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Polymorphism

One fundamental feature of object-oriented programming is polymorphism.

Polymorphism is a fancy term to describe how multiple Concrete types can conform to an Abstract Interface, and be used to switch out behaviours at runtime.

Let’s explore this with an example.

Consider the following code:

class AccountingDocument 
  def initialize(type:, total:)
    @type = type
    @total = total
  end

  def get_journal
    case @type
      when :sale
        [
          :trade_debtors, -@total,
          :revenue, @total
        ] 
      when :purchase
        [
          :trade_creditors, @total,
          :loss, -@total
        ]
      when :expense
        [
          :employee_expenses, @total,
          :bank_account, -@total
        ]
      when :draft
        nil
      else
    end
  end
end

Where an example usage looks like:

AccountingDocument.new(type: :purchase, total: '100.00')

This code is not Polymorphic.

The problem

It violates the Open-Closed Principle. Potentially, also the Single Responsibility Principle too.

Every time we need to introduce a new type of AccountingDocument, we’re going to need to modify this class. What’s more, it will be a magnet for an ever growing number of journal representations, for an ever growing number of those document types.

An alternative

Instead, we can use polymorphism:

class AccountingDocument
  def initialize(journal_disposition:, total:)
    @journal_disposition = journal_disposition
    @total = total
  end

  def get_journal
    @journal_disposition.get_journal(@total)
  end
end

The above class can be said to be “Open for Extension” and “Closed for Modification”.

We can then define classes for each “JournalDisposition”

class SaleJournalDisposition
  def get_journal(total)
    [
      :trade_debtors, -@total,
      :revenue, @total
    ] 
  end
end
class PurchaseJournalDisposition
  def get_journal(total)
    [
      :trade_creditors, @total,
      :revenue, -@total
    ] 
  end
end
class ExpenseJournalDisposition
  def get_journal(total)
    [
      :employee_expenses, total,
      :bank_account, -@total
    ]
  end
end
class NoJournalDisposition
  def get_journal(_); end
end

But wait! We still need something which can convert data into behaviour

If we want to reconstruct polymorphic behaviours from data stored in a database, we need a mapping between the data: type (a string) and the behaviour: the appropriate Ruby object.

class JournalDispositionFactory
  def create(type)
    case type
    when :sale
      SaleJournalDisposition.new
    when :purchase
      PurchaseJournalDisposition.new
    when :expense
      ExpenseJournalDisposition.new
    else
      NoJournalDisposition.new
    end
  end
end

As an example usage:

journal_disposition_factory = JournalDispositionFactory.new
AccountingDocument.new(journal_disposition: journal_disposition_factory.create(:sale), total: '56.23')

But wait, that’s more code!

Sure - there is more code after our refactoring.

However, software design is more than just reducing lines of code.

  • Whenever we change a file, we risk breaking other things in the same file.
  • Whenever we change a file that someone else is working on, we risk a merge.
  • The more things a file does, the harder it is to understand.

Instead, it is desirable to break code apart into lego bricks. (We will cover this in more detail later).

Exercises

  • Do the Video Store Kata, and consider how you could build taking advantage of polymorphism.

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