How to compare two arrays in Kotlin?

Question

Given some arrays in Kotlin

let a = arrayOf("first", "second")
val b = arrayOf("first", "second")
val c = arrayOf("1st", "2nd")

Are there built-in functions to the Kotlin std-lib that tests two arrays for (value) equality for each element?

Thus resulting in:

a.equals(b) // true
a.equals(c) // false

equals() is actually returning false in both cases, but maybe there are built-in functions to Kotlin that one could use?

There is the static function java.utils.Arrays.deepEquals(a.toTypedArray(), b.toTypedArray()) but I would rather prefer an instance method as it would work better with optionals.

That's probably a case for extension functions. – Kirill Rakhman Feb 08 '16 at 17:12 — Kirill Rakhman, Feb 08 '16 at 17:12

mfulton26 · Accepted Answer · 2017-05-01T13:25:00.247

In Kotlin 1.1 you can use contentEquals and contentDeepEquals to compare two arrays for structural equality. e.g.:

a contentEquals b // true
b contentEquals c // false

In Kotlin 1.0 there are no "built-in functions to the Kotlin std-lib that tests two arrays for (value) equality for each element."

"Arrays are always compared using equals(), as all other objects" (Feedback Request: Limitations on Data Classes | Kotlin Blog).

So a.equals(b) will only return true if a and b reference the same array.

You can, however, create your own "optionals"-friendly methods using extension functions. e.g.:

fun Array<*>.equalsArray(other: Array<*>) = Arrays.equals(this, other)
fun Array<*>.deepEqualsArray(other: Array<*>) = Arrays.deepEquals(this, other)

P.S. The comments on Feedback Request: Limitations on Data Classes | Kotlin Blog are worth a read as well, specifically comment 39364.

Thank god. `Arrays.equals` was too confusing. – Jeffrey Blattman Apr 28 '22 at 21:32 — Jeffrey Blattman, Apr 28 '22 at 21:32

score 25 · Answer 2 · answered Apr 19 '17 at 19:06

25

Kotlin 1.1 introduced extensions for comparing arrays by content: contentEquals and contentDeepEquals.

These extensions are infix, so you can use them the following way:

val areEqual = arr1 contentEquals arr2

answered Apr 19 '17 at 19:06

Ilya

21,871
8
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score 19 · Answer 3 · answered Feb 13 '18 at 15:15

19

And if you want to compare contents of two Collections ignoring the order you can add this simple extension:

infix fun <T> Collection<T>.sameContentWith(collection: Collection<T>?)
    = collection?.let { this.size == it.size && this.containsAll(it) }

...and use it like this:

a = mutableListOf<String>()
b = mutableListOf<String>()

isListsHasSameContent = a sameContentWith b

answered Feb 13 '18 at 15:15

Alexander Krol

1,418
15
15

Awesome. This should be the accepted answer for collection (if not using array) – mochadwi Jun 03 '19 at 07:24
3

Watch out for edge condition; `listOf("a", "b", "b").sameContentWith(listOf("a", "a", "b"))` is true when it might be expected to return false. – dnault Sep 20 '21 at 20:02
@dnault thanks for mentioning this. well, looks like this works perfectly for Sets :) – Alexander Krol Sep 06 '22 at 20:08
1

`sameContentWith` unfortunately has even more errors. You can give elements into the left collection that aren't in the right collection, and even then `sameContentWith` might return true - for example in this case: `listOf("a", "b").sameContentWith(listOf("a", "a"))` – fjf2002 Jan 04 '23 at 14:33

score 5 · Answer 4 · answered Aug 04 '16 at 13:17

5

For a simple equals (not deep equals!):

otherArray.size == array.size && otherArray.filter { !array.contains(it) }.isEmpty()

This code will compare the size and the items. The items are compared with .equals().

answered Aug 04 '16 at 13:17

Kevin Robatel

8,025
3
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9

This does not account for the order of the Array; however the general method is good for Sets. – Chris Hatton Feb 01 '17 at 10:06

score 2 · Answer 5 · answered Jan 14 '20 at 07:22

In koltin if your array or ArrayList is type of a data Class you can simply compare array :a and array :b

like this

if(a == b)

it will return simple boolean if it matched all the value of both arrays, but if you are matching other-than data data Class then you can use this extension to compare it with single value

fun <T> Array<T>.isEqual(comparable: Array<T>): Boolean {
    var isChanged = true

    if (this.size == comparable.size) {
        for (index in 0 until comparable.size) {
            if (this[index] != comparable[index]) {
                isChanged = false
                break
            }
        }
    } else {
        isChanged = false
    }

    return isChanged
}

then you can use your array

    val a = arrayOf("first", "second")
    val b = arrayOf("first", "second")

    println(a.isEqual(b)) //true

    a[0] = "third"

    println(a.isEqual(b)) //false

Where do you save this function for future reuse? Github repo? — Geoff Langenderfer, May 14 '20 at 15:56

Ahamadullah Saikat · Answer 6 · 2023-05-15T08:17:59.067

Checking of Two ArrayList equality:

fun isArrayListEquals(
    list1: ArrayList<CustomItem>,
    list2: ArrayList<CustomItem>,
): Boolean {
    if (list1.size != list2.size) return false

    val hash1 = list1.map { it.hashCode() }.toSet()
    val hash2 = list2.map { it.hashCode() }.toSet()

    return (hash1.intersect(hash2)).size == hash1.size
}


import java.util.Objects;

public class CustomItem {
    private String id, title, key;

    // implement getter/setter here

    // Mandatory
    @Override
    public int hashCode() {
        return Objects.hash(id, title, key);
    }
}

Or Simple Solution:

fun isArrayListEquals(
    list1: ArrayList<CustomItem>,
    list2: ArrayList<CustomItem>,
): Boolean {
    val gson = Gson()
    val list1Str = gson.toJson(list1)
    val list2Str = gson.toJson(list2)
    return list1Str == list2Str
}

How to compare two arrays in Kotlin?

6 Answers6

Checking of Two ArrayList equality:

Or Simple Solution:

Linked