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DISCLAIMER: Prior to 0.5.0, this library was org.ajoberstar/ike.cljj with namespaces under ike.cljj.*. It is now org.ajoberstar/cljj with namespaces under org.ajoberstar.cljj.

Why do you care?

Clojure provides some nice Java interop features, but they are missing clean support for newer APIs added to Java 7+.

What is it?

cljj is a Clojure library of wrappers around Java APIs.

Current Support

  • Clojure functions to arbitrary Single Abstract Method (SAM) types (org.ajoberstar.cljj.function -> java.util.function and others)
  • Streams ( ->
  • NIO2 File API (org.ajoberstar.cljj.file -> java.nio.file)


NOTE: cljj requires Java 8+

Reducing on a Stream

Requiring the namespace will add support for two main things:

  • Turning a Stream into an ISeq with stream-seq.
  • Reducing/Transducing/etc over a Stream (due to CollReduce impl)
(let [stream (IntStream/range 0 10)]
  (= 25 (transduce (filter odd?) + stream))))

Converting Clojure functions to Java SAMs

As of Java 8, there is powerful support in the Java language for lambdas and method references. One of these features is that methods that accept an argument which is a Single Abstract Method (SAM) interface can also accept any method reference or lambda of the same shape/type.

This unforunately does not translate to Clojure users.

The org.ajoberstar.cljj.function namespace includes three main helpers for this:

  • sam* - function converting a Clojure function to an arbitrary SAM interface
  • sam - creating an anonymous SAM impl, as it were a Clojure function
  • defsam - defining a named SAM impl, as if it were a Clojure function

WARNING: You may need type hints to avoid IllegalAccessError in Java 9+.

(defsam my-sam
  (= x "it matched"))

;; ignore that I'm not using here
(-> (Stream/of "not a match" "it matched")
    (.filter my-sam)
    (.collect Collectors/toList)

Note that primitive streams require different SAM types.

;; ignore that I'm not using here
(-> (IntStream/range 0 10)
    (.filter (sam* java.util.function.IntPredicate odd?))
    (.collect Collectors/toList)

File API

The NIO2 API for files is much improved over, but has some headaches from Clojure, namely the extensive use of varargs. The org.ajoberstar.cljj.file namespace provides wrappers over these functions for two benefits:

  • more natural variadic functions for Clojure use (no explicit into-array calls)
  • flexible argument types using the Pathish protocol that already converts many common types to Path (e.g. String, File, URI).

Questions, Bugs, and Features

Please use the repo's issues for all questions, bug reports, and feature requests.


Contributions are very welcome and are accepted through pull requests.

Smaller changes can come directly as a PR, but larger or more complex ones should be discussed in an issue first to flesh out the approach.

If you're interested in implementing a feature on the issues backlog, add a comment to make sure it's not already in progress and for any needed discussion.

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