Collections

Kotlin’s standard library provides three core collection types — List, Set, and Map — each in read-only and mutable flavors.

Read-only vs mutable

val readOnly = listOf(1, 2, 3)            // List<Int> — no add/remove
val mutable = mutableListOf(1, 2, 3)      // MutableList<Int>
mutable.add(4)

// readOnly.add(4)  // compile error: List has no `add`

The read-only interfaces (List, Set, Map) do not expose mutators. Prefer them — expose List from a function unless callers genuinely need to mutate.

Read-only is not the same as immutable: a List may be a view over a MutableList that something else can change. It simply means you cannot mutate it through this reference.

Lists

val nums = listOf(10, 20, 30)
nums[0]                 // 10  (indexing)
nums.first()            // 10
nums.last()             // 30
nums.size               // 3
20 in nums              // true

Sets (unique elements)

val s = setOf(1, 2, 2, 3)     // {1, 2, 3}
s.contains(2)                 // true
val m = mutableSetOf<String>()
m.add("a"); m.add("a")        // second add is a no-op

Maps (key → value)

val ages = mapOf("Ada" to 36, "Alan" to 41)
ages["Ada"]                   // 36
ages.getOrDefault("Bob", 0)   // 0
ages.keys                     // [Ada, Alan]
ages.values                   // [36, 41]

val counts = mutableMapOf<String, Int>()
counts["x"] = (counts["x"] ?: 0) + 1

to is an infix function that builds a Pair.

Arrays

Arrays are fixed-size and mostly used for interop or performance:

val arr = intArrayOf(1, 2, 3)
val strs = arrayOf("a", "b")
arr[0] = 9

Prefer List in everyday code; reach for arrays when an API requires them.

Exercises

1. Build a Map<String, Int> of three people to their ages and print each on its own line.

2. Write wordFrequencies(text: String): Map<String, Int> that counts how many times each (lowercased) word appears.

   Solution

   fun wordFrequencies(text: String): Map<String, Int> =
       text.lowercase()
           .split(Regex("\\W+"))
           .filter { it.isNotEmpty() }
           .groupingBy { it }
           .eachCount()
   

</details>

This solution is in examples/core/part4/ and is tested by CI.

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Next: Collection Operations