Duck-typing

Let us say I want to be able to output a variety of objects (say strings, floats and dates) on a web page. With Java I would create a new class with over-loaded methods, like this (warning: my Java is getting rusty, and this is untested):

class Formatter {
  SimpleDateFormat sdf = new SimpleDateFormat("MM/dd/yy");

  public static String show(String s) { return s; }
  public static String show(Date d) { return sdf.format(d); }
  public static String show(double x) { return x < 0.05 ? '<0.1' : "" + (Math.round(x * 10) / 10.0); }
  public static String show(Object o) { return o.toString(); }
}

To invoke, I would use this:

Formatter.show(myObject);

Java will select the method based on the class I send. Note that there is a method for object to catch anything unexpected.

In Ruby, I would approach this quite differently. There is no need for a new class, just modify the existing classes. This is not possible in Java; I could extend Date, but I would have to ensure that every date I sent was of my date class. String and float cannot be extended at all.

class String
  def show
    to_s
  end
end

# Remember the require 'date.rb'
class DateTime
  DATE_FORMAT = '%d/%b/%y'

  def show
    strftime(DATE_FORMAT)
  end
end

class Float
  def show
    self < 0.05 ? '<0.1' : (self * 10).round / 10.0
  end
end

class Object
  def show
    to_s
  end
end

That is more verbose, but the result is much neater, much more object-orientated, as now the method can be invoked like this:

my_object.show

I have a library of useful Java methods. It is a collection of static methods that do various operations on arrays and strings. In Ruby, I am building up a library that changes how arrays and string behave. Just occasionally, that is not the best way - again because of duck-typing, as it happens. Say I have a method to format dates consistently. All it does is invoke strftime with a certain format string. I could define my method as part of the DateTime class, however I would then be unable to use it with Time objects. In this case, I am better adding the method to the Object class, then DateTime, Date and Time objects would all be able to use it, as they all have strftime methods (and duck-typing allows this to work).