I learned java and python in high school and I became very comfortable with python. I have recently started to learn kotlin, mainly for fun (the keyword for defining a function is fun so it has to be a fun language, right), but I have a little problem.
Let's suppose I have a hierarchy of classes for Chess pieces:
abstract class Piece {
...
}
class Rook : Piece() {
...
}
class Bishop : Piece() {
...
}
.
.
.
I am taking input from the user to generate the board, so if the user types r, I need to create a Rook object, if he types b, I need to create a Bishop etc.
In python, I'd probably use a dictionary that maps the input string to the corresponding class, so I can create an object of the correct type:
class Piece:
...
class Rook(Piece):
...
class Bishop(Piece):
...
.
.
.
input_map = {
'r': Rook,
'b': Bishop,
...
}
s = input_map[input()]() # use user input as key and create a piece of the correct type
I was really amazed by this pattern when I discovered it. In java, I had to use a switch case or a bunch of if else if to achieve the same result, which is not the end of the world, especially if I abstract it into a separate function, but it's not as nice as the python approach.
I want to do the same thing in kotlin, and I was wondering if there is a similar pattern for kotlin since it's a modern language like python (I know, I know, python isn't new, but I think it's very modern). I tried to look online, but it seems like I can't store a class (class, not an object) in a variable or a map like I can in python.
Am I wrong about it? Can I use a similar pattern in kotlin or do I have to fall back to the when statement (or expression)?
If I am not mistaken, a similar pattern could be achieved in java using reflection. I never got to learn reflection in java deeply, but I know it's a way to use classes dynamically, what I can do for free in python. I also heard that in java, reflection should be used as a last resort because it's inefficient and it's considered "black magic" if you understand my meaning. Does it mean that I need to use reflection to achieve that result in kotlin? And if so, is it recommended to use reflection in kotlin, and is it efficient?
I'd like to know how I can approach this problem, and I accept multiple answers and additional solutions I didn't come up with. Thanks in advance.
This can be done without reflection.
You can map the input characters to the constructors:
val pieceConstructorsByKeyChar = mapOf(
'r' to ::Rook,
'b' to ::Bishop,
// etc.
)
Getting values from a map gives you a nullable, since it's possible the key you supply isn't in the map. Maybe this is fine, if when you use this you might be passing a character the player typed that might not be supported. Then you would probably handle null by telling the player to try again:
val piece: Piece? = pieceConstructorsByKeyChar[keyPressed]?.invoke()
Or if you do the look-up after you've already checked that it's a valid key-stroke, you can use !! safely:
val piece: Piece = pieceConstructorsByKeyChar[keyPressed]!!()
Yes you can use similiar approach with Kotlin. Kotlin has many features and supports reflection. Let me write an example about your problem.
Firstly create your classes that will be generate by user input.
abstract class Piece
class Rook : Piece()
class Bishop : Piece()
Create your class map
val inputMap = mapOf(
"r" to Rook::class.java,
"b" to Bishop::class.java
)
Create an instance what you want using newInstance function. If your input map doesn't contains key you gave then it will return null.
val rook = inputMap["r"]?.newInstance()
val bishop = inputMap["b"]?.newInstance()
// null
val king = inputMap["k"]?.newInstance()
Also you can write your custom extensions to create new objects.
fun <T> Map<String, Class<out T>>.newInstance(key: String) = this[key]?.newInstance()
// Create an instance with extension function
inputMap.newInstance("r")
Related
I've read about reflection in both Kotlin and Java documentation & getattr in Python examples but they all seem to lack my use case:
I have variables of type multableLiveData<boolean> like repetitionOfElementsInNewExerciseAllowed in a ViewModel class (Android programming) that I need to update from a different class like HomeFragment.
My unsexy approach was the following function in ViewModel:
fun updateBoolElement (boolElementToUpdate:String, valueToUpdateWith:Boolean){
when (boolElementToUpdate){
"repetitionOfElementsInNewExerciseAllowed" -> repetitionOfElementsInNewExerciseAllowed.value=valueToUpdateWith
}
}
Instead of this I'm striving for a recursive approach to extract the variable's name whose value I want to change out of the string boolElementToUpdate like this (pseudo code):
fun updateBoolElement (boolElementToUpdate:String, valueToUpdateWith:Boolean){
getAttributeReferenceByName(boolElementToUpdate).value = valueToUpdateWith
}
The way I would like to reuse the function is by calling it with different varible names then like
sharedViewModel.updateBoolElement("repetitionOfElementsInNewExerciseAllowed",repetitionOfElementsInNewExerciseAllowed)
or more abstractly
sharedViewModel.updateBoolElement("newVariableName",newBoolValue)
Is reflection or any other structure in Kotlin capable of doing such wizardry? Thanks for any hint!
I wonder if a data class with one of the properties being a function, such as:
data class Holder(val x: Data, val f: () -> Unit)
can work at all, since the following test fails.
val a = {}
val b = {}
Assert.assertEquals(a, b)
Update: Use case for this could be to have a
data class ButtonDescriptor(val text: String, val onClick: () -> Unit)
and then flow it to UI whilst doing distinctUntilChanged()
I don't think this is possible, I'm afraid.
You can of course check reference equality (===, or == in this case because functions don't generally override equals()). That would give you a definite answer where you have references to the same function instance. But that doesn't check structural equality, and so reports the two lambdas in the question as different.
You can check whether the two functions are instances of the same class by checking their .javaClass property. If the same, that would imply that they do the same processing — though I think they could still have different variables/captures. However, if different, that wouldn't tell you anything. Even the simple examples in the question are different classes…
And of course, you can't check them as ‘black boxes’ — it's not feasible to try every possible input and check their outputs. (Even assuming they were pure functions with no side effects, which in general isn't true!)
You might be able to get their bytecode from a classloader, and compare that, but I really wouldn't recommend it — it'd be a lot of unnecessary work, you'd have to allow for the difference in class names etc., it would probably have a lot of false negatives, and again I think it could return the same code for two functions which behaved differently due to different parameters/captures.
So no, I don't think this is possible in JVM languages.
What are you trying to achieve with this, and could there be another way? (For example, if these functions are under your control, can you arrange for reference equality to do what you need? Or could you use function objects with an extra property giving an ID or something else you could compare?)
When you create your data class, if you pass the function by reference it will work with DiffUtils and distinctUntilChanged(). Function references do not break the isEquals() method of data classes in the same way that a lambda does.
For example, you create a function for your onClick:
private fun onClick() { // handle click }
and create your data class like
BottomDescriptor("some text", ::onClick)
Is there way to declare a Map that would accept both String and Int in Kotlin? Like:
var map: Map<String, String | Int>
There is no union type in Kotlin, so you can't directly express this.
A few options:
use TreeMap<String, Any>, which is the simplest of all, but it allows more types than just Int and String, and requires additional conversion boilerplate on the consumer side when accessing values.
use a custom sealed class to encapsulate your "Int or String" type, and use it in a regular TreeMap<String, IntOrString>. This will have some runtime overhead for wrapping/unwrapping the int/string objects for each insertion/access. Also, this puts the burden of (un)wrapping on the consumer side. That being said, maybe your domain can be better modeled with clearer names than Int or String, so it might actually improve the consumer's experience to define such types.
use a custom map type, with independent accessors for ints and strings, but backed by a single TreeMap<String, Any> behind the scenes. This has the advantage of encapsulating the logic and exposing a neat API for the consumers. If you extend TreeMap you will save some overhead as opposed to composition, but it shouldn't matter much.
I am wondering about the actual use case at hand, though. Usually the need for union types stems from using plain generic primitive types to represent meaningful domain data that could maybe better be expressed using more specific custom domain types.
Option 2 would therefore usually be my choice, but I would use a sealed class that actually represents my domain data instead of the generic "string or int".
You can use Any here, see this example:
import java.util.TreeMap
fun main() {
var map = TreeMap<String, Any>()
map.put("One", 1)
map.put("Two", "two")
println(map)
}
which outputs
{One=1, Two=two}
Please note:
This will allow more types than just the desired String and Int, because the Kotlin Docs state (...). Every Kotlin class has Any as a superclass..
There are can be two ways of writing helper method in Kotlin
First is
object Helper {
fun doSomething(a: Any, b: Any): Any {
// Do some businesss logic and return result
}
}
Or simply writing this
fun doSomething(a: Any, b: Any): Any {
// Do some businesss logic and return result
}
inside a Helper.kt class.
So my question is in terms of performance and maintainability which is better and why?
In general, your first choice should be top-level functions. If a function has a clear "primary" argument, you can make it even more idiomatic by extracting it as the receiver of an extension function.
The object is nothing more than a holder of the namespace of its member functions. If you find that you have several groups of functions that you want to categorize, you can create several objects for them so you can qualify the calls with the object's name. There's little beyond this going in their favor in this role.
object as a language feature makes a lot more sense when it implements a well-known interface.
There's a third and arguably more idiomatic way: extension functions.
fun Int.add(b: Int): Int = this + b
And to use it:
val x = 1
val y = x.add(3) // 4
val z = 1.add(3) // 4
In terms of maintainability, I find extension functions just as easy to maintain as top-level functions or helper classes. I'm not a big fan of helper classes because they end up acquiring a lot of cruft over time (things people swear we'll reuse but never do, oddball variants of what we already have for special use cases, etc).
In terms of performance, these are all going to resolve more or less the same way - statically. The Kotlin compiler is effectively going to compile all of these down to the same java code - a class with a static method.
I asked a question at How to design a complex class which incude some classes to make expansion easily in future in Kotlin? about how to design a complex class which incude some classes to make expansion easily in future in Kotlin.
A expert named s1m0nw1 give me a great answer as the following code.
But I don't know why he want to change MutableList to List at https://stackoverflow.com/posts/47960036/revisions , I can get the correct result when I use MutableList. Could you tell me?
The code
interface DeviceDef
data class BluetoothDef(val Status: Boolean = false) : DeviceDef
data class WiFiDef(val Name: String, val Status: Boolean = false) : DeviceDef
data class ScreenDef(val Name: String, val size: Long) : DeviceDef
class MDetail(val _id: Long, val devices: List<DeviceDef>) {
inline fun <reified T> getDevice(): T {
return devices.filterIsInstance(T::class.java).first()
}
}
Added
I think that mutableListOf<DeviceDef> is better than ListOf<DeviceDef> in order to extend in future.
I can use aMutableList.add() function to extend when I append new element of mutableListOf<DeviceDef>.
If I use ListOf<DeviceDef>, I have to construct it with listOf(mBluetoothDef1, mWiFiDef1, //mOther), it's not good. Right?
var aMutableList= mutableListOf<DeviceDef>()
var mBluetoothDef1= BluetoothDef(true)
var mWiFiDef1= WiFiHelper(this).getWiFiDefFromSystem()
aMutableList.add(mBluetoothDef1)
aMutableList.add(mWiFiDef1)
// aMutableList.add(mOther) //This is extension
var aMDetail1= MDetail(myID, aMutableList)
Sorry for not giving an explanation in the first place. The differences are explained in the docs.:
Unlike many languages, Kotlin distinguishes between mutable and immutable collections (lists, sets, maps, etc). Precise control over exactly when collections can be edited is useful for eliminating bugs, and for designing good APIs.
It is important to understand up front the difference between a read-only view of a mutable collection, and an actually immutable collection. Both are easy to create, but the type system doesn't express the difference, so keeping track of that (if it's relevant) is up to you.
The Kotlin List<out T> type is an interface that provides read-only operations like size, get and so on. Like in Java, it inherits from Collection<T> and that in turn inherits from Iterable<T>. Methods that change the list are added by the MutableList<T> interface. [...]
The List interface provides a read-only view so that you cannot e.g add new elements to the list which has many advantages for instance in multithreaded environments. There may be situations in which you will use MutableList instead.
I also recommend the following discussion:
Kotlin and Immutable Collections?
EDIT (added content):
You can do this is a one-liner without any add invocation:
val list = listOf(mBluetoothDef1, mWiFiDef1)