Instance Methods in Ruby

To talk about instance methods in Ruby, you have to first talk about classes and objects. A Class in ruby is a factory for making objects. What is an object? In, Ruby, it’s everything. 2 + 2 will return a Fixnum object, 'any strings' will return a string object, true and false are boolean type objects. When you want to program anything with a bit of complexity, it becomes easier/necessary to use a Class.

When you churn out an object created by a class, you are creating an instance of that class. It inherits all the methods defined by that class and a few more you may never have to use. Let’s try making a class and instantiating a new object. For this post, let’s make a Dog and Person class, and create a few instance methods that can be called on newly created objects by that class.

We’ll be coding our solution to the Instance Method Lab. Create a ruby file in your terminal. I’m on a macbook using osx so I’ll create a new ruby file with

touch lib/dog.rb
touch lib/person.rb

We want to create our Dog class. Let’s start by defining it in the dog.rb file

class Dog  # define our class

end

Nice. We have a Dog class, but new instances of Dog wouldn’t have much to do. Let’s change that make our dogs be able to bark with instance methods. We define an instance method like this

class Dog
  def bark  # define our instance method
    puts 'Woof!'
  end
end

Now if we want to make a dog bark, we’d instantiate a new dog with

# lib/dog.rb

fido = Dog.new  # create/instantiate a new dog and assign it to the variable 'fido'
fido.bark  # calls the #bark instance method on the newly created instance of Dog

fido.bark would return "Woof!". How would you get fido.sit to return 'The Dog is sitting' ? With our Person class, we want our new person to respond to a #talk instance method.

class Person  # starts defining our Person class
  def talk  # start defining our #talk method
    puts "Hello World!"  # prints out 'Hello World!' to the output
  end  # closes out our definition of the #talk method. Super important.
end  # closes out our definition of the Person class. Also super important.

Now, if we were to create a new instance of the Person class, dave = Person.new, and then call the #talk instance method on the newly instantiated person dave.talk, we’ll see the message 'Hello World!'. How would you get our new person to respond to a #walk method that prints 'The Person is walking' to the console?

Run your tests with rspec in the terminal, after you coded out the rest of the instance methods!