Null Objects

Null objects are objects that define neutral/null behavior. In many object-oriented languages, references may be null/nil. These references must be check before actually calling a method because methods typically cannot be invoked on nil references.

Null Object are used to express the absence of an object, for example, no subscription. A Null Object implements the expected interface and the expected behavior for the absence of an object of interest.

By returning a null object, there is no need to check conditionals. Calling that method will simply do nothing.

When to use:

• when one notices a lot of duplication in code and logic. It's noticeable when we are constantly checking for nil.

Pros:

• Removes clutter
• Improves readability of the code
• Removes conditionals

Cons:

• Need to create a null class that implements the expected interface of the class of interest
• More code = more maintenance

Without Null Object

class User
  include ActiveModel::Model
  attr_accessor :credit_card, :subscription

  def charge
    unless subscription.nil?
      subscription.charge(credit_card)
    end
  end

  def price
    subscription.try(:price) || 0
  end
end

With Null Object

class NoSubscription
  def price
    0
  end

  def charge(credit_card)
  end
end

class User
  include ActiveModel::Model
  attr_accessor :credit_card, :subscription

  def subscription
    @subscription || NoSubscription.new
  end

  def charge
    subscription.charge(credit_card)
  end

  def price
    subscription.price
  end
end