How to get key for value from Hashmap in Kotlin?

Question

I have HashMap in Kotlin

val map = HashMap<String, String>()

I want to know how to get key for a particular value from this HashMap without iterating through complete HashMap?

Solution 1

Using filterValues {}

val map = HashMap<String, String>()
val keys = map.filterValues { it == "your_value" }.keys

And keys will be the set of all keys matching the given value

Solution 2

In Kotlin HashMap, you can use these ways:

val hashMap = HashMap<String, String>() // Dummy HashMap.

val keyFirstElement = hashMap.keys.first() // Get key.
val valueOfElement = hashMap.getValue(keyFirstElement) // Get Value.
    
val keyByIndex = hashMap.keys.elementAt(0) // Get key by index.
val valueOfElement = hashMap.getValue(keyByIndex) // Get value.

Solution 3

If you are constantly finding keys by values, a possible solution could be to reverse the map, so you can get any key by any value.

For instance:

val reversed = map.entries.associate{(k,v)-> v to k}

val resultKey = reversed[value]

Hope it helps!