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")]
(.addActionListener
convert-button
(reify ActionListener
(actionPerformed
[_ 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))))
(celsius)