Programming language: Kotlin
License: MIT License
Tags: Extensions    
Latest version: v1.1.2

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Successor of Ktoolz.

Collection of Kotlin extension functions and utilities. This library does not have any dependency.

Using Katlib

Katlib is available on the Maven Central. Then to import Katlib to Gradle project use:

implementation("dev.forst", "katlib", "2.2.2")

Or with Groovy DSL

implementation 'dev.forst:katlib:2.2.2'

To import Katlib to Maven project use:



Available online - katlib.forst.dev


Feel free to submit PR with your favourite extension functions and other cool utilities!


The library contains a lot of useful (as well as useless) extensions and functions that were gathered during my (and my colleges) Kotlin career. Please see [tests](src/test/kotlin/dev/forst/katlib) folder for all possible functions and how to use them. Full documentation can be found here.

Please note that some functions seems like duplicates of the standard library - usually it is not like that as they provide similar functionality on different interface. For example there's List.random but not Iterable.random - random on Iterable is then implemented in this library. Some functions are also essentially type aliases (for example equalsIgnoreCase) for standard library as I was having hard time finding correct name.

However, in some cases there might be duplicates, as development of this library started in 2018 and a lot of functions came to standard library in Kotlin 1.4. - so if you find some duplicates, let me know or create PR that deprecates them in the code.

Following functions are the most popular ones.

[Iterable Extensions](src/main/kotlin/dev/forst/katlib/IterableExtensions.kt)
  • Iterable<E>.random - returns the random element from the iterable
  • Iterable<T>.reduction - reduce producing list allowing you to set initial value, useful for cumulative sums
  • Iterable<List<Int>>.sumByIndexes - sums all Lists of integers into single one by indexes
  • Iterable<List<Double>>.sumDoublesByIndexes - same as previous but with doubles
  • Iterable<T>.mapToSet - creates set from iterable, transforms with given function
  • Iterable<T>.dominantValueBy - returns the most frequently occurring value of the given function
  • Iterable<T1>.cartesianProduct - cartesian product between all the elements from two iterables
  • Iterable<T?>.forEachNotNull - performs action on each element that is not null
  • Iterable<Iterable<T>?>.union - creates union of all iterables
  • Iterable<Iterable<T>?>.intersect - creates intersect of all iterables
  • Iterable<T?>.filterNotNullBy - returns list with only not null results using selector
  • Iterable<T>.singleOrEmpty - returns single element or null if it wasn't found, throws exception if there are multiple elements
  • Iterable<Pair<T, V>>.splitPairCollection - returns pair of two lists with left and right values
  • Iterable<T>.setDifferenceBy - returns difference between two iterables
  • Iterable<Pair<K, V>>.assoc - returns map from key value pairs but logs case when key is replaced by same key with different value
  • Iterable<Triple<A, B, C>>.flattenToLists - returns three lists constructed from triples
  • Iterable<T>.toNavigableSet - creates NavigableSet
  • Iterable<T>.isEmpty - determines whether the iterable is empty or not
  • Iterable<Iterable<T>>.cartesianProduct - cartesian product between all the elements from nested iterables
  • Iterable<Iterable<T>>.lazyCartesianProduct - cartesian product between all the elements from nested iterables as sequence
  • Iterable<A>.zip(b: Iterable<B>, c: Iterable<C>, transform: (a: A, b: B, c: C) -> V) - zip with three collections instead of two
  • Iterable<T>.sumByFloat(selector: (T) -> Float) - sums iterable by float selector, because sumOf from stdlib does not have implementation for Floats
  • Iterable<T>.withEach(action: T.() -> Unit) - performs the given action with each element as a receiver
  • Iterable<T>.withEachIndexed(action: T.(index: Int) -> Unit) - performs the given action with each element as a receiver, providing sequential index with the element
[Map Extensions](src/main/kotlin/dev/forst/katlib/MapExtensions.kt)
  • Map<T, Double>.getWeightedRandom - randomly selects item with respect to the current weight distribution
  • Map<K, V>.mergeReduce - two maps together using the given reduce function
  • Map<K, V1>.join - joins two maps together using the given join function
  • Map<K1, Map<K2, V>>.swapKeys(): Map<K2, Map<K1, V>> - swaps keys in two-dimensional maps
  • there are multiple swapKeys implementations for up to three-dimensional maps, just browse [the code](src/main/kotlin/dev/forst/katlib/MapExtensions.kt)
  • Map<Pair<K1, K2>, V>.toTwoLevelMap(): Map<K1, Map<K2, V>> - creates two-dimensional map from the map of pairs
  • Map<Triple<K1, K2, K3>, V>.toThreeLevelMap(): Map<K1, Map<K2, Map<K3, V>>> - creates three-dimensional map from the map of triples
  • Map<K1, Map<K2, V>>.getSecondLevelValues(): Set<V> - collects all the values from the bottom level into set
  • Map<K1, Map<K2, Map<K3, V>>>.getThirdLevelValues(): Set<V> - collects all the values from the bottom level into set
  • Iterable<Map<K, V>>.merge(): Map<K, List<V>> - for each key, merges all the values into one common list
[Set Extensions](src/main/kotlin/dev/forst/katlib/SetExtensions.kt)
  • SortedSet.min - returns minimum of the set or null if empty
  • SortedSet.max - returns maximum of the set or null
[Pair Extensions](src/main/kotlin/dev/forst/katlib/PairExtensions.kt)
  • mapLeft/Right/Pair - applies given block to left/right/all iterable element/s of the pair
val pair = Pair(listOf(1, 2, 3), 0)
assertEquals(Pair(listOf(11, 12, 13), 0), pair.mapLeft { it + 10 })
  • letLeft/Right/Pair - applies given block to left/right/all element/s of the pair
val pair = Pair(10, 20)
assertEquals(Pair("10", 20), pair.letLeft { it.toString() })
[Date Extensions](src/main/kotlin/dev/forst/katlib/DateExtensions.kt)
  • getDateRangeTo - returns list of dates between two LocalDate
  • getDaysInInterval - returns number of days between two LocalDate (inclusive)
  • getDayDifference - returns number of days between two LocalDate (exclusive)
  • getWeekOfYear - returns week of year for given LocalDate and optionaly Locale
[Jackson Extensions](src/main/kotlin/dev/forst/katlib/JacksonExtensions.kt)

To use these, one must include dependency on Jackson

implementation("com.fasterxml.jackson.core", "jackson-databind", jacksonVersion)
implementation("com.fasterxml.jackson.module", "jackson-module-kotlin", jacksonVersion)
  • jacksonMapper - creates jackson mapper with some reasonable settings
  • parseJson - parses JSON from the string/bytes, returns either instance of null, can log exception if some occurs
val obj: MyDataClass? = parseJson<MyDataClass>(myJson)
  • createJson - creates JSON from given object
  • createPrettyJson - creates JSON with pretty print
  • createJsonBytes - creates JSON in bytes from given object
  • prettyPrintJson - returns pretty printed JSON value as string
[Boolean Extensions](src/main/kotlin/dev/forst/katlib/BooleanExtensions.kt)

whenTrue and whenFalse - useful extensions mainly used for logging when the oneliners are used.

fun someFunctionIndicatingSuccess(): Boolean =
        .whenFalse {
            logger.warning { "someComputationReturningBoolean returned false! Computation probably failed" }
[String Extensions](src/main/kotlin/dev/forst/katlib/StringExtensions.kt)
  • startsWithLetter - returns true fi string starts with latin letter a-z or A-Z
  • restrictLengthWithEllipsis - shortens the string to given max length, appends ellipsis
assertEquals("ABCD…", "ABCDEFHG".restrictLengthWithEllipsis(5, "..."))
  • toUuid - converts string to UUID
[Instant Extensions](src/main/kotlin/dev/forst/katlib/InstantExtensions.kt)
  • durationToInMilli - returns absolute difference between two Instant values in milliseconds
[Crypto Extensions](src/main/kotlin/dev/forst/katlib/CryptoExtensions.kt)
  • hashWithSha256 - produces SHA-256 of given string/file/bytes
  • computeMd5 - computes MD5 from given byte array, returns base64 encoded data
[Miscellaneous Extensions](src/main/kotlin/dev/forst/katlib/OtherExtensions.kt)
  • Optional<T>.orNull(): T? - from optional to Kotlin optional
  • T.whenNull - executes block when this is null, useful for logging
fun someFunction(): String? =
        .whenNull { logger.warn { "produceOptionalString returned null value!" } }
  • T.asList - from `this creates one element list
  • ClosedRange<T>.intersects - intersection between ranges
  • T.with - bundles two objects to list
  • validate - executes invalid block if validating block returns false, useful for validation
    { someText.startsWith("something") && someText.endsWith("else") },
    { throw IllegalStateException() }
  • Pair<A?, B?>.propagateNull(): Pair<A, B>? - if left or right is null, returns null, otherwise pair
  • T.applyIf - applies given block only if should apply block returns true
byteBuffer.applyIf(shouldReadInt) { getInt() }
  • isUuid - returns true if given string is UUID
  • isUrl - returns true if give string is URL (with some limitations, see docs)
  • getEnv - shortcut for System.getenv
  • newLine - shortcut for System.lineSeparator
  • ByteArray.toUuid - Read ByteArray as two longs and combine the to UUID
  • TemporalProvider - Interface providing access to current time via now method, very useful when mocking
[Array Extensions](src/main/kotlin/dev/forst/katlib/ArrayExtensions.kt)
  • buildArray(builderAction: MutableList<E>.() -> Unit): Array<E> - builds a new Array by populating a MutableList using the given builderAction and returning an Array with the same elements
  • Array<out T>.map(transform: (T) -> R): Array<R> - Returns an array containing the results of applying the given transform function to each element in the original array
  • Array<out T>.mapIndexed(transform: (index: Int, T) -> R): Array<R> - returns an array containing the results of applying the given transform function to each element and its index in the original array
  • Array<out T>.filter(predicate: (T) -> Boolean): Array<T> - returns an array containing only elements matching the given predicate
  • Array<out T>.filterNot(predicate: (T) -> Boolean): Array<T> - returns an array containing all elements not matching the given predicate
  • Array<out T>.filterIndexed(predicate: (index: Int, T) -> Boolean): Array<T> - returns an array containing only elements matching the given predicate
  • Array<*>.filterIsInstance(): Array<R> - returns an array containing all elements that are instances of specified type parameter R
  • Array<out T?>.filterNotNull(): Array<T> - returns an array containing all elements that are not null
  • Array<out T>.minus(element: T): Array<T> - returns an array containing all elements of the original collection without the first occurrence of the given element
  • Array<out T>.minus(elements: Array<out T>): Array<T> - returns an array containing all elements of the original collection except the elements contained in the given elements array
  • prompt(promptText: String, exceptionHandler: (e: Exception) -> String? = { null }, transform: (input: String) -> R): R - prompts user and applies transform to input, invokes exceptionHandler if transform threw an Exception, and repeats prompt