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Kotlin provides a powerful and expressive Collections API that simplifies working with data structures such as lists, sets, and maps. The Collections API offers both mutable and immutable collections, making it easier to manage data efficiently and safely. In this blog, we will explore the Collections API with examples and best practices.
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1. Immutable vs Mutable Collections
Kotlin distinguishes between immutable and mutable collections.
Immutable collections: Cannot be modified after creation.
Mutable collections: Can be modified (add, remove, update elements).
Creating Immutable Collections
Immutable collections are part of Kotlin's standard library and ensure safety in multi-threaded environments.
val numbers: List<Int> = listOf(1, 2, 3, 4, 5)
val names: Set<String> = setOf("Alice", "Bob", "Charlie")
val userAges: Map<String, Int> = mapOf("Alice" to 25, "Bob" to 30)Creating Mutable Collections
Mutable collections allow modifications, making them useful for dynamic data structures.
val numbers = mutableListOf(1, 2, 3)
numbers.add(4) // [1, 2, 3, 4]
val names = mutableSetOf("Alice", "Bob")
names.add("Charlie") // [Alice, Bob, Charlie]
val userAges = mutableMapOf("Alice" to 25)
userAges["Bob"] = 30 // {Alice=25, Bob=30}2. List in Kotlin
A List is an ordered collection of elements. It can be either immutable (List) or mutable (MutableList).
Common List Operations
val fruits = listOf("Apple", "Banana", "Cherry")
println(fruits[0]) // Apple
println(fruits.size) // 3
println(fruits.contains("Banana")) // trueModifying a MutableList
val numbers = mutableListOf(10, 20, 30)
numbers.add(40) // [10, 20, 30, 40]
numbers.remove(20) // [10, 30, 40]
numbers[0] = 100 // [100, 30, 40]3. Set in Kotlin
A Set is an unordered collection of unique elements.
Common Set Operations
val numbers = setOf(1, 2, 3, 3, 4, 5)
println(numbers) // [1, 2, 3, 4, 5] (duplicates removed)
println(numbers.contains(2)) // trueModifying a MutableSet
val colors = mutableSetOf("Red", "Blue")
colors.add("Green") // [Red, Blue, Green]
colors.remove("Blue") // [Red, Green]4. Map in Kotlin
A Map is a collection of key-value pairs.
Common Map Operations
val ages = mapOf("Alice" to 25, "Bob" to 30)
println(ages["Alice"]) // 25
println(ages.keys) // [Alice, Bob]
println(ages.values) // [25, 30]Modifying a MutableMap
val scores = mutableMapOf("Math" to 90)
scores["Science"] = 85 // {Math=90, Science=85}
scores.remove("Math") // {Science=85}5. Higher-Order Functions in Collections
Kotlin collections support functional programming with built-in higher-order functions.
Filter, Map, and Reduce
val numbers = listOf(1, 2, 3, 4, 5)
val evenNumbers = numbers.filter { it % 2 == 0 } // [2, 4]
val squares = numbers.map { it * it } // [1, 4, 9, 16, 25]
val sum = numbers.reduce { acc, num -> acc + num } // 15Sorting and Reversing
val names = listOf("Charlie", "Alice", "Bob")
println(names.sorted()) // [Alice, Bob, Charlie]
println(names.reversed()) // [Bob, Alice, Charlie]Grouping and Chunking
val words = listOf("apple", "banana", "apricot", "blueberry")
val grouped = words.groupBy { it.first() }
println(grouped) // {a=[apple, apricot], b=[banana, blueberry]}6. Advanced Collection Operations
Kotlin also provides advanced collection functions for better data manipulation.
FlatMap and Flatten
val lists = listOf(listOf(1, 2, 3), listOf(4, 5, 6))
val flatList = lists.flatten()
println(flatList) // [1, 2, 3, 4, 5, 6]
val mapped = lists.flatMap { it.map { num -> num * 2 } }
println(mapped) // [2, 4, 6, 8, 10, 12]Zip and Unzip
val names = listOf("Alice", "Bob")
val ages = listOf(25, 30)
val paired = names.zip(ages)
println(paired) // [(Alice, 25), (Bob, 30)]
val (unzippedNames, unzippedAges) = paired.unzip()
println(unzippedNames) // [Alice, Bob]
println(unzippedAges) // [25, 30]Conclusion
The Collections API in Kotlin provides a robust set of tools for handling data efficiently. Understanding lists, sets, maps, and higher-order functions allows developers to write clean, concise, and efficient Kotlin code. Whether you are working with immutable collections for thread safety or mutable collections for dynamic data, Kotlin offers powerful and expressive solutions.
Want to learn more about Kotlin and Jetpack Compose? Stay tuned for more insightful blogs!
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Akshay Nandwana
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