Push Down Method §

Problem §

Is behavior implemented in a superclass used by only one (or a few) subclasses?

classDiagram
  class Unit {
    getFuel()
  }
  class Soldier
  class Tank

  Unit <|-- Soldier
  Unit <|-- Tank

Solution §

Move this behavior to the subclasses.

classDiagram
  class Unit
  class Soldier
  class Tank {
    getFuel()
  }

  Unit <|-- Soldier
  Unit <|-- Tank

Why Refactor §

At first a certain method was meant to be universal for all classes but in reality is used in only one subclass. This situation can occur when planned features fail to materialize.

Such situations can also occur after partial extraction (or removal) of functionality from a class hierarchy, leaving a method that’s used in only one subclass.

If you see that a method is needed by more than one subclass, but not all of them, it may be useful to create an intermediate subclass and move the method to it. This allows avoiding the code duplication that would result from pushing a method down to all subclasses.

Benefits §

• Improves class coherence. A method is located where you expect to see it.

How to Refactor §

1. Declare the method in a subclass and copy its code from the superclass.

2. Remove the method from the superclass.

3. Find all places where the method is used and verify that it’s called from the necessary subclass.