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; it also provides big (64-bit) arrays, sets and lists, and fast, practical I/O classes for binary and text files. It is free software distributed under the Apache License 2.0.

The classes implement their standard counterpart interface (e.g., Map for maps) and can be plugged into existing code. Moreover, they provide additional features (such as bidirectional iterators) that are not available in the standard classes.

Besides objects and primitive types, fastutil classes provide support for references, that is, objects that are compared using the equality operator rather than the equals() method.

The sources are generated using a C preprocessor, starting from a set of driver files. You can peek at the javadoc-generated documentation. In particular, the overview explains the design choices used in fastutil.

If the standard fastutil jar is too large, there is a core jar containing only data structures specific for integers, longs and doubles. Note that those classes are duplicated in the standard jar, so if you are depending on both (for example, because of transitive dependencies) you should exclude the core jar.

You can also create a small, customized fastutil jar (which you can put in your repo, local maven repo, etc.) using the find-deps.sh shell script. It has mild prerequisites, as only the jdeps tool is required (bundled with JDK 8). It can be used to identify all fastutil classes your project uses and build a minimized jar only containing the necessary classes.

First, you have to make sources to get the actual Java sources. After that, ant jar will generate a single jar file; ant javadoc will generate the API documentation; ant junit will run the unit tests.

If you want to obtain the two jars above, you have to run the script split.sh, and then ant osgi-rest.

The Java sources are generated using a C preprocessor. The gencsource.sh script reads in a driver file, that is, a Java source that uses some preprocessor-defined symbols and some conditional compilation, and produces a (fake) C source, which includes the driver code and some definitions that customize the environment.

fastutil provides in many cases the fastest implementations available. You can find many other implementations of primitive collections (e.g., HPPC, Koloboke, etc.). Sometimes authors are a little bit quick in defining their implementations the “fastest available“: the truth is, you have to take decisions in any implementation. These decisions make your implementation faster or slower in different scenarios. I suggest to always test speed within your own application, rather than relying on general benchmarks, and ask the authors for suggestions about how to use the libraries in an optimal way. In particular, when testing hash-based data structures you should always set explicitly the load factor, as speed is strongly dependent on the length of collision chains.

With fastutil 6, a new set of classes makes it possible to handle very large collections: in particular, collections whose size exceeds 2³¹. Big arrays are arrays-of-arrays handled by a wealth of static methods that act on them as if they were monodimensional arrays with 64-bit indices, and big lists provide 64-bit list access. The size of a hash big set is limited only by the amount of core memory.

There is a discussion group about fastutil. You can join or send a message.