Converting Clojure data structures to Java collections

Question

What is the Clojure-idiomatic way to convert a data structure to a Java collection, specifically:

• [] to a java.util.ArrayList
• {} to a java.util.HashMap
• #{} to a java.util.HashSet
• () to a java.util.LinkedList

Is there a clojure.contrib library to do this?

USE CASE: In order to ease Clojure into my organization, I am considering writing a unit-test suite for an all-Java REST server in Clojure. I have written part of the suite in Scala, but think that Clojure may be better because the macro support will reduce a lot of the boilerplate code (I need to test dozens of similar REST service calls).

I am using EasyMock to mock the database connections (is there a better way?) and my mocked methods need to return java.util.List<java.util.Map<String, Object>> items (representing database row sets) to callers. I would pass in a [{ "first_name" "Joe" "last_name" "Smith" "date_of_birth" (date "1960-06-13") ... } ...] structure to my mock and convert it to the required Java collection so that it can be returned to the caller in the expected format.

Answer 1

Clojure vector, set and list classes implement the java.util.Collection interface and ArrayList, HashSet and LinkedList can take a java.util.Collection constructor argument. So you can simply do:

user=> (java.util.ArrayList. [1 2 3]) #<ArrayList [1, 2, 3]> user=> (.get (java.util.ArrayList. [1 2 3]) 0) 1 

Similarly, Clojure map class implements java.util.Map interface and HashMap takes a java.util.Map constructor argument. So:

user=> (java.util.HashMap. {"a" 1 "b" 2}) #<HashMap {b=2, a=1}> user=> (.get (java.util.HashMap. {"a" 1 "b" 2}) "a") 1 

You can also do the reverse and it is much easier:

ser=> (into [] (java.util.ArrayList. [1 2 3])) [1 2 3] user=> (into #{} (java.util.HashSet. #{1 2 3})) #{1 2 3} user=> (into '() (java.util.LinkedList. '(1 2 3))) (3 2 1) user=> (into {} (java.util.HashMap. {:a 1 :b 2})) {:b 2, :a 1}