Ruby classes, subclasses and factory methods

Question

I'm working on a TestFirst exercise (temperature_object) and have come to a standstill when it comes to integrating a subclass. So far I've got:

class Temperature

    def initialize(opts = {})
        @options = opts
        @c = @options[:c]
        @f = @options[:f]
    end

    def self.from_celsius(num)
        self.new(:c => num)
    end

    def self.from_fahrenheit(num)
        self.new(:f => num)
    end

    def in_celsius
        if @options.has_key?(:c)
            @c
        elsif @options.has_key?(:f)
            ctof(@f)
        end
    end

    def in_fahrenheit
        if @options.has_key?(:f)
            @f
        elsif @options.has_key?(:c)
            ftoc(@c)
        end
    end

    def ftoc(num)
        (((num * 9) / 5.000) + 32)
    end

    def ctof(num)
        (((num - 32) * 5) / 9.0000)
    end


end

class Celsius < Temperature

    def initialize(num)

        @c = num

    end
end

class Fahrenheit < Temperature

    def initialize(num)

        @f = num

        end
end

All of the tests pass until I get to the following:

require "temperature_object"

describe Temperature do

    # Here's another way to solve the problem!
    describe "Temperature subclasses" do
        describe "Celsius subclass" do
            it "is constructed in degrees celsius" do
                Celsius.new(50).in_celsius.should == 50
                Celsius.new(50).in_fahrenheit.should == 122
            end

            it "is a Temperature subclass" do
                Celsius.new(0).should be_a(Temperature)
            end
        end

        describe "Fahrenheit subclass" do
            it "is constructed in degrees fahrenheit" do
                Fahrenheit.new(50).in_fahrenheit.should == 50
                Fahrenheit.new(50).in_celsius.should == 10
            end

            it "is a Temperature subclass" do
                Fahrenheit.new(0).should be_a(Temperature)
            end
        end
    end

end

So, I'm thinking the problem is that I'm trying to go from Temperature.new, which takes a hash, to Celsius.new, which only takes a value. I'm getting an undefined method "has_key?" for nil:NilClass error message. Do I need to set num as a hash value and assign it a key? If so, how do I do that? If not, any suggestions?

Answer 1

Your problem is that you refer to @options, but you don't assign it when creating an instance of Celsius. You should call the super constructor in your inherited classes:

class Celsius < Temperature  
  def initialize(num)
    super(c: num)
  end
end

class Fahrenheit < Temperature 
  def initialize(num)
    super(f: num)
  end
end

Now, when you call Celsius.new(50) the initialize(opts) will be called as if you called Temperature.new(c: 50), and all members will be properly assigned.