Clojure's PersistentHashMap on JRuby

Clojure is an impressive language. Not just succinct syntax, Clojure has immutable data types for concurrency. Such Clojure's persistent data types might be useful in some cases in other languages. A question is whether we can use Clojure data types from JRuby. The answer is yes. Below is what I tried on irb:

$ rvm jruby-1.6.4
$ irb
jruby-1.6.4 :001 > require 'java'
 => true 
jruby-1.6.4 :002 > $CLASSPATH << "/Users/yoko/Tools/clojure-1.3.0"
 => ["file:/Users/yoko/Tools/clojure-1.3.0/"] 
jruby-1.6.4 :003 > require 'clojure-1.3.0-slim'
 => true 
jruby-1.6.4 :004 > h = {"a" => 100, "b" => 300, "c" => 200}
 => {"a"=>100, "b"=>300, "c"=>200} 
jruby-1.6.4 :005 > pmap = Java::clojure.lang.PersistentHashMap.create(h)
 => {"a"=>100, "b"=>300, "c"=>200} 
jruby-1.6.4 :006 > pmap.class
 => Java::ClojureLang::PersistentHashMap

OK. I could successfully create Clojure's PersistentHashMap object. PersistentHashMap implements java.util.Map interface, which means the object has all of Ruby's Hash methods.

jruby-1.6.4 :007 > pmap.each {|k, v| puts "#{k} is #{v}"}
a is 100
b is 300
c is 200
 => {"a"=>100, "b"=>300, "c"=>200} 
jruby-1.6.4 :008 > pmap.select {|k, v| k > "a"}
 => [["b", 300], ["c", 200]] 
jruby-1.6.4 :009 > pmap.has_value?(400)
 => false 

PersistenHasMap is immutable. So, when the method tries to change its data, Clojure raises exception:

jruby-1.6.4 :012 > pmap.delete_if {|k, v| k >= "b"}
Java::JavaLang::UnsupportedOperationException: 
 from clojure.lang.APersistentMap.remove(APersistentMap.java:273)
 from org.jruby.java.proxies.MapJavaProxy$RubyHashMap.internalDelete(MapJavaProxy.java:157)
 from org.jruby.RubyHash.delete(RubyHash.java:1407)
(snip)

This way, we can use Clojure's immutable data types on JRuby.