littlephillips / phase-3-ruby-oo-class-variables-private-methods

ruby-oo-class-variables-private-methods

Private Methods

Learning Goals

• Define private methods
• Understand the context in which you will use such a method
• See how private methods are defined

Background: Instance and Class Methods

As we dive deeper into object orientation in Ruby, we've been working with different types of methods: instance and class methods. Remember that instance methods are called on instances of a class. Let's make the following class called Bartender.

class Bartender
  attr_accessor :name

  def initialize(name)
    @name = name
  end

  def intro
    "Hello, my name is #{self.name}!"
  end
end

phil = Bartender.new("Phil")
phil.intro
#=> "Hello, my name is Phil!"

intro is an instance method because we can call it on an instance of the Bartender class.

And class methods we call on the entire class itself, not the instance. Like if we had a method that kept track of all of the new instances of Bartender:

class Bartender
  attr_accessor :name

  @@all = []

  def initialize(name)
    @name = name
    @@all << self
  end

  def self.all
    @@all
  end

  def intro
    "Hello, my name is #{self.name}!"
  end
end

Here we're shoveling in every new instance of Bartender into a class variable that holds onto all bartenders. Then we have a class method self.all, which we'll call on the class itself to return all of the bartenders.

phil = Bartender.new("Phil")
nancy = Bartender.new("Nancy")

Bartender.all
#=> [#<Bartender:0x007f94cb16bbd0 @name="Phil">, #<Bartender:0x007f94cb16bb58 @name="Nancy">]

Public vs. Private Methods

Defining Public Methods

We've already been writing public methods: intro and self.all. We can call them from outside the scope of the class declaration, like on the instance of the class or the class itself. Public methods are called by an explicit receiver: the instance of phil explicitly receives the method intro.

Defining Private Methods

Private methods cannot be called by an explicit receiver. What does that mean? It means we can only call private methods within the context of the defining class: the receiver of a private method is always self.

Why Use Private Methods?

Private methods are a way of encapsulating functionality within a class. For example, a bartender can make a drink at a customer's request. Part of the process of making a drink includes choosing liquors, mixers, garnish, and stirring everything together. As a customer, you're permitted to ask the bartender for a drink (from a menu of options), but you can't instruct him or her on each step. The smaller steps that make up the bartender's job can be considered private methods.

Private methods also signal to other developers that this method is depended on by other methods in your program. It signals that they should beware of removing such a method for fear of breaking other parts of the program that they may not realize rely on it.

Building Private Methods

We've already written a private method in our Bartender class: #initialize.

phil.initialize
#=>NoMethodError: private method `initialize' called for #<Bartender:0x007fafb4257dd8 @name="Phil">

The #initialize method is special for a couple reasons:

• We don't invoke it directly; we use ClassName.new to create a new instance and then invoke the #initialize method on that instance.
• It is a private method by default.

Private methods, aside from initialize, are usually written with the word private above them. Let's make some private methods: #choose_liquor, #choose_mixer, and #choose_garnish.

We'll also create a public method #make_drink that calls on those private methods.

class Bartender
  attr_accessor :name

  @@all = []

  def initialize(name)
    @name = name
    @@all << self
  end

  def self.all
    @@all
  end

  def intro
    "Hello, my name is #{name}!"
  end

  def make_drink
    @cocktail_ingredients = []
    choose_liquor
    choose_mixer
    choose_garnish
    "Here is your drink. It contains #{@cocktail_ingredients}"
  end

  private

  def choose_liquor
    @cocktail_ingredients << "whiskey"
  end

  def choose_mixer
    @cocktail_ingredients << "vermouth"
  end

  def choose_garnish
    @cocktail_ingredients << "olives"
  end

end

If we try to call #choose_liquor with an instance of a bartender, we get an error:

phil.choose_liquor
# NoMethodError: private method `choose_liquor' called for #<Bartender:0x007f9f5b03d318 @name="Phil">

Again, private methods cannot be called by an explicit receiver. Because phil is the explicit receiver of choose_liquor, the method errors out.

However, if we call make_drink, the choose_liquor method works. What gives?

phil.make_drink
# Here is your drink. It contains ["whiskey", "vermouth", "olives"]

The #choose_liquor method was called without any receiver. Ruby sees the missing receiver and assumes it to be self, or the current object. When #choose_liquor is called, self is an instance of Bartender. Only a Bartender object can tell itself to choose a liquor, a mixer, and a garnish. Phil can tell himself to choose a liquor, garnish, etc., but we cannot instruct Phil to do so.

Private methods restrict an outsider from calling methods that belong to an object. However, we, as customers, are free to ask a bartender to make us a drink (#make_drink).

Note: Prior to version 2.7 of Ruby, if we tried to call our private methods from #make_drink using self (e.g., self.choose_liquor rather than just choose_liquor), we would get that same 'private method' error. However, beginning with version 2.7 of Ruby, private methods can be called from other methods in the Class with or without the self keyword.

Conclusion

When defining a class in Ruby, it's important to consider what kind of interface the class exposes to the rest of your code base. By using a mix of private and public methods, you can more carefully choose what methods are callable from outside the class, and what methods are only callable from within the class. Methods that are meant to be helper methods within your class, for example, are good candidates to be private methods.

Resources

• Ruby Method Visibility