TIL: fastutil - fast & compact type-speciic collections for JVM (no autobox!)

Discusses what autoboxing is, why it might hit your performance (and memory). Finally some alternatives are also provided. Learn how to use effective data collections today!

Hampus Londögård


September 3, 2020

fastutil - how to optimize your collections by primitives

fastutil extends the Java™ Collections Framework by providing type-specific maps, sets, lists and queues with a small memory footprint and fast access and insertion

Homepage of fastutil


So what does the quote above actually mean?
First we need to dive into, what is a Java Collection, and why are they "bad" for performance and memory requirements?

Java Collections (Collection<E>) only works with objects, meaning that if we have a List<E> which we populate with int it’ll actually "autobox" the int into a Integer, i.e. the class, rather than the primitive type.
What does this mean for you as a user?

  • It’s done through "autoboxing" which means automatic casting to the Integer-type, so nothing required for you
  • It allocates more memory than the primitive int.

So how do you create an effective List that contains primitives, such as int, boolean and float? You can’t.
What you can do is to create an array, int[], which will contain the actual primitives, no autoboxing applied.

But what if you want to have the methods from List, such as find and "auto-resizing"? Then you’ll have to research and find a library, fastutil to the rescue!


fastutil implements their own versions of List, HashMap and so on which actually use the raw primitives, thereby increasing throughput while lowering memory used (as we’re not allocating as many objects anymore, when using primitives).

These types of libraries are only required once you hit an enormous amount of data or very strict requirement.


Using gradle

implementation: 'it.unimi.dsi:fastutil:8.4.1' (latest version as of Aug 2020, mvnrepository)



val d2dMap: Double2DoubleMap = Double2DoubleOpenHashMap().apply {
    put(2.0, 5.5)
    put(3.0, 6.6)
assertEquals(5.5, d2dMap.get(2.0))

This map is not only less memory-hungry (because using double rather than Double) but is also faster with insertions & get, than the Java Collections counterpart.


Less space used & faster - it is as simple as that!

  • No "AutoBoxing"
  • No Object allocations for primitives

Side-note Something I noticed while working on my Language Model in Kotlin, with some strict requirements and a lot of data, was that even when using fastutil I wasn’t gaining that much as I was mainly using views of my Lists, further optimizing memory. Views are what the name implies, a view of the List. It never creates a copy but just the indices and make use of the original structure. Using immutable data this is very effective, but if you’d been using mutable data it could prove dangerous as someone can change the structure and data of your view (even if your view is immutable the underlying List might not be).


Goldman Sachs Collection - now Eclipse Collections - Probably the best alternative, in my opinion. HPPC - Carrot Search Labs Trove4j - Not as active as other alternatives, but who cares when it’s performant and "done"?

Find a 2015 benchmark of the libraries here At least both fastutil& Eclipse Collections are updated for Java 8 streams!

-Hampus Londögård