Sometimes is very useful to add methods to ruby classes that are not on your code base.
Let's say you have an utility class with an utility methods called sub_comma_by_space that substitute all commas on a String by spaces:
1 def self.sub_comma_by_space(str) 2 str.sub ",", " " 3 end
This could be more object oriented if we can use this method like this:
In Java, for example, you would have to create a new "PowerString" with all String methods and add this method to your "PowerString" and then change all your references to use your new "PowerString" instead of String class.
Ruby has the concept of open classes so you can easily extend an existing ruby class, without changing the String source file:
1 class String 2 def sub_comma_by_space 3 sub ",", " " 4 end 5 end
Now you can call sub_comma_by_space method on all you Strings.