How to compare two arrays in Kotlin?

问题:

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.

回答1:

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.

回答2:

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

回答3:

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

回答4:

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().