groupBy with +

class Dna(val sequence: String) {
    init {
        require(sequence.all { it in "ACGT" })
    }
    val nucleotideCounts
        get() = mapOf('A' to 0, 'C' to 0, 'G' to 0, 'T' to 0) + sequence.groupingBy { it }.eachCount()
}

Verification

We receive a sequence as input, which we have to verify. Because the sequence is part of the primary constructor, which can't contain runnable code, we have to place the check in an initializer block. In Kotlin it's idiomatic to use the require keyword for verification. require will throw an IllegalArgumentException if the given value is false.

init {
    require(sequence.all { it in "ACGT" })
}

Base Map

We define a base map with a zero value for every nucleotide. We do this to ensure that every nucleotide will be present in the output, even if there is no count. Afterward we can overwrite some of the values by using the + operator. Note that the + operator on maps does not add the values but overwrites them.

val nucleotideCounts
    get() = mapOf('A' to 0, 'C' to 0, 'G' to 0, 'T' to 0)

Counting Nucleotides

We use the .groupingBy function to create a Grouping of the nucleotides. By using the .eachCount() function we turn the Grouping in a Map. For example: "ACA".groupingBy { it }.eachCount() results in {A=2, C=1}. After adding the maps together we receive our final answer.

val nucleotideCounts
    get() = mapOf('A' to 0, 'C' to 0, 'G' to 0, 'T' to 0) + sequence.groupingBy { it }.eachCount()