Just started using Kotlin in our projects. To initialise an immutable map or list (possibly any collections in Kotlin) I could see two options mapOf() and emptyMap() (listOf() and emptyList() for a list).
Basically, the mapOf is nothing but an inline function that returns emptyMap().
#kotlin.internal.InlineOnly
public inline fun <K, V> mapOf(): Map<K, V> = emptyMap()
What is preferred over another and why does Kotlin expose both?
It's a specialized overload of mapOf(vararg Pair<K, V>) - there is no need to perform the size check if you're calling that function without any arguments.
As for "what's preferred over another" - whatever makes the code it's used in more readable. Performance-wise, there's no difference (as mapOf() is inline), though for the sake of consistency you might want to choose one and stick with it.
I have class that internally maintains a mutable list, and I want to provide an immutable view on this list. Currently I'm using the following:
/**The list that actually stores which element is at which position*/
private val list: MutableList<T> = ArrayList()
/**Immutable view of [list] to the outside.*/
val listView: List<T> get() = list.toList()
First question: Can this be done easier
Second question: How can I test that listView is actually immutable. I guess reflections are necessary?
If you only needed the compile-time type to be immutable, you could simply upcast your list:
val listView: List<T> get() = list
(Though if you checked and downcast that to MutableList, you could make changes — and those would affect the original list.)
However, if you want full immutability, that's tricky. I don't think there are any general-purpose truly immutable lists in the Kotlin stdlib.
Although List and MutableList look like two different types, in Kotlin/JVM they both compile down to the same type in the bytecode, which is mutable. And even in Kotlin, while Iterable.toList() does return a new list, the current implementation* actually gives a MutableList that's been upcast to List. (Though mutating it wouldn't change the original list.)
Some third-party libraries provide truly immutable collections, though; see this question.
And to check whether a List is mutable, you could use a simple type check:
if (listView is MutableList)
// …
No need to use reflection explicitly. (A type check like that is considered to be implicit reflection.)
(* That can change, of course. It's usually a mistake to read too much into the current code, if it's not backed up by the documentation.)
I have a Java function that has a Map<String, String and needs to pass it to a Kotlin function for adding values to the map.
The problem is that if I have:
fun updateMap(map: Map<String, String>)
It seems that the map is immutable and I can't do: map[KEY] = VALUE as I get compilation error.
It would work if I did: fun updateMap(map: HashMap<String, String>) but in that case I can't pass the original map from the Java code without some casting which I would like to avoid if possible.
What is the solution for this?
Kotlin, unlike Java, has separate interfaces for mutable and read-only collections, see Kotlin Collections Overview.
The Map interface in Kotlin doesn't expose any modifying functions (including the operator map[key] = value), but MutableMap does.
On the JVM, the Kotlin Map and MutableMap interfaces are both represented by the java.util.Map, so you can freely change your parameter type to MutableMap<String, String>:
fun updateMap(map: MutableMap<String, String>) {
map["foo"] = "bar"
}
Note that you might need to change Map to MutableMap in some other places in your Kotlin code, as the compiler won't allow you to pass a read-only Map as a MutableMap argument.
As for HashMap, given that it's a concrete implementation, it also implements the MutableMap and therefore exposes the mutating functions. However, using interfaces and not implementation classes is more preferable.
private val repositories = mutableListOf<String>()
private val repositories = ArrayList<String>()
Both are mutable list, then what is the point of two keywords mutableListOf or ArrayList?
or is there any major difference?
The only difference between the two is communicating your intent.
When you write val a = mutableListOf(), you're saying "I want a mutable list, and I don't particularly care about the implementation". When you write, instead, val a = ArrayList(), you're saying "I specifically want an ArrayList".
In practice, in the current implementation of Kotlin compiling to the JVM, calling mutableListOf will produce an ArrayList, and there's no difference in behaviour: once the list is built, everything will behave the same.
Now, let's say that a future version of Kotlin changes mutableListOf to return a different type of list.
Likelier than not, the Kotlin team would only make that change if they figure the new implementation works better for most use cases. mutableListOf would then have you using that new list implementation transparently, and you'd get that better behaviour for free. Go with mutableListOf if that sounds like your case.
On the other hand, maybe you spent a bunch of time thinking about your problem, and figured that ArrayList really is the best fit for your problem, and you don't want to risk getting moved to something sub-optimal. Then you probably want to either use ArrayList directly, or use the arrayListOf factory function (an ArrayList-specific analogue to mutableListOf).
mutableListOf<T>() is an inline function invocation that returns a
MutableList<T>. As of today, mutableListOf does return an instance of ArrayList.
ArrayList<String>() is a constructor invocation and cannot be inlined.
In other words, given:
val a = mutableListOf<String>()
val b = ArrayList<String>()
a is of type MutableList<String>
b is of type ArrayList<String>
At runtime, both a and b will hold an instance of ArrayList.
Note that inlining is particularly useful when combined with type reification, which justifies the existence of listOf, mutableListOf and the like.
Under the covers, both mutableListOf() and arrayListOf() create an instance of ArrayList. ArrayList is a class that happens to implement the MutableList interface.
The only difference is that arrayListOf() returns the ArrayList as an actual ArrayList. mutableListOf() returns a MutableList, so the actual ArrayList is "disguised" as just the parts that are described by the MutableList interface.
The difference, in practice, is that ArrayList has a few methods that are not part of the MutableList interface (trimToSize and ensureCapacity).
The difference, philosophically, is that the MutableList only cares about the behaviour of the object being returned. It just returns "something that acts like a MutableList". The ArrayList cares about the "structure" of the object. It allows you to directly manipulate the memory allocated by the object (trimToSize).
The rule of thumb is that you should prefer the interface version of things (mutableListOf()) unless you actually have a reason to care about the exact details of the underlying structure.
Or, in other words, if you don't know which one you want, choose mutableListOf first.
As you can see in sources:
public inline fun <T> mutableListOf(): MutableList<T> = ArrayList()
So, there is no difference, just a convenience method.
I am learning Kotlin and it is looking likely I may want to use it as my primary language within the next year. However, I keep getting conflicting research that Kotlin does or does not have immutable collections and I'm trying to figure out if I need to use Google Guava.
Can someone please give me some guidance on this? Does it by default use Immutable collections? What operators return mutable or immutable collections? If not, are there plans to implement them?
Kotlin's List from the standard library is readonly:
interface List<out E> : Collection<E> (source)
A generic ordered collection of elements. Methods in this interface
support only read-only access to the list; read/write access is
supported through the MutableList interface.
Parameters
E - the type of elements contained in the list.
As mentioned, there is also the MutableList
interface MutableList<E> : List<E>, MutableCollection<E> (source)
A generic ordered collection of elements that supports adding and
removing elements.
Parameters
E - the type of elements contained in the list.
Due to this, Kotlin enforces readonly behaviour through its interfaces, instead of throwing Exceptions on runtime like default Java implementations do.
Likewise, there is a MutableCollection, MutableIterable, MutableIterator, MutableListIterator, MutableMap, and MutableSet, see the stdlib documentation.
It is confusing but there are three, not two types of immutability:
Mutable - you are supposed to change the collection (Kotlin's MutableList)
Readonly - you are NOT supposed to change it (Kotlin's List) but something may (cast to Mutable, or change from Java)
Immutable - no one can change it (Guavas's immutable collections)
So in case (2) List is just an interface that does not have mutating methods, but you can change the instance if you cast it to MutableList.
With Guava (case (3)) you are safe from anybody to change the collection, even with a cast or from another thread.
Kotlin chose to be readonly in order to use Java collections directly, so there is no overhead or conversion in using Java collections..
As you see in other answers, Kotlin has readonly interfaces to mutable collections that let you view a collection through a readonly lens. But the collection can be bypassed via casting or manipulated from Java. But in cooperative Kotlin code that is fine, most uses do not need truly immutable collections and if your team avoids casts to the mutable form of the collection then maybe you don't need fully immutable collections.
The Kotlin collections allow both copy-on-change mutations, as well as lazy mutations. So to answer part of your questions, things like filter, map, flatmap, operators + - all create copies when used against non lazy collections. When used on a Sequence they modify the values as the collection as it is accessed and continue to be lazy (resulting in another Sequence). Although for a Sequence, calling anything such as toList, toSet, toMap will result in the final copy being made. By naming convention almost anything that starts with to is making a copy.
In other words, most operators return you the same type as you started with, and if that type is "readonly" then you will receive a copy. If that type is lazy, then you will lazily apply the change until you demand the collection in its entirety.
Some people want them for other reasons, such as parallel processing. In those cases, it might be best to look at really high performance collections designed just for those purposes. And only use them in those cases, not in all general cases.
In the JVM world it is hard to avoid interop with libraries that want standard Java collections, and converting to/from these collections adds a lot of pain and overhead for libraries that do not support the common interfaces. Kotlin gives a good mix of interop and lack of conversion, with readonly protection by contract.
So if you can't avoid wanting immutable collections, Kotlin easily works with anything from the JVM space:
Guava (https://github.com/google/guava)
Dexx a port of the Scala collections to Java (https://github.com/andrewoma/dexx) with Kotlin helpers (https://github.com/andrewoma/dexx/blob/master/kollection/README.md)
Eclipse Collections (formerly GS-Collections) a really high performance, JDK compatible, top performer in parallel processing with immutable and mutable variations (home: https://www.eclipse.org/collections/ and Github: https://github.com/eclipse/eclipse-collections)
PCollections (http://pcollections.org/)
Also, the Kotlin team is working on Immutable Collections natively for Kotlin, that effort can be seen here:
https://github.com/Kotlin/kotlinx.collections.immutable
There are many other collection frameworks out there for all different needs and constraints, Google is your friend for finding them. There is no reason the Kotlin team needs to reinvent them for its standard library. You have a lot of options, and they specialize in different things such as performance, memory use, not-boxing, immutability, etc. "Choice is Good" ... therefore some others: HPCC, HPCC-RT, FastUtil, Koloboke, Trove and more...
There are even efforts like Pure4J which since Kotlin supports Annotation processing now, maybe can have a port to Kotlin for similar ideals.
Kotlin 1.0 will not have immutable collections in the standard library. It does, however, have read-only and mutable interfaces. And nothing prevents you from using third party immutable collection libraries.
Methods in Kotlin's List interface "support only read-only access to the list" while methods in its MutableList interface support "adding and removing elements". Both of these, however, are only interfaces.
Kotlin's List interface enforces read-only access at compile-time instead of deferring such checks to run-time like java.util.Collections.unmodifiableList(java.util.List) (which "returns an unmodifiable view of the specified list... [where] attempts to modify the returned list... result in an UnsupportedOperationException." It does not enforce immutability.
Consider the following Kotlin code:
import com.google.common.collect.ImmutableList
import kotlin.test.assertEquals
import kotlin.test.assertFailsWith
fun main(args: Array<String>) {
val readOnlyList: List<Int> = arrayListOf(1, 2, 3)
val mutableList: MutableList<Int> = readOnlyList as MutableList<Int>
val immutableList: ImmutableList<Int> = ImmutableList.copyOf(readOnlyList)
assertEquals(readOnlyList, mutableList)
assertEquals(mutableList, immutableList)
// readOnlyList.add(4) // Kotlin: Unresolved reference: add
mutableList.add(4)
assertFailsWith(UnsupportedOperationException::class) { immutableList.add(4) }
assertEquals(readOnlyList, mutableList)
assertEquals(mutableList, immutableList)
}
Notice how readOnlyList is a List and methods such as add cannot be resolved (and won't compile), mutableList can naturally be mutated, and add on immutableList (from Google Guava) can also be resolved at compile-time but throws an exception at run-time.
All of the above assertions pass with exception of the last one which results in Exception in thread "main" java.lang.AssertionError: Expected <[1, 2, 3, 4]>, actual <[1, 2, 3]>. i.e. We successfully mutated a read-only List!
Note that using listOf(...) instead of arrayListOf(...) returns an effectively immutable list as you cannot cast it to any mutable list type. However, using the List interface for a variable does not prevent a MutableList from being assigned to it (MutableList<E> extends List<E>).
Finally, note that an interface in Kotlin (as well as in Java) cannot enforce immutability as it "cannot store state" (see Interfaces). As such, if you want an immutable collection you need to use something like those provided by Google Guava.
See also ImmutableCollectionsExplained · google/guava Wiki · GitHub
NOTE: This answer is here because the code is simple and open-source and you can use this idea to make your collections that you create immutable. It is not intended only as an advertisement of the library.
In Klutter library, are new Kotlin Immutable wrappers that use Kotlin delegation to wrap a existing Kotlin collection interface with a protective layer without any performance hit. There is then no way to cast the collection, its iterator, or other collections it might return into something that could be modified. They become in effect Immutable.
Klutter 1.20.0 released which adds immutable protectors for existing collections, based on a SO answer by #miensol provides a light-weight delegate around collections that prevents any avenue of modification including casting to a mutable type then modifying. And Klutter goes a step further by protecting sub collections such as iterator, listIterator, entrySet, etc. All of those doors are closed and using Kotlin delegation for most methods you take no hit in performance. Simply call myCollection.asReadonly() (protect) or myCollection.toImmutable() (copy then protect) and the result is the same interface but protected.
Here is an example from the code showing how simply the technique is, by basically delegating the interface to the actual class while overriding mutation methods and any sub-collections returned are wrapped on the fly.
/**
* Wraps a List with a lightweight delegating class that prevents casting back to mutable type
*/
open class ReadOnlyList <T>(protected val delegate: List<T>) : List<T> by delegate, ReadOnly, Serializable {
companion object {
#JvmField val serialVersionUID = 1L
}
override fun iterator(): Iterator<T> {
return delegate.iterator().asReadOnly()
}
override fun listIterator(): ListIterator<T> {
return delegate.listIterator().asReadOnly()
}
override fun listIterator(index: Int): ListIterator<T> {
return delegate.listIterator(index).asReadOnly()
}
override fun subList(fromIndex: Int, toIndex: Int): List<T> {
return delegate.subList(fromIndex, toIndex).asReadOnly()
}
override fun toString(): String {
return "ReadOnly: ${super.toString()}"
}
override fun equals(other: Any?): Boolean {
return delegate.equals(other)
}
override fun hashCode(): Int {
return delegate.hashCode()
}
}
Along with helper extension functions to make it easy to access:
/**
* Wraps the List with a lightweight delegating class that prevents casting back to mutable type,
* specializing for the case of the RandomAccess marker interface being retained if it was there originally
*/
fun <T> List<T>.asReadOnly(): List<T> {
return this.whenNotAlreadyReadOnly {
when (it) {
is RandomAccess -> ReadOnlyRandomAccessList(it)
else -> ReadOnlyList(it)
}
}
}
/**
* Copies the List and then wraps with a lightweight delegating class that prevents casting back to mutable type,
* specializing for the case of the RandomAccess marker interface being retained if it was there originally
*/
#Suppress("UNCHECKED_CAST")
fun <T> List<T>.toImmutable(): List<T> {
val copy = when (this) {
is RandomAccess -> ArrayList<T>(this)
else -> this.toList()
}
return when (copy) {
is RandomAccess -> ReadOnlyRandomAccessList(copy)
else -> ReadOnlyList(copy)
}
}
You can see the idea and extrapolate to create the missing classes from this code which repeats the patterns for other referenced types. Or view the full code here:
https://github.com/kohesive/klutter/blob/master/core-jdk6/src/main/kotlin/uy/klutter/core/common/Immutable.kt
And with tests showing some of the tricks that allowed modifications before, but now do not, along with the blocked casts and calls using these wrappers.
https://github.com/kohesive/klutter/blob/master/core-jdk6/src/test/kotlin/uy/klutter/core/collections/TestImmutable.kt
Now we have https://github.com/Kotlin/kotlinx.collections.immutable.
fun Iterable<T>.toImmutableList(): ImmutableList<T>
fun Iterable<T>.toImmutableSet(): ImmutableSet<T>
fun Iterable<T>.toPersistentList(): PersistentList<T>
fun Iterable<T>.toPersistentSet(): PersistentSet<T>