Maps in Kotlin are powerful collection types that store key-value pairs. They provide an efficient way to organize and retrieve data based on unique keys.

Creating Maps

Kotlin offers several ways to create maps:


// Using mapOf() function
val readOnlyMap = mapOf("key1" to "value1", "key2" to "value2")

// Using mutableMapOf() for mutable maps
val mutableMap = mutableMapOf<String, Int>()

// Using HashMap
val hashMap = HashMap<String, Double>()

Accessing and Modifying Map Elements

You can access map elements using square bracket notation or the get() function:


val map = mapOf("name" to "John", "age" to 30)

println(map["name"]) // Output: John
println(map.get("age")) // Output: 30

// For mutable maps, you can modify values
val mutableMap = mutableMapOf("x" to 1, "y" to 2)
mutableMap["x"] = 10
mutableMap.put("z", 3)

Useful Map Operations

size: Returns the number of entries in the map
isEmpty(): Checks if the map is empty
containsKey(key): Checks if the map contains a specific key
containsValue(value): Checks if the map contains a specific value
keys: Returns a set of all keys in the map
values: Returns a collection of all values in the map

Iterating Over Maps

Kotlin provides convenient ways to iterate over maps:


val fruitInventory = mapOf("apples" to 5, "bananas" to 8, "oranges" to 3)

// Iterate over entries
for ((fruit, quantity) in fruitInventory) {
    println("We have $quantity $fruit")
}

// Iterate over keys
for (fruit in fruitInventory.keys) {
    println("Fruit: $fruit")
}

// Iterate over values
for (quantity in fruitInventory.values) {
    println("Quantity: $quantity")
}

Map Transformations

Kotlin's standard library offers various functions for map transformations:


val numbers = mapOf("one" to 1, "two" to 2, "three" to 3)

// Filter
val evenNumbers = numbers.filter { (_, value) -> value % 2 == 0 }

// Map transformation
val doubled = numbers.mapValues { (_, value) -> value * 2 }

// Merge two maps
val map1 = mapOf("a" to 1, "b" to 2)
val map2 = mapOf("b" to 3, "c" to 4)
val merged = map1 + map2 // Result: {"a" to 1, "b" to 3, "c" to 4}

Best Practices

Use immutable maps (mapOf()) when the content doesn't need to change
Prefer mutableMapOf() over HashMap for better interoperability
Use Kotlin Nullable Types for keys or values that might be null
Consider using Kotlin Data Classes as map keys for complex objects

Related Concepts

To further enhance your understanding of Kotlin collections, explore these related topics:

Maps are essential data structures in Kotlin programming. They offer efficient key-value storage and retrieval, making them ideal for various applications such as caching, data organization, and complex data relationships.