cljj

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.

Getting Help or Contributing

Send questions, bug reports, or patches to the mailing list.

You DO NOT need a SourceHut account to contribute or ask questions.

Why do you care?

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

As of Clojure 1.12, this is less true, so I expect the value of these library to continue to wane over time. The only real gap left is varargs method support, which is the main reason the org.ajoberstar.cljj.file namespace would be useful over native java.nio.file package.

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 (org.ajoberstar.cljj.stream -> java.util.stream)
• NIO2 File API (org.ajoberstar.cljj.file -> java.nio.file)

Usage

NOTE: cljj requires Java 8+

See the CHANGELOG.md for updates on releases.

Reducing on a Stream

Requiring the org.ajoberstar.cljj.stream 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
  java.util.function.Predicate
  [x]
  (= x "it matched"))

;; ignore that I'm not using org.ajoberstar.cljj.stream 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 org.ajoberstar.cljj.stream 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 java.io.File, 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).