7. Java Interop


Sitting in my favorite coffeehouse with a new notebook and a hot cup of java is my idea of Heaven.
— Libba Bray

Clojure syntax for Java constructors

(ns training.core
  (:import (java.util Date)))
(Date. 2018 02 17)

Which is equivalent to the less used variant:

(new Date)
(new Date 2018 02 17)

Calling methods on a Java object

(.length "hello world")
(.isDirectory (java.io.File. "my-dir"))

Equivalent to the less used variant:

(. "hello world" length)
(. (java.io.File. "my-dir") isDirectory)

Static and inner

Java static method calls:

(Math/pow 1 2)
(.print System/out "hi")

Inner classes:


Initializing objects with doto

(ns training.core
  (:import (java.util HashMap)))
(doto (HashMap.)
  (.put "a" 1)
  (.put "b" 2))
=> {"a" 1, "b" 2}

We get the constructed object, with side-effects applied


reify creates an object that conforms to an interface:

(.listFiles (java.io.File. ".")
    (accept [this f]
      (.isDirectory f))))

Notice that we did not define a class?

gen-class and proxy

gen-class creates a class.

proxy extends a concrete superclass.

Rarely needed, refer to manual.

Including Java classes in Clojure projects

You can define Java classes in Java in a separate directory and add

:java-source-paths ["java-src"]

To your project.clj file

lein compile

Java code in that directory will be usable from Clojure

End Java Interop