Kotlin convert arraylist to array

Kotlin Program to Convert List (ArrayList) to Array and Vice-Versa

Solution 2: Well first, that is not how to define a list in Kotlin. There is also extension to most subclasses in Kotlin In this program, you’ll learn to convert a list to an array using toArray() and array to list using asList() in Kotlin.

How to Convert List into ArrayList in Kotlin

[ [Data(name1, good) ,Data(name2,good)] , [Data(name2,good), Data(name2,bad)] ] 

How to convert this List into ArrayList please?

var list = [ [Data(name1, good) ,Data(name2,good)] , [Data(name2,good), Data(name2,bad)] ] var arraylist = ArrayList(list) 

Well first, that is not how to define a list in Kotlin. Since there are no list literals in Kotlin.

Instead, it’s like listOf(1, 2, 3, 4,) for a normal list, and mutableListOf(1, 2, 3, 4,) for a mutable (editable) list.

MutableList is basically an ArrayList in Kotlin. But there is still arrayListOf() in Kotlin.

There is also toMutableList() extension to most Collection subclasses in Kotlin

Kotlin list : Arraylist, ArrayList class is used to create a dynamic array in Kotlin. Dynamic array states that we can increase or decrease the size of an array as per requisites. It also provide read and write functionalities. ArrayList may contain duplicates and is non-synchronized in nature. We use ArrayList to access the index of the …

Kotlin Program to Convert List (ArrayList) to Array and Vice-Versa

In this program, you’ll learn to convert a list to an array using toArray() and array to list using asList() in Kotlin.

Example 1: Convert array list to array

fun main(args: Array) < // an arraylist of vowels val vowels_list: List= listOf("a", "e", "i", "o", "u") // converting arraylist to array val vowels_array: Array = vowels_list.toTypedArray() // printing elements of the array vowels_array.forEach < System.out.print(it) >>

In the above program, we have defined an array list, vowels_list . To convert the array List to an Array, we have used the toTypedArray() method.

Finally, the elements of the array are printed by using the forEach() loop.

Example 2: Convert array to array list

fun main(args: Array) < // vowels array val vowels_array: Array= arrayOf("a", "e", "i", "o", "u") // converting array to array list val vowels_list: List = vowels_array.toList() // printing elements of the array list vowels_list.forEach < System.out.print(it) >>

To convert an array to an array list, we have used the toList() method.

Here’s the equivalent Java code: Java program to convert list to array and vice-versa.

Kotlin Lists and Arrays, Array is equivalent to Java’s array, nothing special for it either, IntArray and other primitive array company is used to make up for the lack of primitive types in kotlin, Array is same as Integer [] in Java, while IntArray is same as int []. Same logic is applied to all other variants.

How to convert a type-erased list to an array in Kotlin?

A function toArray should convert type-erased list to T that is Array now.

inline fun toArray(list: List): T < return list.toTypedArray() as T >toArray>(listOf("a", "b", "c")) // should be arrayOf("a", "b", "c") 

However, toArray throws this error.

Problem here, is that you actually trying cast Object[] to String[] in terms of Java, or Array to Array in terms of Kotlin, but this is different objects.

returns Array , and then you trying to cast it to Array and get ClassCastException .

How to fix?

I suggest pass type parameter itself, and cast List :

inline fun toArray(list: List): Array < return (list as List).toTypedArray() > toArray(listOf("1", "2")) 

To understand why this happens I recommend reading difference between list and array types in kotlin and Java Practices -> Prefer Collections over older classes.

In general I recommend using Lists instead of Arrays but if you want to convert a type-erased list to an array in kotlin the easiest way to do so is first mapping to a typed list and then converting to a typed array:

If you don’t actually need an array but a typed list will do then you can simply map the type-erased list to a typed list:

If you find yourself often converting a type-erased list to an array and performance matters then I recommend creating a map function optimized for doing this while avoiding an intermediary list:

inline fun List.mapToTypedArray(transform: (T) -> R): Array  < return when (this) < is RandomAccess ->Array(size) < index ->transform(this[index]) > else -> with(iterator()) < Array(size) < transform(next()) >> > > 

You can then efficiently convert a List to a typed array:

For some reason, I was getting an invalid argument number exception when I was trying to do this:

val argumentTypeMirrors = mutableListOf() . // add items to argumentTypeMirrors val array = argumentTypeMirrors.toTypedArray() 

And I ended up doing it this way:

val argumentTypeMirrors = mutableListOf() . // add items to argumentTypeMirrors val array = Array(argumentTypeMirrors.size)

Then I was able to destruct my array with *array for passing it as a varargs parameter.

Copy one arraylist to another arraylist in kotlin, You can simply pass another List instance into the constructor of your new List. val originalList = arrayListOf (1,2,3,4,5) val orginalListCopy = ArrayList (originalList) Share. answered Oct 28, 2020 at 11:15. trOnk12.

Kotlin — Convert Collection to Array List

I’m converting a Java application to Kotlin. In one area it’s using apache IO’s fileutils listfiles functions.

These return collections and I’m having problems converting/casting the collections into ArrayList

 val myFiles = FileUtils.listFiles(mediaStorageDir, extensions, true) as ArrayList

Whilst this compiles I get a runtime error as follows:

java.util.LinkedList cannot be cast to java.util.ArrayList

What’s the correct way to convert a collection object into an ArrayList?

You can easily create ArrayList in Kotlin from any Collection

val collection = FileUtils.listFiles(mediaStorageDir, extensions, true) val arrayList = ArrayList(collection) 

ArrayList is a class with implementation details. FileUtils is returning a LinkedList and that is why you are having issues. You should do a cast to List instead since it is an interface that both ArrayList and LinkedList implement.

If you would need to modify the returned list you can use MutableList instead the immutable one.

But this is not the best approach because if the creators of FileUtils later change the implementation details of their collections your code may start crashing.

A better soulition if explicitly need it to be an arrayList instead of a generic list could be:

val myFiles = FileUtils.listFiles(mediaStorageDir, extensions, true) val array = arrayListOf() array.addAll(myFiles) 

The as operator is a cast. But a cast does not convert a value to a given type; a cast claims that the value is already of that type.

A LinkedList is not an ArrayList , so when you claim it is, the computer will call you out on your mistake — as you see!

Instead, you need* to convert the value. Perhaps the best way is to call the extension function .toArrayList() . Alternatively, you could call the constructor explicitly: ArrayList(…) .

(* This assumes you actually need an ArrayList . In practice, it’s usually much better to refer only to the interface, in this case List or MutableList . As per other comments, the function you’re calling is declared to return a Collection ; the current implementation may return a LinkedList for you now, but that could change, so it’s safest not to assume it implements either of those interfaces, and instead to call .toList() or .toMutableList() .)

How to convert intArray to ArrayList in Kotlin?, You can get List with a simple toList call like so: val list = intArrayOf (5, 3, 0, 2).toList () However if you really need ArrayList you can create it too: val list = arrayListOf (*intArrayOf (5, 3, 0, 2).toTypedArray ()) or using more idiomatic Kotlin API as suggested by @Ilya:

Источник

Convert a List Into an Array in Kotlin

As a seasoned developer, you’re likely already familiar with Spring. But Kotlin can take your developer experience with Spring to the next level!

• Add new functionality to existing classes with Kotlin extension functions.
• Use Kotlin bean definition DSL.
• Better configure your application using lateinit.
• Use sequences and default argument values to write more expressive code.

By the end of this talk, you’ll have a deeper understanding of the advanced Kotlin techniques that are available to you as a Spring developer, and be able to use them effectively in your projects.

1. Overview

In this quick tutorial, we’ll explore how to convert a List into an Array in Kotlin. Further, we’ll discuss the preference for the choice between Arrays and ArrayList in our daily work.

We’ll use assertions in unit tests to verify the conversion result for simplicity.

2. Converting a List Into an Array

When we want to convert a List into an Array in Kotlin, the extension function Collection.toTypedArray() is the way. Let’s see an example:

val myList = listOf("one", "two", "three", "four", "five") val expectedArray = arrayOf("one", "two", "three", "four", "five") assertThat(myList.toTypedArray()).isEqualTo(expectedArray)

If we execute the test above, it passes. Since the toTypedArray is an extension function on the Collection interface, it’s available on all Collection objects, such as Set and List.

We know that in Kotlin, except for Array , we have primitive type array types, such as IntArray, LongArray, and more. As primitive type arrays are more efficient than their typed arrays, we should prefer using the primitive type arrays. Kotlin has provided different convenient extension functions on the corresponding Collection interfaces, for example Collection.toLongArray():

val myList = listOf(1L, 2L, 3L, 4L, 5L) val expectedArray = longArrayOf(1L, 2L, 3L, 4L, 5L) assertThat(myList.toLongArray()).isEqualTo(expectedArray)