I have a data class I want to populate, where in one constructor I have the data already, and in another I would like to fetch it only when it is required, which is rare.
A sample code would be:
data class Source1(val str1: String)
data class Source2(val str2: String)
data class DTO(val data1: String, val data2: String) {
// ctor which does not need laziness
constructor(source1: Source1) : this(
data1 = source1.str1,
data2 = source1.str1
)
// ctor which needs costly data
constructor(source2: Source2, costlyData: String) : this(
data1 = source2.str2,
data2 = costlyData
)
}
fun demo() {
val source1 = Source1("some str - 1")
DTO(source1)
val source2 = Source2("some str - 2")
val costlyData: String = costlyOperation() // this is the operation I'd like to execute lazily
DTO(source2, costlyData)
}
I'd say the easiest way would be to accept a function as a constructor parameter, something like this:
class DTO(provider:()->String){
constructor(data: String):this({data})
val data by lazy{ provider()}
}
So you can use it in both ways:
val eager = DTO("some str - 1")
val lazy = DTO(::costlyOperation)
A bit nicer way is to have a Source abstraction with different implementations for providing a constant value and performing an operation. But the overall idea would be the same.
Although I wouldn't call this DTO anymore and it loses its data class capabilities regarding the content.
Related
#file:Suppress("UNCHECKED_CAST")
data class Element<T>(
val key: String,
val valueOne: T,
val valueTwo: T,
val comparator: Comparator<T>,
val comparatorValue: CompareResult
)
enum class CompareResult(
val value: Int
) {
LESS(-1),
EQUAL(0),
GREATER_THAN(1)
}
fun <T> matchesComparison(list:Collection<Element<T>>): Pair<Boolean, List<String>> {
val failedComparisons = mutableListOf<String>()
for (element in list) {
val compareValue = element.comparator.compare(element.valueOne, element.valueTwo)
if (element.comparatorValue.value != compareValue) {
failedComparisons.add(element.key)
}
}
return Pair(failedComparisons.isEmpty(), failedComparisons)
}
val stringComparator = Comparator.comparing(String::toString)
val intComparator = Comparator.comparing(Int::toInt)
val elementsToCompare = listOf(
Element("number", 1, 2, intComparator, CompareResult.LESS),
Element("first name", "a", "a", stringComparator, CompareResult.EQUAL),
Element("last name", "a", "b", stringComparator, CompareResult.EQUAL)
)
matchesComparison(elementsToCompare).second.joinToString(", ","Failed elements: \"","\"")
I often get faced with comparing two different object properties with the same values.
As an example object A has props number,firstname,lastname. What i want to do is create a list have and have a function which goes over these Elements and returns which props have failed the comparison. I've managed to use generics for both the object and the matchesComparison function which returns the failed comparisons. The problem begins when i want to pass this list which is of type Collection<Element<out Any>> to this function is i get a type missmatch. instead of using unchecked casts to force the Comparator to be of type Any i would like to do this
val stringComparator = Comparator.comparing(String::toString)
val intComparator = Comparator.comparing(Int::toInt)
The result value that of the script above should be Failed elements: "last name"
I tried changing the signature of the function to out any but then the comparator.compare method has both params as of type Nothing. I really want to avoid unsing unchecked casts.
matchesComparison() doesn't need to be generic in this case. It doesn't really care what is the type of the whole input collection, so we can simply use * here.
Then we have another problem. The compiler isn't smart enough to notice that while we perform operations on a single element, all its properties are of matching types. As a result, it doesn't allow to use element.comparator on element.valueOne and element.valueTwo. To fix this problem, we simply need to create a separate function which works on a single Element, so it understand the type for all properties is the same:
fun matchesComparison(list:Collection<Element<*>>): Pair<Boolean, List<String>> {
fun <T> Element<T>.matches() = comparatorValue.value == comparator.compare(valueOne, valueTwo)
val failedComparisons = mutableListOf<String>()
for (element in list) {
if (!element.matches()) {
failedComparisons.add(element.key)
}
}
return Pair(failedComparisons.isEmpty(), failedComparisons)
}
Also, I believe such matches() function should be actually a member function of Element. It seems strange that while Element is pretty independent and it contains everything that is needed to perform a comparison, it still requires to use external code for this. If it would have a matches() function then we wouldn't need to care about its T. matches() would work with any Element.
How to get the string "hi" from the CharBuffer? toString() does not seem to work.
val a = CharBuffer.allocate(10);
a.put('h');
a.put('i');
val b = a.toString();
Variable states after running the code above:
CharBuffer is pretty low-level and really meant for I/O stuff, so it may seem illogical at first. In your example it actually returned a string containing remaining 8 bytes that you didn't set. To make it return your data you need to invoke flip() like this:
val a = CharBuffer.allocate(10);
a.put('h');
a.put('i');
a.flip()
val b = a.toString();
You can find more in the docs of the Buffer
For more typical use cases it is much easier to use StringBuilder:
val a = StringBuilder()
a.append('h')
a.append('i')
val b = a.toString()
Or even use a Kotlin util that wraps StringBuilder:
val b = buildString {
append('h')
append('i')
}
I don't know how to get the string in readable format in my app. My code is:
val allergyList = recipeItem.allergies
allergyList.joinToString()
var allergyString: String = ""
for (allergy in allergyList) {
allergyList[1]
allergyString += " ${allergy}"
println(allergy.toString())
}
holder.recipeSearchPageAllergies.text = allergyString
When I print this I get the allergy string memory space?
Result for each one is something like this:
Allergy#4e8f238
How do I 'decode' it into something readable for a human? It should say 'nut allergy'.
you have some options. If you have the control over the Allergy source code,
you could override toString method:
class Allergy(val name: String) {
override fun toString(): String = "Allergy[name=${name}]"
}
println(Allergy("flowers"))
// Allergy[name=flowers]
also, you can make a data class of it. Data class has sane toString by default. It also has a few nice perks, like by default equals/hashCode generation, and deconstruction to components, allowing you to use it in destructing:
data class Allergy(val name: String)
println(Allergy("peanuts"))
// Allergy(name=peanuts)
otherwise, if you can't modify the source of the Allregy, you can make up an extension method for that:
class Allergy(val name: String)
fun Allergy.readable() = "Allergy[name=${name}]"
println(Allergy("cats").readable())
// Allergy[name=cats]
in your case, you could also make an extension method for collections of allergies to have the format you need:
fun Collection<Allergy>.readable() = joinToString { "Allergy[name=${it.name}]" }
println(
listOf(Allergy("cats"), Allergy("peanuts"), Allergy("flowers")).readable()
)
// Allergy[name=cats], Allergy[name=peanuts], Allergy[name=flowers]
// in your case:
holder.recipeSearchPageAllergies.text = recipeItem.allergies.readable()
// or just
holder.recipeSearchPageAllergies.text = recipeItem.allergies.joinToString { "Allergy[name=${it.name}]" }
You can make it simplier:
val allergiesStr = recipeItem.allergies.map { allergy ->
// your allergy name from allergy variable
}.joinToString(separator = " ")
I have a list of objects with an optional id as String and I want to make a map out of it.
I want to have the keys of my map as non nullable: so something like this:
data class Foo(
val id: String? = null
val someStuff: String? = null,
)
val foo = listOf(Foo("id1"), Foo())
val bar = foo.filterNot { it.id == null }.associateBy { it.id }
Here bar type is Map<String?, Foo> but not Map<String, Foo>
My workaround is to add a non null asserted call: !!, but it doesn't seem clean.
Is there an easy and safe way to do this?
This looks like something that contracts could help with, but currently a contract expression can't access properties of the class in use.
As a workaround, you could define a 2nd class that has a non-null id, like so
data class Foo(
val id: String? = null,
val someStuff: String? = null
)
data class Foo2(
val id: String,
val someStuff: String? = null
)
val foo = listOf(Foo("id1"), Foo())
val bar = foo
.mapNotNull { if (it.id != null) Foo2(it.id, it.someStuff) else null }
.associateBy { it.id }
There's a six-year-old open feature request for Map.filterNotNullKeys() and a four-year old open feature request for Map.associateByNotNull().
In my opinion, the associateBy { it.id!! } would be cleanest for readability. But you could do it like this:
val bar = foo.mapNotNull { it.id?.run { it.id to it } }.toMap()
As for your actual question, that logic is way too many steps for the compiler to infer. Your last function call to associateBy sees a nullable, so it infers a nullable. For the compiler to figure this out, it would have to step back and see that the List that you call associateBy on happens to have filtered out certain objects in a way that happens to ensure that a certain nullable property won't be null within this specific list, and it's the same property that you are associating with. Now imagine it has to do this for every call to any generic function, and the various lambdas involved could potentially have multiple lines of code. Compile times would skyrocket.
When trying to 'add' a custom object to an ArrayList, the ArrayList remains null
I've tried modifying the data class initialization by using MutableLists, among other things
Here is the data class that I'm using:
data class WYRStatistics(val team: String, val league: String, val gp: String, val g: String, val a: String, val pt: String, val pim: String, val pm: String)
This is where I'm using the data class to construct on object and try to add the object to an ArrayList
var n = 0
var statsArr: ArrayList<WYRStatistics>? = null
while (n < tempStats.length()) {
val statObject = tempStats.getJSONObject(n)
val singleStat = WYRStatistics(statObject.getString("Team"),
statObject.getString("League"),
statObject.getString("GamesPlayed"),
statObject.getString("Goals"),
statObject.getString("Assists"),
statObject.getString("Points"),
statObject.getString("PenaltyMinutes"),
statObject.getString("PlusMinus")
)
println(singleStat)
statsArr?.add(singleStat)
println(statsArr)
tempPlayer.stats?.add(singleStat)
println(tempPlayer)
n++
}
The utilization is inside of a function that handles the asynchronous task of decoding JSON. This is all working fine.
tempStats is a JSONArray - when printed it is formatted properly
When I println singleStat, the object is printed properly
However, when I 'add' singleStat to statsArr, the statsArr returns null
You are initializing arrayList to nullas what JB mentioned.
The correct way should be
val statsArr = arrayListOf<WYRStatistics>()