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Welcome to fastutil

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 provides also big (64-bit) arrays, sets, and lists, sorting algorithms, fast, practical I/O classes for binary and text files, and facilities for memory mapping large files.

Since version 8.5.5, fastutil is split into two jars for convenience:

  • fastutil-core.jar contains data structures based on integers, longs, doubles, and objects;

  • fastutil.jar is the classic distribution, containing all classes.

Note that core 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.

Previous split versions would provide different classes in different jars, but managing sensibly dependencies turned out to be impossible.

You can also create a small, customized fastutil jar (which you can put in your repo, local maven repo, etc.) using the 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 three jars above, you have to run the script, and then ant osgi-rest.

The Java sources are generated using a C preprocessor. The 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.