Factory Methods

Part of the Object Oriented Programming section of Coddy's Ruby journey. Lesson 15 of 57.

A factory method is a class method that creates and returns an instance. It gives a name to one way of building an object. Inside a class method, new means self.new:

class Temperature
  attr_reader :celsius

  def initialize(celsius)
    @celsius = celsius
  end

  def self.from_fahrenheit(f)
    new((f - 32) * 5 / 9.0)
  end

  def self.freezing
    new(0)
  end
end

puts Temperature.from_fahrenheit(212).celsius
puts Temperature.freezing.celsius

Output:

100.0
0

Turning text into an object is a common job for a factory. The parsing lives next to the class, and initialize stays simple:

class Duration
  attr_reader :seconds

  def initialize(seconds)
    @seconds = seconds
  end

  def self.parse(text)
    new(text.split(":").map(&:to_i).reduce(0) { |acc, part| acc * 60 + part })
  end
end

puts Duration.parse("1:30").seconds
puts Duration.parse("2:00:05").seconds

Output:

90
7205

A factory can refuse bad input. Returning nil lets the caller test the result with if; raising an error stops the program unless the caller rescues it:

class Color
  attr_reader :hex

  def initialize(hex)
    @hex = hex
  end

  def self.parse(text)
    return nil unless text.match?(/\A#[0-9a-f]{6}\z/i)

    new(text.downcase)
  end
end

["#FF8800", "orange"].each do |text|
  color = Color.parse(text)
  puts color ? "ok #{color.hex}" : "not a color: #{text}"
end

Output:

ok #ff8800
not a color: orange

private_class_method :new makes new callable only inside the class, so the factories become the only way to create an object:

class Money
  attr_reader :cents

  def initialize(cents)
    @cents = cents
  end
  private_class_method :new

  def self.from_cents(cents)
    new(cents)
  end
end

puts Money.from_cents(250).cents
begin
  Money.new(250)
rescue NoMethodError => e
  puts e.message
end

Output:

250
private method `new' called for class Money

Challenge

Easy

Complete the class Duration. initialize(seconds) stores a number of seconds. The factory Duration.parse(text) reads m:ss or h:mm:ss, the factory Duration.from_minutes(m) takes whole minutes, plus(other) returns a new Duration with the sum, and to_s returns h:mm:ss with two-digit minutes and seconds.

The supplied code reads durations: lines like 45m go through from_minutes, the rest through parse. It prints each duration and finally total 1:47:10.

Your code goes in duration.rb. main.rb holds the supplied input/output code and cannot be edited.

Try it yourself

require_relative 'duration'

# Supplied input/output code: keep it as it is
input = []
while (line = gets)
  input << line.chomp
end
durations = input.map do |line|
  line.end_with?("m") ? Duration.from_minutes(line.to_i) : Duration.parse(line)
end
durations.each { |d| puts d }
puts "total #{durations.reduce(Duration.new(0)) { |sum, d| sum.plus(d) }}"