simple_interface.rb

class Wrapper
  include Java::java.lang.Comparable

  attr_reader :a

  def initialize(a)
    @a = a
  end

  def compare_to(x)
    puts 'called'
    a <=> x.a
  end

  def to_s
    "|#{a}|"
  end
end

describe Wrapper do
  it do
    a = [Wrapper.new(0), Wrapper.new(-10)].to_java
    Java::java.util.Arrays.sort(a)
    puts a.inspect
  end
end