Ruby User's Guide

Redefinition of methods

In a subclass, we can change the behavior of the instances by redefining superclass methods.

ruby> class Human
    |   def identify
    |     puts "I'm a person."
    |   end
    |   def train_toll(age)
    |     if age < 12
    |       puts "Reduced fare.";
    |     else
    |       puts "Normal fare.";
    |     end
    |   end
    | end
   nil
ruby> Human.new.identify
I'm a person.
   nil
ruby> class Student1<Human
    |   def identify
    |     puts "I'm a student."
    |   end
    | end
   nil
ruby> Student1.new.identify
I'm a student.
   nil

Suppose we would rather enhance the superclass's identify method than entirely replace it. For this we can use super.

ruby> class Student2<Human
    |   def identify
    |     super
    |     puts "I'm a student too."
    |   end
    | end
   nil
ruby> Student2.new.identify
I'm a person.
I'm a student too.
   nil

super lets us pass arguments to the original method. It is sometimes said that there are two kinds of people...

ruby> class Dishonest<Human
    |   def train_toll(age)
    |     super(11) # we want a cheap fare.
    |   end
    | end
   nil
ruby> Dishonest.new.train_toll(25)
Reduced fare.
   nil

ruby> class Honest<Human
    |   def train_toll(age)
    |     super(age) # pass the argument we were given
    |   end
    | end
   nil
ruby> Honest.new.train_toll(25)
Normal fare.
   nil

Copyright (c) 2005-2008 Mark Slagell

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