Inheritance: pull up duplicated behavior
Write shared behavior once in a superclass, so every subclass gets each change.
Watch the narrated video
Every step
Problem
Steps 1–3Refactor
Steps 4–6Payoff
Steps 7–8Recap
Steps 9–10The code: before and after
Before
class Shipment
def price
raise NotImplementedError
end
end
class DomesticShipment < Shipment
def initialize(tracking, weight_kg)
@tracking = tracking
@weight_kg = weight_kg
end
def price
5 + @weight_kg
end
def label
parts = [@tracking, "#{@weight_kg} kg", "$#{price}"]
parts.join(" | ")
end
end
class InternationalShipment < Shipment
def initialize(tracking, weight_kg)
@tracking = tracking
@weight_kg = weight_kg
end
def price
20 + @weight_kg * 4
end
def label
parts = [@tracking, "#{@weight_kg} kg", "$#{price}"]
parts.join(" | ")
end
end
home = DomesticShipment.new("DM-1", 2)
abroad = InternationalShipment.new("IN-7", 2)
puts home.label # DM-1 | 2 kg | $7
puts abroad.label # IN-7 | 2 kg | $28After
class Shipment
def initialize(tracking, weight_kg)
@tracking = tracking
@weight_kg = weight_kg
@fragile = false
end
def price
raise NotImplementedError
end
def mark_fragile
@fragile = true
end
def label
parts = [@tracking, "#{@weight_kg} kg", "$#{price}"]
parts << "FRAGILE" if @fragile
parts.join(" | ")
end
end
class DomesticShipment < Shipment
def price
5 + @weight_kg
end
end
class InternationalShipment < Shipment
def price
20 + @weight_kg * 4
end
end
home = DomesticShipment.new("DM-1", 2)
abroad = InternationalShipment.new("IN-7", 2)
abroad.mark_fragile
puts home.label # DM-1 | 2 kg | $7
puts abroad.label # IN-7 | 2 kg | $28 | FRAGILETranscript, step by step
Problem
Step 1: A courier's two shipments Before
A parcel courier's dispatch desk works with two kinds of shipment. Shipment is the superclass, the class the others extend, and each subclass, a class that extends it, says what its parcel costs. Shipment's own price raises an error, so a subclass that forgets to define one fails when the label asks for it. Both subclasses also set a tracking code and a weight in their own initialize, and both build a label from them. The script at the bottom prints one label for a domestic parcel and one for an international one.
Step 2: Fragile parcels Code smell
The courier now needs the word fragile on the label of fragile parcels. The quick way adds a fragile variable, a mark fragile method and a label check to Domestic Shipment, and then the same three again to International Shipment. The script marks the parcel going abroad, so only the second label ends in fragile.
Step 3: Two copies of everything Code smell
Duplicated code across subclasses is when two classes that share a superclass each write the same fields and methods on their own. You can see the fields here and here, and the label here and here. So every change to the label is made twice, and the day someone edits only one copy, a fragile parcel ships without the word on it. A third shipment type would copy all of it a third time.
Refactor
Step 4: Refactor first Back it out
Let's take the fragile flag out again. We'll refactor first, so that the shared parts move up into Shipment, and then we'll add the flag in one place.
Step 5: One initialize Pull Up Constructor Body
Ruby has no field declarations, and an instance variable comes into being when initialize assigns it, so the fields and the constructor body move up together. Shipment's initialize now sets the tracking code and the weight, once. The two subclass constructors only repeated those lines, so both are deleted, and each subclass inherits initialize from Shipment. Instance variables belong to the object, so each subclass label still reads the tracking code and weight that Shipment set. The script still builds both parcels with a tracking code and a weight, and prints the same two labels.
Step 6: One label Pull Up Method
Now the two label bodies are identical, so one copy moves up into Shipment, and both subclass copies are deleted. Label still works because it calls price, and each subclass still answers price its own way. This is inheritance, where a subclass gets the fields and methods of its superclass, so the shared parts are written once, in Shipment. What's left in each subclass is its price. A superclass method that calls a step each subclass fills in, like label calling price, is the Template Method pattern.
Payoff
Step 7: Fragile, once Add the flag
Now the fragile flag comes back, and this time it goes into Shipment only. Shipment gains one fragile variable and one mark fragile method, and label adds the word once. Domestic and International Shipment didn't change, and the script only gains one call that marks the parcel going abroad.
Step 8: Shared code lives once Payoff
The fields now exist once, here, and the label exists once, here. The fragile rule is written once too, so the two labels can't drift apart, and the run prints the word only on the parcel going abroad. It fits because both classes are shipments, each with a tracking code, a weight and a label, and only the price differs. Abstraction hid how a class stores its data, and inheritance shares what two classes both do.
Recap
Step 9: Where we started, where we ended Before / after
On the left, each subclass keeps its own initialize and its own label. On the right, Shipment holds the initialize, the fragile flag and the label, each written once. A subclass like Domestic Shipment keeps only its own price.
Step 10: When to reach for it Takeaways
Reach for it when two or more subclasses of one superclass repeat the same fields or the same method body. Skip it when two classes look alike today but will change for different reasons, because a superclass ties them together, so use Extract Class and delegate to it. You'll spot the need by the same method copied into sibling subclasses, and the same initialize lines in each. Ruby's own errors work this way, because a custom error that subclasses Standard Error writes nothing and still inherits its message and backtrace.