Kotlin For Android - Collections APIs (part 6 of 7)
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This document discusses various collection APIs in Kotlin including functions for filtering collections like filter, transforming collections like map, reducing collections to a single value using fold, and other common collection operations like sorting, joining, finding min/max. It provides examples of how to use functions like filter, map, fold, joinToString and others to manipulate and work with collections in Kotlin.
![drop, take
COLLECTIONS
val a = arrayOf(-1, 0, 1, 2)
a.drop(2) == a.dropWhile { it <= 0 } // [1, 2]
a.take(2) == a.takeWhile { it <= 0 } // [-1, 0]
y/kotlin-workbook
● drop to skip a few elements
● take to drop the rest](https://image.slidesharecdn.com/kotlinforandroid-part6-collections-181019150028/85/Kotlin-For-Android-Collections-APIs-part-6-of-7-11-320.jpg)
Kotlin For Android - Collections APIs (part 6 of 7)
- 1. Kotlin for Android May 24, 2018 y/kotlin-workbook
- 3. all, none COLLECTIONS fun <T> Iterable<T>.all( predicate: (T) -> Boolean ): Boolean fun <T> Iterable<T>.none( predicate: (T) -> Boolean ): Boolean y/kotlin-workbook ● Checks if all values in a collection evaluate correctly ● all breaks at the first false ● none breaks at the first true
- 4. any, find, single COLLECTIONS fun <T> Iterable<T>.any( predicate: (T) -> Boolean ): Boolean y/kotlin-workbook ● Checks if any values in a collection evaluate to true ● any and find break at the first true ● single throws exception if 0 or more than 1 element match ● find and single return the element that matched
- 5. asList toList toSet ... COLLECTIONS fun <T, C : MutableCollection<in T>> any_collection<out T>.toCollection( destination: C ): C y/kotlin-workbook ● Converts one collection to another
- 6. associate associateBy associateByTo COLLECTIONS fun <T, K, V> Iterable<T>.associate( transform: (T) -> Pair<K, V> ): Map<K, V> {} fun <T, K> Iterable<T>.associateBy( keySelector: (T) -> K ): Map<K, T> {} fun <T, K, V> Iterable<T>.associateBy( keySelector: (T) -> K, valueTransform: (T) -> V ): Map<K, V> {} y/kotlin-workbook ● Map a collection
- 7. component1-5 COLLECTIONS val pixel = arrayOf(0, 5, 10, 25) val (_, r, _, b) = pixel val hasYelpPotential = pixel.let { (a, r) -> a == 0xff && r == 0xd3 } y/kotlin-workbook ● Destructing ● Can only be used to initialize, but not assign
- 8. val day = 1..24 val has5 = 5 in day || day.contains(5) val no30 = 30 !in day || !day.contains(30) contains in COLLECTIONS y/kotlin-workbook ● Prefer in over contains
- 9. filter*** COLLECTIONS val day = 1..24 val workingHours = day.filter { it in 9..17 } val workingHours = day.filter((9..17)::contains) y/kotlin-workbook ● A whole family of fuctions ● Create a subset of the original collection ● Can transform to antoher collection type
- 10. distinct COLLECTIONS y/kotlin-workbook ● Like toSet(), but returns a List (maintaining the order of elements)
- 11. drop, take COLLECTIONS val a = arrayOf(-1, 0, 1, 2) a.drop(2) == a.dropWhile { it <= 0 } // [1, 2] a.take(2) == a.takeWhile { it <= 0 } // [-1, 0] y/kotlin-workbook ● drop to skip a few elements ● take to drop the rest
- 12. flatMap flatten COLLECTIONS fun <T, R> Iterable<T>.flatMap( transform: (T) -> Iterable<R> ): List<R> fun <T> Iterable<Iterable<T>>.flatten(): List<T> y/kotlin-workbook ● Flatten a collection of composite objects to a list
- 13. fold, reduce COLLECTIONS fun <T, R> Iterable<T>.fold( initial: R, operation: (acc: R, T) -> R ): R val day = 1..24 day.fold("") { acc, next -> "$acc,$next" } y/kotlin-workbook ● Flatten a collection of objects to a single object
- 14. forEach onEach COLLECTIONS y/kotlin-workbook ● Replacement for a for loop ● onEach returns the collection after the loop has been evaluated
- 15. joinTo joinToString COLLECTIONS fun <T, A: Appendable> Iterable<T>.joinTo( buffer: A, separator: CharSequence = ", ", prefix: CharSequence = "", postfix: CharSequence = "", limit: Int = -1, truncated: CharSequence = "...", transform: (T) -> CharSequence = null ): A joinToString(...) = joinTo(StringBuilder()).toString() (1..24).joinToString() y/kotlin-workbook ● Comma separation out-of-the box ● Quite configurable
- 16. min, max, average COLLECTIONS y/kotlin-workbook ● As you’d expect
- 17. plus, minus COLLECTIONS y/kotlin-workbook ● As you’d expect
- 18. orEmtpy COLLECTIONS val maybe: List<String>? = null maybe.joinToString() // Compile Error maybe.orEmpty().joinToString() // Empty Non-Null String y/kotlin-workbook ● Extension on nullable collections ● Returns empty collection instead of null
- 19. reverse COLLECTIONS y/kotlin-workbook ● Reverses the order of elements
- 20. zip, unzip COLLECTIONS y/kotlin-workbook ● zip 2 collections into a List<Pair> ● unzip a list of Pairs into a Pair of lists ● zip into a custom composite - unzip it via associate
- 21. Questions? Next Up: Custom DSLs
Editor's Notes
- #3: Add a slide about asSequence() Mark terminal vs transfomation functions Callout that all of these are extension functions
- #8: Not limited to interables Auto-generated for data classes - so choose your first 5 constructor arguments carefully
- #10: Be mindful of allocations inside your looped functions
- #13: Example a list of businesses, each business has a list of phone numbers -> a list of phone numbers
- #21: ALMOST END OF SLIDES - NEXT SLIDE IS LAST - Q&A STARTS