jruby.rb

require "java"

class Item
  include java.util.concurrent.Delayed

  def initialize(deliver_at)
    @deliver_at = deliver_at
  end

  def get_delay
    @deliver_at
  end

  def compare_to(other_item)
    get_delay <=> other_item.get_delay
  end
end

class QueueWrapper
  import java.util.concurrent.DelayQueue

  def initialize
    @queue = DelayQueue.new
  end

  def <<(item)
    queue.offer(item)
  end

  def pop
    queue.poll
  end

  def size
    queue.size
  end

  def peek
    queue.peek
  end

  private

  attr_reader :queue
end

item = Item.new(Time.now)
queue = QueueWrapper.new
queue << item
p "Peek: " + queue.peek.inspect
p "Pop: " + queue.pop.inspect

output.sh

jp@oeuf:~/workspace/transporter(master*)$ ruby -v
jruby 1.6.7.2 (ruby-1.9.2-p312) (2012-05-01 26e08ba) (Java HotSpot(TM) 64-Bit Server VM 1.6.0_33) [darwin-x86_64-java]

jp@oeuf:~/workspace/transporter(master*)$ ruby test.rb 
"Peek: #<Item:0x6d87275d @deliver_at=2012-08-23 10:48:26 -0400>"
ArgumentError: wrong number of arguments (1 for 0)
     pop at test.rb:31
  (root) at test.rb:51

This turned out to be a faulty implementation of get_delay which takes a parameter of a TimeUnit. Turns out that jruby 1.6 doesn't return useful information but 1.7's stack trace properly identifies this problem. See http://docs.oracle.com/javase/6/docs/api/java/util/concurrent/Delayed.html