mapFromDbNames(row:Row): Map<String,Any?>{
}
fun identity(param : List<Map<String, Any?>>): List<Map<String, Any?>>{
}
This is still not correct and has a lots of red lines in it. but what you can do is keep this as a starter in a separate file, compare and fix the code accordingly and then maybe we can tell what would be a better way:
replace inline functions (fun xyz(...) = someValue ) to block functions. (alt+enter in windows, cmd+n in mac)
instead of typeAlias, use map directly
::something means a function is passed as parameter . its similar to how we pass runnables in java 8, but even more shorthand. you can do ctrl+click( for mac its cmd+click) on that function and goto that function to check what its params are, what its return type are. do the same for various classes/ extension fucntions, variables too. this will help the most
instead of passing something into something which is being passed into another thing (like val bot = Robot(Petrol("5Litres") ) ) , split into different lines to make it understandable ( val amount = "5litres"; val equipment = Petrol(amount) ; val bot = Robot(equipment) )
try to not use 3rd party library/ replace with your own understandable code.
repeat steps 1-5
Hope this gives someplace to start. kotlin is a beautiful language but is also very easy to make unreadable.
Mapping Map values
I wonder how can I get from a FlatResp a property of an object, that looks like this:
So, for example if I want to get just name from this object what would be the best way to do this?
TL;DR
Without data to work with, here's my guess:
val extractedNames: Map<Long, String?> = destinationDepartment
.mapValues { (_, userData: Map<String, Any?>) ->
when (val name = userData["name"]) {
is String -> name
else -> null
}
}
println(extractedNames)
// {1=Bergen, 2=Cindy, 3=Dave}
Intro
Kotlin is great for manipulating collections. For a more general of how to work with collections in Kotlin, I think the docs are really clear Collection transformation operations#Map.
Let's see how that works for this example. You want to extract a specific element, so for that we can use map().
From your screenshot it looks like this is a Map<Long, Map>, where the value is a Map<String, Any?>. I'll assume you want to change the Map<Long, Map> to a Map<Long, String>, where the key is the database ID and the value is user's name.
Test data
So I've got something to test with, I made a new Map:
val destinationDepartment: Map<Long, Map<String, Any?>> =
mapOf(
1L to mapOf(
"id" to 1,
"name" to "Bergen",
"createdAt" to LocalDateTime.now(),
"updatedAt" to LocalDateTime.now(),
),
2L to mapOf(
"id" to 2,
"name" to "Cindy",
"createdAt" to LocalDateTime.now(),
"updatedAt" to LocalDateTime.now(),
),
3L to mapOf(
"id" to 3,
"name" to "Dave",
"createdAt" to LocalDateTime.now(),
"updatedAt" to LocalDateTime.now(),
),
)
Basic noop
First, set up the basics. A Map can be converted to a list of Entries. When we call map(), it will iterates over each Entry, and applies a lambda - which is something we must write. In this instance, the lambda receives the key and value of the Map, and must return a new value.
Aside: the Java equivalent is map.entrySet().stream().map(...)...
Here, the lambda just returns a pair (created with to).
val extractedNames = destinationDepartment
.map { (id: Long, userData: Map<String, Any?>) ->
id to userData
}
println(extractedNames)
// Output: [(1, {id=1, name=Bergen, createdAt=2021-08-19T11:00:07.447660, updatedAt=2021-08-19T11:00:07.449969}),
// (2, {id=2, name=Cindy, createdAt=2021-08-19T11:00:07.463813, updatedAt=2021-08-19T11:00:07.463845}),
// (3, {id=3, name=Dave, createdAt=2021-08-19T11:00:07.463875, updatedAt=2021-08-19T11:00:07.463890})]
Pretty boring! But now we're set up for the next step - extracting name from userData: Map<String, Any?>.
Extracting name
val extractedNames = destinationDepartment
.map { (id: Long, userData: Map<String, Any?>) ->
val name = userData["name"]
id to name
}
println(extractedNames)
// Output: [(1, Bergen), (2, Cindy), (3, Dave)]
Now there's loads of ways to improve this. Making sure that name is a String, not Any?, filtering out blank or null names, mapping to DTOs, sorting. Again, the Kotlin documentation would be a good start. I'll start by listing one really good improvement.
Converting List<Pair<>> to Map<>
If you look at the type of val extractedNames, you'll see that it's a list, not a map.
val extractedNames: List<Pair<Long, Any?>> = ...
That's because the lambda we wrote in the map() function is returning a Pair<Long, String>. Kotlin doesn't know that this is still considered a Map. We can convert any List<Pair<>> back to a map with toMap()
toMap()
val extractedNames: Map<Long, Any?> = destinationDepartment
.map { (id: Long, userData: Map<String, Any?>) ->
val name = userData["name"]
id to name
}
.toMap() // convert List<Pair<>> to a Map<>
println(extractedNames)
// Output: {1=Bergen, 2=Cindy, 3=Dave}
But this is also not great. Why is id: Long in the lambda if we're not using it? Because we're only extracting the name from userData, we're only mapping the values of the Map. We don't need id: Long at all. Fortunately Kotlin has another useful method: mapValues() - and it returns a Map<>, so we can drop the toMap(). Let's use it.
mapValues()
val extractedNames: Map<Long, Any?> = destinationDepartment
.mapValues { (id: Long, userData: Map<String, Any?>) ->
val name = userData["name"]
id to name
}
println(extractedNames)
// {1=(1, Bergen), 2=(2, Cindy), 3=(3, Dave)}
Umm weird. Why are the ids in the values? That's because the mapValues() lambda should return the new value, and in our lambda we're returning both the id and name - oops! Let's only return the name.
Fixing mapValues()
val extractedNames: Map<Long, Any?> = destinationDepartment
.mapValues { (_, userData: Map<String, Any?>) ->
userData["name"]
}
println(extractedNames)
// {1=(1, Bergen), 2=(2, Cindy), 3=(3, Dave)}
Better! Because id is not used, an underscore can be used instead
Aside: Note that the lambda doesn't have a return. Read Returning a value from a lambda expression for an explanation.
I am playing with Kotlin and I am trying to convert a working Scala code to Kotlin. Everything seems to go pretty well but the compiler gives me this error and I dont know how to handle it.
Type mismatch: inferred type is Any but ExQuestion was expected for this line: return makeMap(questions, add2)
I am using a generic function because I need to access members of type A when building the map and the members would be visible through the lambda function provided.
Here's the code which you can copy into the Kotlin sandbox:
data class ExQuestion(val area: String, val rId: String, val text: String, val rIdAnswer: String, val line: Long)
fun main() {
fun <A> makeMap(list: List<A>, addValue: (A, MutableMap<String, A>) -> Unit): Map<String, A> {
val map = mutableMapOf<String, A>()
for( item in list) {
addValue(item, map)
}
return map
}
val add2: (ExQuestion, MutableMap<String, ExQuestion>) -> Unit =
{ question: ExQuestion, map: MutableMap<String, ExQuestion> ->
val key = question.rId
if (map[key] == null) {
map[key] = question
} else {
println("Id Frage mehrfach vorhanden - " + key)
}
}
val questions = listOf(ExQuestion("Area", "Q01", "text", "A01",1))
return makeMap(questions, add2)
}
Working code:
data class ExQuestion(val area: String, val rId: String, val text: String, val rIdAnswer: String, val line: Long)
fun main() {
fun <A> makeMap(list: List<A>, addValue: (A, MutableMap<String, A>) -> Unit): Map<String, A> {
val map = mutableMapOf<String, A>()
for( item in list) {
addValue(item, map)
}
return map
}
val add2: (ExQuestion, MutableMap<String, ExQuestion>) -> Unit =
{ question: ExQuestion, map: MutableMap<String, ExQuestion> ->
val key = question.rId
if (map[key] == null) {
map[key] = question
} else {
println("Id Frage mehrfach vorhanden - " + key)
}
}
val questions = listOf(ExQuestion("Area", "Q01", "text", "A01",1))
val map = makeMap(questions, add2)
println(map.values)
}
I'm not sure what your question is, but you can convert your list of questions to a map keyed on rId by doing:
val map = questions.map { it.rId to it }.toMap()
println(map)
Result:
{Q01=ExQuestion(area=Area, rId=Q01, text=text, rIdAnswer=A01, line=1)}
Update in response to comments.
You can achieve that without a mutable map by doing something like this:
val map = questions
.groupBy { it.rId }
.mapValues { (key, values) ->
if (values.size > 1) println("Id Frage mehrfach vorhanden - $key")
values.first()
}
However, I think your mutable map solution is fine and arguably clearer, so this is just for demonstration.