Inheritance

We already have three classes: Person, Dog, and Cat. Let's add some properties and methods to cat and dog. Both cat and dog will have following properties:

• legs count
• color

The Dog will also have size property, and the Cat will have cuteness_rate property. Both classes should be able to make some sound. However, the sound they both do should be different.

Ok, let's start:

        class Person
          # person class code here
          # ...
        end

        class Dog
          attr_accessor :legs, :color, :size

          def initialize(opts={})
            @legs = 4
            @color = opts[:color] || "Black"
            @size = opts[:size] || "Medium"
          end

          def make_sound
            p "Bark!"
          end
        end

        class Cat
          attr_accessor :legs, :color, :cuteness_rate

          def initialize(opts={})
            @legs = 4
            @color = opts[:color] || "Red"
            @cuteness_rate = opts[:cuteness_rate] || 100
          end

          def make_sound
            p "Meow!"
          end
        end

        # Let's comment everything about Person for now:
        # john = Person.new
        # p john.learned_languages
        # john.learn "Ruby"
        # john.learn "Python"
        # john.learn "JavaScript"
        # p john.learned_languages

        cat = Cat.new color: "White"
        dog = Dog.new size: "Big"
        p "Cat's color: #{cat.color}"
        cat.make_sound
        p "Dog's color: #{dog.color}"
        dog.make_sound
      

We should be able to see something like this:

Let's come back to Inheritance now. Inheritance is a kind of relation between classes where one class is based on another one. You can use class X as a base for a class Y. For example, we could use our class Person as a base class for new classes Student and Teacher because both would share same basic properties and methods. If something in a child class (the one that inherits from a base class) is different, it can always be overridden.

I think you already understand that we can simplify our Dog and Cat classes since both are animals. Let's create a third class called Animal. It would have following properties:

• legs count
• color
• size

And it would have a make_sound method as well:

        class Animal
          attr_accessor :legs, :color, :size

          def initialize(opts={})
            @legs = 4
            @color = opts[:color] || "Black"
            @size = opts[:size] || "Medium"
          end

          def make_sound
            p "hmmm..."
          end
        end
      

This code looks very similar to our Dog and Cat classes. We can now reuse class Animal - Cat and Dog would "derive" (or will be a subclass) from Animal, and they will be its exact copies for now. To make Dog and Cat inherit Animal class, we just need to do next:

        class Animal
          # animal class code here ...
        end

        class Dog < Animal
        end

        class Cat < Animal
        end
        
      

As you can see, to make some class inherit from another, we just need to add < symbol, and specify parent class.

Right now both Dog and Cat classes are just copies of an Animal class - Dog won't bark, and Cat won't meow. Cat also lost its cuteness_level parameter.

If we try to execute the code, it will work, but not as we would expect: instead of meow or bark, animals will say "hmmm...". However app will not crash - even without specified parameters and initialization code for Cat and Dog, Ruby will know that both classes have make_sound method and that both have color or size parameters:

So, how to make them work? Very easy, we just add whatever we want, and change whatever is wrong:

        class Animal
          attr_accessor :legs, :color, :size

          def initialize(opts={})
            @legs = 4
            @color = opts[:color] || "Black"
            @size = opts[:size] || "Medium"
          end

          def make_sound
            p "hmmm..."
          end
        end

        class Dog < Animal
          def make_sound
            p "Bark!"
          end
        end

        class Cat < Animal
          attr_accessor :cuteness_level

          def make_sound
            p "Meow!"
          end
        end
        
      

We have learned almost all essential Ruby features. To start working on Rubymotion apps we just need to learn one last thing - Modules.

Book Index | Next