[admin] Clojure: Integrating With Java

Currently I am learning Clojure. It is a functional programming language, but not a pure one, since you can both write code that share state (mutable) and also ones that doesn’t.

Why Clojure?

The main reason why I chose Clojure is its easy interoperability with Java, still one of the most used languages, bringing to it the power of Lisp. It’s fast, since the code is compiled, and it supplements some of Java’s weakness, such as the Collections framework and concurrent programming. It is pretty straightforward to write concurrent programs, everything is automatic, no manual lock management!

Integrating With Java

Importing classes

A single class:

(import java.util.List)

Multiple classes from the same package:

(import '(java.util List Set))

Creating instances

Using Java’s new keyword:

(new java.util.ArrayList)
(new ArrayList) ; after importing

Assigning a new List to a Clojure variable:

(def list (new java.util.List))
-> #'user/list

Syntactic Sugar:

(ArrayList.)

Accessing fields

Static fields:

(. Math PI)

Syntactic Sugar:

Math/PI

Invoking methods

Static Methods

(.currentTimeMillis System)

Syntactic Sugar:

(System/currentTimeMillis)

Non-static Methods

(. list size)
(. list get 0) ; returns the object stored at index 0

Syntactic Sugar:

(.size list)

Mixing Them All

Clojure provides a macro called memfn that makes possible execute Java methods as functions. So, for a list of String objects, if I want to make all of them upper-case, all I have to do is:

(map (memfn toUpperCase) ["a" "short" "message"])

The map function applies the function/method toUpperCase to each element in ["a" "short" "message"]

You can also use the bean function to wrap a Java bean in an immutable Clojure map.

(bean (new Person "Alexandre" "Martins"))
-> {:firstName "Alexandre", :lastName "Martins"}

Once converted, you can manipulate the new map using any of Clojure’s map functions, like:

(:firstName (bean (new Person "Alexandre" "Martins")))
-> Alexandre

The code above extracts the firstName key, originally from the Person object.