Functional Java is an open source library that seeks to improve the experience of using the Java programming language in a production environment. The library implements several advanced programming concepts that assist in achieving composition-oriented development. Functional Java is written using vanilla Java 1.5 syntax and requires no external supporting libraries. The JAR file will work with your Java 1.5 project without any additional effort.

Functional Java also serves as a platform for learning functional programming concepts by introducing these concepts using a familiar language. The library is intended for use in production applications and is thoroughly tested using the technique of automated specification-based testing with ScalaCheck.

Functional Java includes the following features:

  * Fully operational Actors for parallel computations (fj.control.parallel) and layered abstractions such as parallel-map, map-reduce, parallel-zip.
  * A package (fj.data.fingertrees) providing 2-3 finger trees for a functional representation of persistent sequences supporting access to the ends in amortized O(1) time.
  * Type-safe heterogeneous list (fj.data.hlist) for lists of elements of differing types without sacrificing type-safety.
  * Monadic parser combinators for writing parsers by combining smaller parsers using composition.
  * Conversion of data types to/from standard Java types.
  * Immutable, in-memory singly linked list (fj.data.List).
  * Immutable lazy singly linked list (fj.data.Stream).
  * Array wrapper (fj.data.Array).
  * Optional value — type-safe null (fj.data.Option).
  * Disjoint union data type — compositional exception handling (fj.data.Either).
  * Monoid (fj.Monoid).
  * Functions with arity 1 to 8.
  * Products of 1 to 8.
  * Configurable equality and hash-code for HashMap and HashSet.
  * Natural number data type (fj.data.Natural).
  * Immutable set implementation using a red/black tree.
  * Immutable multi-way tree — aka rose tree (fj.data.Tree).
  * Immutable tree-map using a red/black tree implementation (fj.data.TreeMap).
  * Zipper implementations for streams and trees.
  * Automated specification-based testing framework (fj.test).