Instance Methods

Learning Goals

Describe an instance method.
Call instance methods on an object.
Build instance methods for an object.

The Behavior of Objects

We know that classes act as a factory for our objects, capable of instantiating new instances.

class Dog
end

fido = Dog.new #=> #<Dog:0x007fc52c2cc588>

But what can this instance of a dog stored in the local variable fido do? In fact, how do we even ask this object to do something?

Dot Notation

We send objects messages asking them to perform an operation or task through a syntax known as "Dot Notation".

class Dog
end

fido = Dog.new #=> #<Dog:0x007fc52c2cc588>

fido.object_id #=> 70173135795280

In the example above, we send the fido instance a message #object_id by separating the receiving object, fido and the message, #object_id by a dot (.).

When we send an object a message through dot notation, we are invoking the corresponding method on the object. We are calling the #object_id method on fido. (Note: the #object_id you get if you test out the above code will be different.)

The #object_id method simply tells you the object's identifier in your computer's memory (the place where all things live in your computer).

I thought of objects being like biological cells and/or individual computers on a network, only able to communicate with messages. - Alan Kay

In dot notation, we call the object that received the method the receiver, and we call the method the message.

# The receiver is this very string      # reverse is the message
"Strings are instances and objects too".reverse

Note: As in JavaScript, dot notation can be used both to call a method and to access an attribute of an object. (We'll talk about object attributes in Ruby in the next lesson.) Unlike JavaScript, however, Ruby does not require the () to be appended to a method's name when you call it. For example, these two lines of code are equivalent:

fido.object_id
fido.object_id()

What this means is that the syntax for accessing an attribute and calling a method can look the same in Ruby. It is important to keep this in mind as you program in Ruby and stay aware of which one you're doing in a particular case.

Exploring Instance Methods

All objects respond to methods (messages), like #object_id in the example above. One interesting method that is available on all objects in Ruby is the #methods method. Calling #methods on an object returns an array of all the methods (messages) an object responds to. We can invoke this method via dot-notation.

One of the great things you can ask every object in Ruby is "What methods do you respond to?" To see for yourself, try this out in IRB:

class Dog
end

fido = Dog.new
fido.methods
#=> [:psych_to_yaml, :to_yaml, :to_yaml_properties, :local_methods, :try, :nil?,
# :===, :=~, :!~, :eql?, :hash, :<=>, :class, :singleton_class, :clone, :dup,
# :itself, :taint, :tainted?, :untaint, :untrust, :untrusted?, :trust, :freeze,
# :frozen?, :to_s, :inspect, :methods, :singleton_methods, :protected_methods,
# :private_methods, :public_methods, :instance_variables,
# :instance_variable_get, :instance_variable_set, :instance_variable_defined?,
# :remove_instance_variable, :instance_of?, :kind_of?, :is_a?, :tap, :send,
# :public_send, :respond_to?, :extend, :display, :method, :public_method,
# :singleton_method, :define_singleton_method, :object_id, :to_enum, :enum_for,
# :==, :equal?, :!, :!=, :instance_eval, :instance_exec, :__send__, :__id__]

As you can see, out of the box, our objects can do a lot of things. Where these things come from and what they do are not so important right now because all of that functionality is very low level and not interesting to our Dogs.

Building Your Own Instance Methods

If you haven't already, fork and clone this lab to your local environment and run learn test. Follow along below to get the first few tests for the Dog class to pass. Once you have those passing, you will complete the remaining tasks on your own.

How do we add our own methods to our classes? In our Dog example, can we teach our Dog a new trick? Can we teach our Dog to bark for example?

We can. We're used to defining methods already with the def keyword. If we place this method definition within the body of a class, that method becomes a specific behavior of instances of that class, not a generic procedure we can just call whenever we want.

We call the methods defined within the object's class Instance Methods because they are methods that belong to any instance of the class.

Let's create our Dog class in lib/dog.rb. It's a convention among Rubyists to define each class in its own file, using the class name to determine the file name.

In the Dog class, let's define our #bark instance method:

class Dog
  # Class body

  # Instance Method Definition
  def bark
    puts "Woof!"
  end
end

With this code in place, if you run the tests you should now be passing the first three tests.

Now let's create an instance of Dog and verify that our Dog object knows how to bark:

class Dog
  def bark
    puts "Woof!"
  end
end

fido = Dog.new
fido.bark #=> "Woof!"

If you execute this code in the terminal by running ruby lib/dog.rb, you should see "Woof!" written out.

By defining #bark within the Dog class, bark becomes a method of all instances of Dogs. Let's create a second instance of Dog, snoopy, and verify that snoopy also knows how to bark by running ruby lib/dog.rb again.

class Dog
  def bark
    puts "Woof!"
  end
end

fido = Dog.new
fido.bark #=> "Woof!"

snoopy = Dog.new
snoopy.bark #=> "Woof!"

Objects can only do what we teach them to do via the code we write and the methods we define. For example, currently, Dogs do not know how to sit.

class Dog
  def bark
    puts "Woof!"
  end
end

fido = Dog.new
fido.bark #=> "Woof!"
fido.sit #=> NoMethodError: undefined method `sit' for #<Dog:0x007fa4e9a9e8a0>

In the same manner, instance methods, the methods that belong to particular instances of particular classes, cannot be used globally like procedural methods. They cannot be called without an instance.

class Dog
  def bark
    puts "Woof!"
  end
end

fido = Dog.new

# Let's try just calling bark without fido
bark #=> NameError: undefined local variable or method `bark' for main:Object

Instructions

Complete the following tasks to get the rests of the tests passing:

Add an instance method #sit to your Dog class that will puts "The Dog is sitting".
Define a Person class in lib/person.rb
Add an instance method #talk to your Person class that will puts "Hello World!"
Add an instance method #walk to your Person class that will puts "The Person is walking".

Additional Note on Lab Testing

In this lab, we asked that you code your two classes in separate dog.rb and person.rb files. You could, in theory, code both classes in the same file, or even code them in opposite files and still pass all tests. Why do you think that is?

...

When the tests are run in this lab, RSpec loads both the dog.rb and person.rb files (this happens in the first two lines of spec/spec_helper.rb using require_relative). As long as you place your classes in one of the files that RSpec loads, the tests will have access to them.

While it isn't enforced, we do encourage you to separate classes into individual, accurately named files. In a larger application, you might not always need to load the Dog class when loading the Person class. As classes get larger, it also becomes easier to manage your code if you know each file contains one class. Keeping to these conventions makes it easier in the future to go back and read code you've previously written.

Conclusion

With all tests passing, you have successfully written multiple instance methods and two different classes!

The ability to define methods and behaviors in our classes for our instances makes Ruby classes behave not just as factories, capable of instantiating new individual instances, but also as a blueprint, defining what those instances can do.

GordonGichini/phase-3-ruby-oo-basics-instance-methods-lab