Suppose we have the following code:
#ExperimentalCoroutinesApi
fun ProducerScope<DownloadableDataDto<out User>>.findInteresting(input: ReceiveChannel<DownloadableDataDto<out User>>,
communitiesCount: Int,
userCountMap: MutableMap<User, Int> = ConcurrentHashMap()) = createProducer(input) {
if (userCountMap.compute(it.data!!) { _, value ->
if (value == null) 1 else value + 1
} == communitiesCount) send(it)
}
This code checks if a user is a part of all of the communitiesCount communities. But this logic is enclosed inside a createChannel() higher order function, which I would not want to test at the moment. Is there a way to test only the internals? I assume I could probably extract that to a separate function as well, right?
And if I do it that way, let's say we have this instead:
#ExperimentalCoroutinesApi
fun ProducerScope<DownloadableDataDto<out User>>.findInteresting(input: ReceiveChannel<DownloadableDataDto<out User>>,
communitiesCount: Int,
userCountMap: MutableMap<User, Int> = ConcurrentHashMap()) = createProducer(input) {
sendIfInteresting(it, communitiesCount, userCountMap)
}
#ExperimentalCoroutinesApi
private suspend fun ProducerScope<DownloadableDataDto<out User>>.sendIfInteresting(userDto: DownloadableDataDto<out User>,
communitiesCount: Int,
userCountMap: MutableMap<User, Int>) {
if (userCountMap.compute(userDto.data!!) { _, value ->
if (value == null) 1 else value + 1
} == communitiesCount) send(userDto)
}
How would I mock the send(userDto) call? I can mock the ProducerScope object, but how would I call the real sendIfInteresting() method?
Related
I have a function in Kotlin which takes a particular string as input. Depending on the input, I want to create a variable of a specific type and do some computations on it.
For example,
fun compute(input: String): Any{
if(input=="2d"){
var point: Point2D;// Points2D - x: int, y: int
//initilize and do some computations
return point.findDistanceFromOrigin()
}else if(input=="2d-1"){
var point: Point2DWithP1AsOrigin;// Point2DWithP1AsOrigin - x: int, y: int
//initilize and do some computations
return point.findDistanceFromOrigin()
}else if(input=="2d-2"){
var point: Point2DWithP2AsOrigin;
//initilize and do some computations
return point.findDistanceFromOrigin()
}
.
.
.
}
You can see in the above example, I want to initilize the type of point depending on the input and do computation and return.
All the if-else conditions have the same code except for the definition of the variable. How can I put all this in a single block with something like this:
var point: if(input=="2d) Point2D::class else if(input=="2d-1") Point2DWithP1AsOrigin::class.....
How can I do that?
You could do something like this
fun compute(input: String): Any{
val point: MyPoint = when(input) {
"2d" -> Point2D()
"2d-1" -> Point2DWithP1AsOrigin()
"2d-2" -> Point2DWithP2AsOrigin()
else -> Point2D() //fallback is necessary
}
//initilize and do some computations
return point.findDistanceFromOrigin()
}
But then it's essential that all those classes share the same interface. Because they need to have the same methods in order to do the same operations on them.
For example like this:
class Point2D : MyPoint {
override fun findDistanceFromOrigin() = 5
}
class Point2DWithP1AsOrigin : MyPoint{
override fun findDistanceFromOrigin() = 6
}
class Point2DWithP2AsOrigin : MyPoint{
override fun findDistanceFromOrigin() = 7
}
interface MyPoint {
fun findDistanceFromOrigin() : Int
}
You can store constructor references and then invoke required one
fun main() {
val constructors = mapOf(
"2d" to ::Point2D,
"2d-1" to ::Point2DWithP1AsOrigin,
"2d-2" to ::Point2DWithP2AsOrigin,
)
val type = "2d-2"
val constructor = constructors[type] ?: throw IllegalArgumentException("$type not supported")
val point = constructor()
println(point::class)
}
Output
class Point2DWithP2AsOrigin
write a method "lastWhere" that accepts a function called "where" of type (T) -> Boolean. The method returns the last element of type T to which the "where" function applies. If no matching element is found, null is returned.
call the method "lastwhere" on the linked list below. Find the last game that is more than 10 euros.
So far I've got this Code going for me.
I assume the only important piece of Code I need to edit is the "fun lastWhere" for task number 1)
the second task wants me to implement a way on the main function to find the last Game that is cheaper than 10 Euros.
class LinkedList<T> {
data class Node<T>(val data: T, var next: Node<T>?)
private var first: Node<T>? = null
override fun toString(): String = first?.toString() ?: "-"
fun isEmpty() = first == null
fun addLast(data: T) {
if (first == null) {
first = Node(data, first)
return
}
var runPointer = first
while (runPointer?.next != null) {
runPointer = runPointer.next
}
runPointer?.next = Node(data, null)
}
fun lastWhere (where: (T) -> Boolean): T? { // "where" function needs to be implemented
if (isEmpty()) return null
else {
var runPointer = first
while (runPointer?.next != null ) {
runPointer = runPointer.next
}
return runPointer?.data
}
}
}
data class Game(val title: String, val price: Double)
fun main() {
val list = LinkedList<Game>()
list.addLast(Game("Minecraft", 9.99))
list.addLast(Game("Overwatch", 29.99))
list.addLast(Game("Mario Kart", 59.99))
list.addLast(Game("World of Warcraft", 19.99))
var test = list.lastWhere ({it.price >= 10.00}) // This is probably wrong too, since I haven't got task 1) working
println (test)
}
Would appreciate any help!
Since you only store a reference to first node, you don't have any choice but to start at first and iterate. you will also have to keep a reference to last item that satisfied the where predicate, and keep updating this reference with every iteration.
fun lastWhere (where: (T) -> Boolean): T? {
var runPointer = first
var item: T? = null // init item to null, if nothing is found we return null
while (runPointer != null ) {
// For every node, execute the where function and if it returns true
// then update the return value
if(where(runPointer.data)) { item = runPointer.data }
runPointer = runPointer.next
}
return item
}
I made a function which returns all factors of an integer. But the problem is that it gives an error that the return type is Unit instead of mutableListOf. my code:
fun get_factors(num: Int) {
var factors = mutableListOf<Int>()
for (x in 1..num) {
if (x % num == 0) {
factors.add(x)
}
}
return factors
}
fun main() {
print(get_factors(18))
}
I tried doing:
fun get_factors(num: Int): mustableListOf {
var factors = mutableListOf<Int>()
for (x in 1..num) {
if (x % num == 0) {
factors.add(x)
}
}
return factors
}
fun main() {
print(get_factors(18))
}
but it says mutableListOf not defined.
I just started learning Kotlin today so I am a total beginner with Kotlin. Please help me out with this.
You seem to have confused the type MutableList<Int>, with the function mutableListOf.
Since the type name is MutableList<Int>, you should do:
fun get_factors(num: Int) : MutableList<Int> {
...
}
Or, if the caller doesn't need to modify the list, you can just return List<Int> too:
fun get_factors(num: Int) : List<Int> {
...
}
I can see why this is confusing - to create an instance of a type, you normally just add () to the end of the type name, so if mutableListOf() creates a list, you'd think that mutableListOf is the type name. However, this is actually calling the global function called mutableListOf, which returns an instance of MutableList<T>.
One way to distinguish between these is to look at the first letter. Type names usually begin with a capital letter, whereas function names begin with a small letter.
I have the function below. However, when I pass a string to it, I get the following error:
error: operator call corresponds to a dot-qualified call 'charCountMap.get(c).plus(1)' which is not allowed on a nullable receiver 'charCountMap.get(c)'. charCountMap.put(c, charCountMap.get(c) + 1)
private fun characterCount(inputString:String) {
val charCountMap = HashMap<Char, Int>()
val strArray = inputString.toCharArray()
for (c in strArray)
{
if (charCountMap.containsKey(c))
{
charCountMap.put(c, charCountMap.get(c) + 1)
}
else
{
charCountMap.put(c, 1)
}
}
}
The Kotlin Standard Library has groupingBy and eachCount for this purpose, you don't need to do any of this manually:
private fun characterCount(inputString:String) {
val charCountMap : Map<Char, Int> = inputString.groupingBy { it }.eachCount()
}
Note that I put the type on charCountMap for clarity, but it can be left off and inferred.
There is nice compute method in HashMap for this:
private fun characterCount(inputString:String) = hashMapOf<Char, Int>().also { charCountMap ->
inputString.forEach { charCountMap.compute(it) { _, v -> if (v == null) 1 else v + 1 } }
}
Both the other answers are correct. Todd's answer is right, you don't need to write a function for this. Just use the standard library. And if you are going to write a function that updates maps, Михаил Нафталь's suggestion to use compute() to handle updating existing values is also good.
However, if you're just doing this an an exercise, here are three suggestions to fix/improve your algorithm:
Instead of get(), use getValue(), which does not return null. It will raise an exception if the element does not exist, but you already checked for that.
Use the [] operator instead of put() (no need to, it's just nicer syntax).
You don't need to call toCharArray() because Strings are already iterable.
if (charCountMap.containsKey(c))
{
charCountMap[c] = charCountMap.getValue(c) + 1
}
else
{
charCountMap[c] = 1
}
Rewriting the whole thing using standard formatting:
fun characterCount(inputString: String): Map<Char, Int> {
val charCountMap = mutableMapOf<Char, Int>()
for (c in inputString) {
if (charCountMap.containsKey(c)) {
charCountMap[c] = charCountMap.getValue(c) + 1
} else {
charCountMap[c] = 1
}
}
return charCountMap
}
Given the following Kotlin class:
data class Test(val value: Int)
How would I override the Int getter so that it returns 0 if the value negative?
If this isn't possible, what are some techniques to achieve a suitable result?
After spending almost a full year of writing Kotlin daily I've found that attempting to override data classes like this is a bad practice. There are 3 valid approaches to this, and after I present them, I'll explain why the approach other answers have suggested is bad.
Have your business logic that creates the data class alter the value to be 0 or greater before calling the constructor with the bad value. This is probably the best approach for most cases.
Don't use a data class. Use a regular class and have your IDE generate the equals and hashCode methods for you (or don't, if you don't need them). Yes, you'll have to re-generate it if any of the properties are changed on the object, but you are left with total control of the object.
class Test(value: Int) {
val value: Int = value
get() = if (field < 0) 0 else field
override fun equals(other: Any?): Boolean {
if (this === other) return true
if (other !is Test) return false
return true
}
override fun hashCode(): Int {
return javaClass.hashCode()
}
}
Create an additional safe property on the object that does what you want instead of having a private value that's effectively overriden.
data class Test(val value: Int) {
val safeValue: Int
get() = if (value < 0) 0 else value
}
A bad approach that other answers are suggesting:
data class Test(private val _value: Int) {
val value: Int
get() = if (_value < 0) 0 else _value
}
The problem with this approach is that data classes aren't really meant for altering data like this. They are really just for holding data. Overriding the getter for a data class like this would mean that Test(0) and Test(-1) wouldn't equal one another and would have different hashCodes, but when you called .value, they would have the same result. This is inconsistent, and while it may work for you, other people on your team who see this is a data class, may accidentally misuse it without realizing how you've altered it / made it not work as expected (i.e. this approach wouldn't work correctly in a Map or a Set).
You could try something like this:
data class Test(private val _value: Int) {
val value = _value
get(): Int {
return if (field < 0) 0 else field
}
}
assert(1 == Test(1).value)
assert(0 == Test(0).value)
assert(0 == Test(-1).value)
assert(1 == Test(1)._value) // Fail because _value is private
assert(0 == Test(0)._value) // Fail because _value is private
assert(0 == Test(-1)._value) // Fail because _value is private
In a data class you must to mark the primary constructor's parameters with either val or var.
I'm assigning the value of _value to value in order to use the desired name for the property.
I defined a custom accessor for the property with the logic you described.
The answer depends on what capabilities you actually use that data provides. #EPadron mentioned a nifty trick (improved version):
data class Test(private val _value: Int) {
val value: Int
get() = if (_value < 0) 0 else _value
}
That will works as expected, e.i it has one field, one getter, right equals, hashcode and component1. The catch is that toString and copy are weird:
println(Test(1)) // prints: Test(_value=1)
Test(1).copy(_value = 5) // <- weird naming
To fix the problem with toString you may redefine it by hands. I know of no way to fix the parameter naming but not to use data at all.
I have seen your answer, I agree that data classes are meant for holding data only, but sometimes we need to make somethings out of them.
Here is what i'm doing with my data class, I changed some properties from val to var, and overid them in the constructor.
like so:
data class Recording(
val id: Int = 0,
val createdAt: Date = Date(),
val path: String,
val deleted: Boolean = false,
var fileName: String = "",
val duration: Int = 0,
var format: String = " "
) {
init {
if (fileName.isEmpty())
fileName = path.substring(path.lastIndexOf('\\'))
if (format.isEmpty())
format = path.substring(path.lastIndexOf('.'))
}
fun asEntity(): rc {
return rc(id, createdAt, path, deleted, fileName, duration, format)
}
}
I know this is an old question but it seems nobody mentioned the possibility to make value private and writing custom getter like this:
data class Test(private val value: Int) {
fun getValue(): Int = if (value < 0) 0 else value
}
This should be perfectly valid as Kotlin will not generate default getter for private field.
But otherwise I definitely agree with spierce7 that data classes are for holding data and you should avoid hardcoding "business" logic there.
I found the following to be the best approach to achieve what you need without breaking equals and hashCode:
data class TestData(private var _value: Int) {
init {
_value = if (_value < 0) 0 else _value
}
val value: Int
get() = _value
}
// Test value
assert(1 == TestData(1).value)
assert(0 == TestData(-1).value)
assert(0 == TestData(0).value)
// Test copy()
assert(0 == TestData(-1).copy().value)
assert(0 == TestData(1).copy(-1).value)
assert(1 == TestData(-1).copy(1).value)
// Test toString()
assert("TestData(_value=1)" == TestData(1).toString())
assert("TestData(_value=0)" == TestData(-1).toString())
assert("TestData(_value=0)" == TestData(0).toString())
assert(TestData(0).toString() == TestData(-1).toString())
// Test equals
assert(TestData(0) == TestData(-1))
assert(TestData(0) == TestData(-1).copy())
assert(TestData(0) == TestData(1).copy(-1))
assert(TestData(1) == TestData(-1).copy(1))
// Test hashCode()
assert(TestData(0).hashCode() == TestData(-1).hashCode())
assert(TestData(1).hashCode() != TestData(-1).hashCode())
However,
First, note that _value is var, not val, but on the other hand, since it's private and data classes cannot be inherited from, it's fairly easy to make sure that it is not modified within the class.
Second, toString() produces a slightly different result than it would if _value was named value, but it's consistent and TestData(0).toString() == TestData(-1).toString().
Seems to be an old but interesting question.
Just want to contribute an option:
data class Test(#JvmField val value: Int){
fun getValue() = if(value<0) 0 else value
}
Now you can override getValue, and still have component1() working.
This seems to be one (among other) annoying drawbacks of Kotlin.
It seems that the only reasonable solution, which completely keeps backward compatibility of the class is to convert it into a regular class (not a "data" class), and implement by hand (with the aid of the IDE) the methods: hashCode(), equals(), toString(), copy() and componentN()
class Data3(i: Int)
{
var i: Int = i
override fun equals(other: Any?): Boolean
{
if (this === other) return true
if (other?.javaClass != javaClass) return false
other as Data3
if (i != other.i) return false
return true
}
override fun hashCode(): Int
{
return i
}
override fun toString(): String
{
return "Data3(i=$i)"
}
fun component1():Int = i
fun copy(i: Int = this.i): Data3
{
return Data3(i)
}
}
You can follow the Builder Pattern for this I think it'd be much better.
Here is an example:
data class Test(
// Fields:
val email: String,
val password: String
) {
// Builder(User):
class Builder(private val email: String) {
// Fields:
private lateinit var password: String
// Methods:
fun setPassword(password: String): Builder {
// Some operation like encrypting
this.password = password
// Returning:
return this
}
fun build(): Test = Test(email, password)
}
}