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
| 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 human.
I'm a student too.
nil
| def identify
| super
| puts "I'm a student too."
| end
| end
nil
ruby> Student2.new.identify
I'm a human.
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
| 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