I have a bunch of lookup tables indexed by key that I would like instantiate lazily (i.e. the tables are expensive to compute and I only expect some of them to be used on any given execution of the code).
private var myLazyMap: Map<KeyClass, TableClass> by lazy { ...}
Doesn't work as that makes the map object itself lazy, which isn't right. I think I may need to write a custom delegate, but I still can't see how to embed that into the map object.
I could wrap TableClass with something like
class LazyTable(val param: TableClassParameter) {
private var table: TableClass by lazy { TableClass(param) }
fun wrappedTableFun(): ResultClass {
return table.tableFun()
}
}
But this does mean the class is wrong and it feels like a hack. Can this be done in a neater way?
It could be implemented in multiple ways, depending on your needs. Probably the easiest is to use a map of lazy values directly:
val map = mutableMapOf<KeyClass, Lazy<TableClass>>()
map[myKey1] = lazy { createTable1() }
map[myKey2] = lazy { createTable2() }
val table = map[myKey1]?.value
If we want to not expose Lazy to the users of the map, we need to create our own LazyMap. One way is to use a map similar to above and just hide Lazy from the user:
class LazyMap<K, V>(
private val map: Map<K, Lazy<V>>
) {
operator fun get(key: K): V? = map[key]?.value
}
Another solution is to use a function that creates values when needed:
class LazyMap<K, V>(
private val compute: (K) -> V
) {
private val map = mutableMapOf<K, V>()
operator fun get(key: K): V? = map.getOrPut(key) { compute(key) }
}
We can also use a separate compute function per each key, as in the answer by #tibtof .
A partial implementation could be something like this:
class LazyMap<K, V>(val lazyVals: Map<K, () -> V>, val cache: MutableMap<K, V> = mutableMapOf()) : Map<K, V> by cache {
companion object {
fun <K, V> lazyMapOf(vararg entries: Pair<K, () -> V>): LazyMap<K, V> = LazyMap(mapOf(*entries))
}
override fun get(key: K): V? = cache[key] ?: lazyVals[key]?.let { cache[key] = it(); cache[key] }
}
fun main() {
val lazyMap = lazyMapOf(
1 to { println("computing one"); "one" },
2 to { println("computing two"); "two" }
)
println("get 2")
println(lazyMap[2])
println("get 1")
println(lazyMap[1])
println("get 0")
println(lazyMap[0])
println("get 2 again")
println(lazyMap[2])
}
An we can observe the lazyness in the output:
get 2
computing two
two
get 1
computing one
one
get 0
null
get 2 again
two
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.
In javascript we can do something like this
function putritanjungsari(data){
console.log(data.name)
}
let data = {
name:"putri",
div:"m4th"
}
putritanjungsari(data)
In kotlin, i'am creating a function that accept an object as parameter then read it's properties later, how to do that in kotlin that targeting JVM?
If I understood your question correct, you are trying to have a variable that associates keys with some value or undefined(null in kt) if none are found. You are searching for a Map
If you don't know what types you want, you can make a map of type Any? So
Map<String, Any?>
Which is also nullable
Map<String, Any>
If you don't want nullables
Your code for example:
fun putritanjungsari(data: Map<String, Any?>){
print(data["name"])
}
val data: Map<String, Any?> =mapOf(
"name" to "putri",
"div" to "m4th"
)
putritanjungsari(data)
Note that you can't add new keys or edit any data here, the default map is immutable. There is MutableMap (which is implemented the same, only it has a method to put new data)
You can apply the property design pattern to solve your problem.
Here is its implementation in Kotlin:
interface DynamicProperty<T> {
fun cast(value: Any?): T
fun default(): T
companion object {
inline fun <reified T> fromDefaultSupplier(crossinline default: () -> T) =
object : DynamicProperty<T> {
override fun cast(value: Any?): T = value as T
override fun default(): T = default()
}
inline operator fun <reified T> invoke(default: T) = fromDefaultSupplier { default }
inline fun <reified T> required() = fromDefaultSupplier<T> {
throw IllegalStateException("DynamicProperty isn't initialized")
}
inline fun <reified T> nullable() = DynamicProperty<T?>(null)
}
}
operator fun <T> DynamicProperty<T>.invoke(value: T) = DynamicPropertyValue(this, value)
data class DynamicPropertyValue<T>(val property: DynamicProperty<T>, val value: T)
class DynamicObject(vararg properties: DynamicPropertyValue<*>) {
private val properties = HashMap<DynamicProperty<*>, Any?>().apply {
properties.forEach { put(it.property, it.value) }
}
operator fun <T> get(property: DynamicProperty<T>) =
if (properties.containsKey(property)) property.cast(properties[property])
else property.default()
operator fun <T> set(property: DynamicProperty<T>, value: T) = properties.put(property, value)
operator fun <T> DynamicProperty<T>.minus(value: T) = set(this, value)
}
fun dynamicObj(init: DynamicObject.() -> Unit) = DynamicObject().apply(init)
You can define your properties these ways:
val NAME = DynamicProperty.required<String>() // throws exceptions on usage before initialization
val DIV = DynamicProperty.nullable<String>() // has nullable type String?
val IS_ENABLED = DynamicProperty(true) // true by default
Now you can use them:
fun printObjName(obj: DynamicObject) {
println(obj[NAME])
}
val data = dynamicObj {
NAME - "putri"
DIV - "m4th"
}
printObjName(data)
// throws exception because name isn't initialized
printObjName(DynamicObject(DIV("m4th"), IS_ENABLED(false)))
Reasons to use DynamicObject instead of Map<String, Any?>:
Type-safety (NAME - 3 and NAME(true) will not compile)
No casting is required on properties usage
You can define what the program should do when a property isn't initialized
Kotlin is statically typed language, so it required a param type to be precisely defined or unambiguously inferred (Groovy, for instance, addresses the case by at least two ways). But for JS interoperability Kotlin offers dynamic type.
Meanwhile, in your particular case you can type data structure to kt's Map and do not argue with strict typing.
You have to use Any and after that, you have to cast your object, like this
private fun putritanjungsari(data : Any){
if(data is Mydata){
var data = data as? Mydata
data.name
}
}
Just for the sake of inspiration. In Kotlin, you can create ad hoc objects:
val adHoc = object {
var x = 1
var y = 2
}
println(adHoc.x + adHoc.y)
I am experimenting with Lenses in Kotlin, and I was wondering if there is an elegant way to change multiple attributes at the same time for one object. Let's say my domain looks something like this:
#optics
data class Parameters(
val duration: Int,
val length: Int) {
companion object
}
#optics
data class Calculation(
val product: String
val parameters: Parameters) {
companion object
}
thanks to the #optics annotations, editing single fields is easy to do:
val calculation = Calculation(product = "prod", Parameters(duration = 10, length = 15))
Calculation.product.modify(calculation) { selectedProduct }
Calculation.parameters.duration(calculation) { newDuration() }
Calculation.parameters.length(calculation) { 10 }
These lenses work perfectly in isolation, but what is the right pattern to use when I want to apply the three transformations at once? I can use a var and just overwrite calculation every time, but that does not feel very idiomatic to me.
Arrow currently does not expose such functionality but you can easily write a generic solution yourself.
The snippet below demonstrates how it can be achieved, you can add additional methods to compose from Lens<S, Tuple2<FA, FB>> to Lens<S, Tuple3<FA, FB, FC>> etc.
#optics data class Char(val name: String, val health: Int) {
companion object
}
infix fun <S, FA, FB> Lens<S, FA>.aside(other: Lens<S, FB>): Lens<S, Tuple2<FA, FB>> = object : Lens<S, Tuple2<FA, FB>> {
override fun get(s: S): Tuple2<FA, FB> = Tuple2(this#aside.get(s), other.get(s))
override fun set(s: S, b: Tuple2<FA, FB>): S = other.set(this#aside.set(s, b.a), b.b)
}
fun main() {
val original = Char("", 0)
val charName: Lens<Char, String> = Char.name
val charHealth: Lens<Char, Int> = Char.health
val charNameAndHealth: Lens<Char, Tuple2<String, Int>> = charName.aside(charHealth)
charNameAndHealth.modify(original) { Tuple2("Test", 30) }
}