Ruby Classes and Instances

March 16, 2016

What is a Class? What is an Instance?

In this blog post, we'll talk about Ruby classes and instances. Let's take a look at the above image. In Ruby, a class is a blueprint from which instances or objects are created. We can see a building is a class while the Empire State Building is an instance (object) created from building which makes sense because Empire State is a building right? The World Trade Center and Chrysler Building comes from the building class too because they are buildings as well.

Let's take a look at the dog example. A dog is a class and it creates Lassie, a famous dog and an instance of the dog class. If you want to know more about Lassie then check out Wikipedia. If you happen to have a dog at home, well your dog came from the dog class too. If instead you have a cat then it came from the cat class and not dog class.

                        
                                                class Puppy
                                                    def initialize(name)
                                                        @name = name 
                                                    end
    
                                                    def wag_tail
                                                        puts "#{@name} wags his tail!" 
                                                    end
    
                                                    def puppy_dog_eyes
                                                        puts "#{@name} looks at you with puppy dog eyes" 
                                                    end
                                                end

                                                ruby = Puppy.new("Ruby")
                                                ruby.wag_tail
                                                ruby.puppy_dog_eyes

Ruby Class Example

The above is an example of how a class looks like in Ruby. We created a class called Puppy and it has 3 methods, basically behaviors or actions all instances of the Puppy class can use. Every class comes with an initialize method which allows us to put default values into all new instances of the class. So when we make a new Puppy instance, we have to initialize with a name because that's how we defined the initialize method using def initialize(name). Let's say we want to name our puppy "Ruby" so we would create a variable to save "Ruby" using ruby = Puppy.new("Ruby")

Then we save the Puppy's name into an instance variable called @name. We know this is an instance variable because it has the @ character. Instance variables are variables that can be used and reused throughout all methods of the class. Next we have the wag_tail method that outputs an action using the name we intialized with. We can then call this method using the variable we created using ruby.wag_tail and we can call the last method using ruby.puppy_dog_eyes. Try these codes out on repl.it and create more class methods for your Puppy.

Classes and instances are great for organizing and reusing our codes. The methods we created in the Puppy class won't be accessible by the Kitten class or vice versa. And by making our Puppy class, we can make many puppy instances and all of them can use the methods we created in the Puppy class. Learn more about Ruby class here.