Working with Java classes from Clojure code is easy. If we want to invoke methods or access fields on instances of Java classes we must first create an instance by invoking the constructor. In Clojure we can do that using the special form new or using a dot (.) notation. The new special form has the class name of the Java class we want to create an instance of as argument followed by arguments for the Java class constructor. With the dot notation we place a . after the class name followed by arguments for the class constructor to create a new instance.

In the following example we see several ways to create an instance of a Java class.

(ns mrhaki.java.new-instance
  (:require [clojure.test :refer [is]])
  (:import (java.net URI)
           (java.util Map TreeMap)))

;; Using a dot after the class name to invoke the constructor.
(is (instance? String (String.)))

;; Or using the new special form to invoke the constructor.
(is (instance? String (new String)))

;; Constructor arguments can be used.
(is (instance? URI (URI. "https://www.mrhaki.com")))
(is (instance? URI (new URI "https" "www.mrhaki.com" "/" "")))

;; We can use Clojure data structures in constructors.
(is (instance? Map (TreeMap. {:language "Clojure"})))

Written with Clojure 1.10.1.

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