collections

KTX: Collection utilities

Utilities and extension function for custom libGDX collections.

Why?

Unfortunately, libGDX collections do not implement interfaces or extend any abstract classes from the java.util package. Although standard Kotlin library features superb utilities, some of them can be used along with the libGDX collections simply because they do not implement the Collection interface. Not to mention libGDX collections could use some extensions and factory methods, as well as fully benefit from the syntax sugar (like the square bracket operator) that Kotlin comes with.

Guide

Arrays

libGDX features Array class (with unfortunate naming), which works similarly to ArrayList, but it reuses its iterators. Its name clashes with kotlin.Array, so it is advised to use the GdxArray type alias provided by KTX, or alternatively import it with import com.badlogic.gdx.utils.Array as GdxArray when necessary.

• Array instances can be constructed with gdxArrayOf methods, similarly to how you create native arrays in Kotlin.
• Null-safe isEmpty(), isNotEmpty() and size() methods where added. They allow you to inspect the collection even if the variable is a possible null.
• Null-safe inlined extension property lastIndex was added. It returns index of last element in the list - or -1 if the list is null or empty.
• + and - operators were overridden: they allow adding or removing elements from the collection. They can be invoked with a compatible element type or another collection storing values of the same type. Both operator invocations can be chained. These operators return a new collection with specified added or removed elements.
• += and -= operators were overridden: they allow adding or removing elements from an existing collection, modifying its content.
• getLast and removeLast utility extension methods were added.
• Utility sortDescending, sortBy and sortByDescending extension methods were added to ease list sorting.
• Get-or-else extension method was added and is available through array[index, alternative] syntax.
• Missing addAll and removeAll methods for arrays and iterables were added.
• iterate method allows iterating over collection's elements, while providing reference to MutableInterator. Can be used to easily remove collection elements during iteration.
• removeAll and retainAll higher-order functions that work like collection extensions in Kotlin stdlib. A Pool can optionally be passed to automatically free the removed items.
• transfer extension method can be used to move elements from one array to another using a lambda predicate.
• map, filter, flatten and flatMap methods that work like methods in Kotlin stdlib but return GdxArray.
• Every iterable and array can be converted to Array using toGdxArray method.
• IntArray, BooleanArray and FloatArray can be converted to corresponding libGDX primitive collections using toGdxArray method.
• Type aliases added for libGDX collections to avoid collisions with the standard library:
  • GdxArray: com.badlogic.gdx.utils.Array
  • GdxIntArray: com.badlogic.gdx.utils.IntArray
  • GdxFloatArray: com.badlogic.gdx.utils.FloatArray
  • GdxBooleanArray: com.badlogic.gdx.utils.BooleanArray
  • GdxCharArray: com.badlogic.gdx.utils.CharArray
  • GdxLongArray: com.badlogic.gdx.utils.LongArray
  • GdxShortArray: com.badlogic.gdx.utils.ShortArray
  • GdxByteArray: com.badlogic.gdx.utils.ByteArray
• Factory methods for libGDX arrays storing primitives:
  • gdxBooleanArrayOf
  • gdxByteArrayOf
  • gdxCharArrayOf
  • gdxShortArrayOf
  • gdxIntArrayOf
  • gdxLongArrayOf
  • gdxFloatArrayOf

Sets

libGDX features ObjectSet class, which works similarly to HashSet, but it reuses its iterators.

• ObjectSet instances can be constructed with gdxSetOf methods, similarly to how you create native arrays in Kotlin.
• Null-safe isEmpty(), isNotEmpty() and size() methods where added. They allow you to inspect the collection even if the variable is a possible null.
• + and - operators were overridden: they allow adding or removing elements from the collection. They can be invoked with a compatible element type or another collection storing values of the same type. Both operator invocations can be chained. These operators return a new collection with specified added or removed elements.
• += and -= operators were overridden: they allow adding or removing elements from an existing collection, modifying its content.
• Missing addAll and removeAll methods for arrays and iterables were added.
• iterate method allows iterating over collection's elements, while providing reference to MutableIterator. Can be used to easily remove collection elements during iteration.
• map, filter, flatten and flatMap methods that work like methods in Kotlin stdlib but return GdxSet.
• Every iterable and array can be converted to ObjectSet using toGdxSet method.
• IntArray can be converted to IntSet using toGdxSet method.
• Type alias added for consistency with other collections: GdxSet - com.badlogic.gdx.utils.ObjectSet.

Maps

libGDX features ObjectMap class, which works similarly to HashMap, but it reuses its iterators.

• ObjectMap instances can be constructed with gdxMapOf methods, similarly to how you create java.util maps in Kotlin.
• IdentityMap instances can be constructed with gdxIdentityMapOf methods.
• Basic support for optimized primitive IntIntMap, IntFloatMap and IntMap collections.
• Null-safe isEmpty(), isNotEmpty() and size() methods where added. They allow you to inspect the collection even if the variable is a possible null.
• in operator can be used to check if a particular key is stored in the map.
• Square bracket syntax can be used to add new elements to the maps: map[key] = value is an equivalent to map.put(key, value).
• iterate method allows iterating over map elements with a reference to MutableIterator. Can be used to easily remove elements from the map.
• map, filter, flatten and flatMap methods that work like methods in Kotlin stdlib but return GdxMap and GdxArray.
• Keys stored in the map can be quickly converted to an ObjectSet using toGdxSet method.
• Every iterable and array can be converted to ObjectMap using toGdxMap method. A lambda that converts values to keys has to be provided - since the method is inlined, no new lambda object will be created at runtime.
• Type aliases were added for consistency with other collections:
  • GdxMap: com.badlogic.gdx.utils.ObjectMap
  • GdxIdentityMap: com.badlogic.gdx.utils.IdentityMap
  • GdxArrayMap: com.badlogic.gdx.utils.ArrayMap
• All libGDX map entries now feature component1() and component2() operator extension methods, so they can be destructed into a key and a value.
• getOrPut for ObjectMap, IdentityMap, ArrayMap and IntMap method to get an existing value to a given key or if it does not exist, create a default value, add it to the map and return it.

Note

It is highly advised to use ktx.collections.* import when working with libGDX collections. Kotlin standard library comes with multiple unoptimized utility methods for Iterable instances - like - operator that iterates over the whole collection to remove an element, which would be highly inefficient in case of ObjectSet, for example. By using a wildcard import of all KTX utilities, you can make sure that you're using the correct extension method implementations.

IntelliJ allows marking packages for automatic wildcard import at Settings > Editor > Code Style > Kotlin > Imports.

Usage examples

Working with libGDX Array:

import ktx.collections.*

val array = gdxArrayOf("zero", "one", "two")
array[0] // "zero"
"one" in array // true
array += "three" // array[3] == "three"; array.size == 4
array -= "three" // "three" in array == false; array.size == 3
array += arrayOf("three", "four") // array[3] == "three", array[4] = "four"

val empty = gdxArrayOf<String>()

val arrayOfPrimitives = gdxIntArrayOf(1, 2, 3)

Working with libGDX ObjectSet:

import ktx.collections.*

val set = gdxSetOf("zero", "one", "two")
"one" in set // true
set += "three" // "three" in set == true; set.size = 4
set -= "three" // "three" in set == false; set.size == 3
set += arrayOf("three", "four") // "three" in set == true; "four" in set == true

val empty = gdxSetOf<String>()

Working with libGDX ObjectMap:

import ktx.collections.*

val map = gdxMapOf(0 to "zero", 1 to "one", 2 to "two")
0 in map // true
map[0] // "zero"
map[3] = "three" // 3 in map == true; map[3] == "three"

val empty = gdxMapOf<Int, String>()

Iterating over libGDX maps with destructing syntax:

import ktx.collections.*

val map = gdxMapOf(0 to "zero", 1 to "one", 2 to "two")
map.forEach { (key, value) ->
  println("$value was mapped to $key.")
}

Alternatives

• Kotlin standard library provides utilities for default Java collections, although you should be aware that java.util collections can cause garbage collection issues on slower devices.
• Kiwi contains a module which helps with libGDX collections - but since it is written with Java, ktx-collections is arguably easier to use in Kotlin applications. It is still worth looking into for its lazy, disposable and immutable collections.
• Koloboke contains efficient implementations of sets and maps that can use unboxed primitive types as keys or values. Its API design and implementation is most likely significantly better and more efficient than libGDX collections, it is also a major dependency due to the sheer amount of available collections and its code generators. Note that Koloboke collections are compatible with java.util collections API, while libGDX collections are not - Koloboke maps and sets can fully benefit from Kotlin standard library utilities.

Additional documentation

• Official libGDX collections article.