Refactoring: Refactorings/Change Value to Reference

A data object can sometimes accumulate many identical instances. These identical instances should be replaced with references to a single, unique object.

Mechanics

1. Use Replace Constructor with Factory Method
2. Test
3. Decide what object is responsible for providing access to the objects (maybe one or more hashes or registry objects)
4. Decide whether objects are already created (in which case they need to be retrieved from memory) or created on the fly.
5. Alter the factory method to return the reference object.
6. Test

Example

Before

class Customer
  attr_reader :name

  def initialize(name)
    @name = name
  end
end

class Order
  def initialize(customer_name)
    @customer = Customer.new(customer_name)
  end

  def customer=(customer_name)
    @customer = Customer.new(customer_name)
  end

  def customer_name
    @customer.name
  end
end

# usage

def self.number_of_orders_for(orders, customer)
  orders.select { |order| order.customer_name == customer.name }.size
end

After

class Customer
  attr_reader :name

  Instances = {}

  def initialize(name)
    @name = name
  end

  def self.with_name(name)
    Instances[name]
  end

  def self.load_customers
    new("Lemon Car Hire").store
    new("Associated Coffee Machines").store
    new("Bilston Gasworks").store
  end

  def store
    Instances[name] = self
  end
end

class Order
  def initialize(customer_name)
    @customer = Customer.with_name(customer_name)
  end

  def customer=(customer_name)
    @customer = Customer.with_name(customer_name)
  end

  def customer_name
    @customer.name
  end
end