gistfile1.txt

@JRubyClass(name = "Date")
public class RubyDate extends RubyObject {

    public static class RubyDateSingleton {
        @JRubyMethod(name = "jd_to_civil", required = 1, optional = 1, visibility = Visibility.PRIVATE)
        public IRubyObject jd_to_civil(ThreadContext ctx, IRubyObject[] args) {
            System.out.println("LOOOOL");
            return ctx.nil;
        }
    }

    private static final ObjectAllocator DATE_ALLOCATOR = new ObjectAllocator() {
        public IRubyObject allocate(Ruby runtime, RubyClass klass) {
            return new RubyDate(runtime, klass);
        }
    };

    public static RubyClass createDate(Ruby runtime) {
        RubyClass result = runtime.defineClass("Date", runtime.getObject(), DATE_ALLOCATOR);
        result.defineAnnotatedMethods(RubyDate.class);
        result.defineAnnotatedConstants(RubyDate.class);

        RubyModule module = RubyModule.newModule(runtime);
        module.defineAnnotatedMethods(RubyDateSingleton.class);
        result.include(new IRubyObject[] { module });
        result.extend(new IRubyObject[] { module });
        result.getSingletonClass().include(new IRubyObject[] { module });
        result.getSingletonClass().extend(new IRubyObject[] { module });
        return result;
    }

    public RubyDate(Ruby runtime, RubyClass klass) {
        super(runtime, klass);
    }

gistfile2.txt

> require 'date'
> Date.new                                                                              
NoMethodError: undefined method `jd_to_civil' for Date:Class
        from /Volumes/v2/git/jruby/lib/ruby/1.9/date.rb:625:in `_valid_civil?'
        from /Volumes/v2/git/jruby/lib/ruby/1.9/date.rb:808:in `civil'
        from (irb):2:in `evaluate'
        from org/jruby/RubyKernel.java:1087:in `eval'
        from org/jruby/RubyKernel.java:1412:in `loop'
        from org/jruby/RubyKernel.java:1199:in `catch'
        from org/jruby/RubyKernel.java:1199:in `catch'
        from /Volumes/v2/git/jruby/bin/jirb:13:in `(root)'

gistfile3.txt

t = Module.new do
  private
  def jd_to_civil(a,b)
    ...
  end
end
Date.extend(t)