Hosted on the JVM

Clojure is designed to be a hosted language, sharing the JVM type system, GC, threads etc. It compiles all functions to JVM bytecode. Clojure is a great Java library consumer, offering the dot-target-member notation for calls to Java. Class names can be referenced in full, or as non-qualified names after being imported. Clojure supports the dynamic implementation of Java interfaces and classes using reify and proxy:

Tapping into a Java class is as easy as:

(import '[java.time LocalDate])

(defn add-week ^LocalDate [^LocalDate date]
  (.plusDays date 7))

(defn day->str [^LocalDate date]
  (format "%s %s, %s" (.getMonth date) (.getDayOfMonth date) (.getYear date)))

;; infinite sequence of weekly dates, starting from today
(def future-weeks (iterate add-week (LocalDate/now)))

(map day->str (take 4 future-weeks))
;; ("JULY 24, 2017" "JULY 31, 2017" "AUGUST 7, 2017" "AUGUST 14, 2017")

Here’s a small Swing app as a more involved example:

(import '(javax.swing JFrame JLabel JTextField JButton)
        '(java.awt.event ActionListener)
        '(java.awt GridLayout))
(defn celsius []
  (let [frame (JFrame. "Celsius Converter")
        temp-text (JTextField.)
        celsius-label (JLabel. "Celsius")
        convert-button (JButton. "Convert")
        fahrenheit-label (JLabel. "Fahrenheit")]
     (reify ActionListener
             [_ evt]
             (let [c (Double/parseDouble (.getText temp-text))]
               (.setText fahrenheit-label
                         (str (+ 32 (* 1.8 c)) " Fahrenheit"))))))
    (doto frame
      (.setLayout (GridLayout. 2 2 3 3))
      (.add temp-text)
      (.add celsius-label)
      (.add convert-button)
      (.add fahrenheit-label)
      (.setSize 300 80)
      (.setVisible true))))