Sometimes you may want to redefine a method that already exists in some class. You’ve done this before when, for example, you created classes with their own to_s methods to return a string representation. Every Ruby class, from Object downward, has a to_s method. The to_s method of the Object class returns the class name and a hexadecimal representation of the object’s unique identifier. However, many Ruby classes have their own special versions of to_s. For example, Array.to_s concatenates and returns the values in the array.

When a method in one class replaces a method of the same name in an ancestor class, it is said to override that method. You can override methods that are defined in the standard class library such as to_s as well as methods defined in your own classes. If you need to add new behavior to an existing method, remember to call the superclass’s method using the super keyword at the start of the overridden method.

Here is an example:

override.rb

class MyClass
   def sayHello
      return "Hello from MyClass"
   end

   def sayGoodbye
      return "Goodbye from MyClass"
   end
end

class MyOtherClass < MyClass
    def sayHello            #overrides (and replaces) MyClass.sayHello
        return "Hello from MyOtherClass"
    end

        # overrides MyClass.sayGoodbye   but first calls that method
        # with super. So this version "adds to" MyClass.sayGoodbye
    def sayGoodbye
        return super << " and also from MyOtherClass"
    end

        # overrides default to_s method
    def to_s
        return "I am an instance of the #{self.class} class"
    end
end