MultiKeyMap Implementation in Kotlin

MultiKeyMap is a map implementation that uses multiple keys to map the value. This post will provide MultiKeyMap implementation in Kotlin.

The idea is to construct a class that consists of all keys and uses an instance of that class in our map as a key to map the value. The class should override equals() and hashCode() methods to test equality on a hash-based map.

This approach is demonstrated below for two keys, but we can easily extend it to an arbitrary number of keys.

internal class Key<K1, K2>(key1: K1, key2: K2) {

private var key1: K1? = key1

private var key2: K2? = key2

override fun equals(other: Any?): Boolean {

if (this === other) {

return true

}

if (other == null || javaClass != other.javaClass) {

return false

}

val key = other as Key<*, *>

if (if (key1 != null) key1 != key.key1 else key.key1 != null) {

return false

}

return !if (key2 != null) key2 != key.key2 else key.key2 != null

}

override fun hashCode(): Int {

var result = if (key1 != null) key1.hashCode() else 0

result = 31 * result + if (key2 != null) key2.hashCode() else 0

return result

}

override fun toString(): String {

return "[$key1, $key2]"

}

fun main() {

val multiKeyMap: MutableMap<Key<*, *>, String> = HashMap()

// [key1, key2] -> value1

val key1plus2: Key<*, *> = Key<Any?, Any?>("key1", "key2")

multiKeyMap[key1plus2] = "value1"

// [key3, key4] -> value2

val key3plus4: Key<*, *> = Key<Any?, Any?>("key3", "key4")

multiKeyMap[key3plus4] = "value2"

// print multikey map

println(multiKeyMap)

}

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Output:

{[key1, key2]=value1, [key3, key4]=value2}

Kotlin’s data class already provides an implementation of equals() and hashCode() methods. The following example shows usage of data class to implement a MultiKeyMap:

data class Key<K1, K2>(val key1: K1, val key2: K2) {