MockingJavaInJRuby.rb

require 'java'
require 'out.jar'

include_class Java::ImplementingMockableClass
i = ImplementingMockableClass.new(v)
puts i.output_string

class ClassWithInterface
  def get_string
    "mocked" 
  end
end

i = ImplementingMockableClass.new(v)
puts i.output_string

class Faked 
  include Java::IInterface
  
  def get_string
    "Jruby Mocked"
  end
end

i = ImplementingMockableClass.new(Faked.new)
puts i.output_string

Output

ImplementingMockableClass - ClassWithInterface - GetString()
ImplementingMockableClass - ClassWithInterface - GetString()
ImplementingMockableClass - Jruby Mocked