Convert list to array kotlin

Kotlin — Convert Collection to Array List

Both lists allow you to change properties of those elements. Will divide this answer into three answers: is the interface which extends interface, provides basic common list operations, extends interface as well as interface adding methods needed to change elements of that list, is function which creates and fills it with given arguments, by using we don’t need to specify which implementation of will be used, for example on JVM is backed by (not same as ), while on JavaScript side it is probably backed up by (take this statement with grain of salt, as I have never worked with Kotlin for JS) is typealias to , there is nothing special about it, it is implemenentation of Java’s interface, is backed by this implementation on JVM. is equivalent to Java’s array, nothing special for it either, and other primitive array company is used to make up for the lack of primitive types in kotlin, is same as in Java, while is same as . From memory perspective, is continuous region in memory which size doesn’t change, that is why you can’t change the size of after it is created, but you can change its elements, on other hand can be implemented in different ways, meaning that memory structure can be different, most common implementations are where array is used to store elements, and once array is filled, its changed with bigger array with contents of old one being added to new one, another implementation is , where you have nodes pointing to next element on list.

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() .)

Copy one arraylist to another arraylist in kotlin, 1k 10 204 207. Add a comment. 2. 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 Lists and Arrays [duplicate]

Just started working with Kotlin and I love it but. I can not make any sense of Lists and Arrys in this language. I’m not new to programming and do not need an explanation on what arrays are. What I do not understand is.

What is the difference between a list and an Array? They seem very much the same you access both using a[index] and use them in much the same way. If a list is immutable they are even more the same, so. What is the difference? Assuming the list is not a linked list they both work in O(1) access time.

Sorry for the long question,

1. First difference is that List is interface describing some common list operations, while Array is a class. From memory perspective, Array is continuous region in memory which size doesn’t change, that is why you can’t change the size of Array after it is created, but you can change its elements, on other hand List can be implemented in different ways, meaning that memory structure can be different, most common implementations are ArrayList where array is used to store elements, and once array is filled, its changed with bigger array with contents of old one being added to new one, another implementation is LinkedList , where you have nodes pointing to next element on list. From performance perspective Array is always faster than any implementation of List but it is also much more limited.
2. Difference between List and MutableList is that when you use MutableList you can change elements of that list(add or remove elements from it), while when using immutable List you can’t add or remove elements from it. Both lists allow you to change properties of those elements.
3. Will divide this answer into three answers:
   • List is the interface which extends Collection interface, provides basic common list operations, MutableList extends List interface as well as MutableCollection interface adding methods needed to change elements of that list, listOf is function which creates List and fills it with given arguments, by using listOf we don’t need to specify which implementation of List will be used, for example on JVM List is backed by java.util.Arrays.ArrayList (not same as java.util.ArrayList ), while on JavaScript side it is probably backed up by Array (take this statement with grain of salt, as I have never worked with Kotlin for JS)
   • ArrayList is typealias to java.util.ArrayList , there is nothing special about it, it is implemenentation of Java’s List interface, MutableList is backed by this implementation on JVM.
   • 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. Using primitive types you get better performance, but difference can be neglected in most cases on modern computers, still if you have really a lot of data you should go for primitive types where possible.

You can see yourself all collections hierarchy on kotlin repository

1. Use built-in Kotlin functions like listOf , arrayOf , mutableListOf , this isn’t a must, but its always good to follow best practices.

Coming from C/C++ the multitude of different names is very confusing.

Then maybe this can give C++ analogy specifically:

1. Array is like std::array (though length doesn’t need to be known at compile time), or like C arrays, except it stores the length and all accesses are bounds-checked.
2. ArrayList is like std::vector (again, all accesses are bounds-checked).
3. MutableList is the interface to ArrayList (like SequenceContainer ).
4. List is the read-only part of MutableList .

Generics work very differently from C++ templates, in particular there’s no specialization: in C++, there is separate code generated for std::vector and std::vector , in Java and Kotlin there isn’t. (Actually, Kotlin has a form of it with reified type parameters, but it doesn’t apply here.) So e.g. Array and List have to work with boxed java.lang.Integer s instead of primitive types. But Java does have arrays of primitives, and that’s what Kotlin calls IntArray .

Kotlin Program to Convert List (ArrayList) to Array and, 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

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.

Using Kotlin’s MutableList or ArrayList where lists are, I am trying to learn the «Kotlin native way» to do things in Android, while being an expert in neither Kotlin, Java, nor Android development. Specifically, when to use ArrayList versus MutableList. It seems to me that MutableList should be chosen whenever possible.Yet, if I look at Android examples, they seem to …

Convert Array<String> to ArrayList<String&gt

How do I convert Array to ArrayList in Kotlin?

var categoryList : ArrayList?=null val list = arrayOf("None", "ABC") categoryList = ArrayList(Arrays.asList(list)) 

Type inference failed. Expected type mismatch: inferred type is kotlin.collections.ArrayList!> /* = java.util.ArrayList!> */ but kotlin.collections.ArrayList? /* = java.util.ArrayList? */ was expected 

You may be interested in toCollection to add the elements to any collection you pass:

categoryList = list.toCollection(ArrayList()) 

Note that to fix your specific problem you just need the spread operator ( * ). With the * -fix applied it would have worked too:

categoryList = ArrayList(Arrays.asList(*list)) 

var categoryList : ArrayList?=null val list = arrayOf("None", "ABC") categoryList = list.toCollection(ArrayList()) 

Another approach is use arrayListOf

 categoryList =arrayListOf("None", "ABC") 

The spread operator and arrayListOf is another option:

var categoryList : ArrayList?=null val list = arrayOf("None", "ABC") categoryList = arrayListOf(*list) 

Kotlin: How to save an arraylist in SharedPreferences, 0. You can use GSON library for this. At first, you have to convert the ArrayList to string. For example: val arrayString = Gson ().toJson (arrayList) Then, you can store the string in SharedPreference. To retrieve the String, you have to retrieve the String first and then convert it again to ArrayList. For example:

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Convert a list to an array in Kotlin

This article explores different ways to convert a list to an array in Kotlin.

1. Using toTypedArray() function

List interface provides a toTypedArray() function that returns a typed array containing the list elements.

To convert between two types, you can use the map() function. For example, the following code converts from a list of Integer to an array of String:

2. Using Java 8 Stream

You can also use Stream to convert a list to an array. The trick is to convert the given list to Stream using the stream() function and use the toArray() function to return an array containing the Stream elements. The conversion between the two types can be done using a lambda expression.

That’s all about converting a list to an array in Kotlin.

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