Push Down Field §

Problem §

Is a field used only in a few subclasses?

classDiagram
  class Unit {
    fuel
  }
  class Soldier
  class Tank

  Unit <|-- Soldier
  Unit <|-- Tank

Solution §

Move the field to these subclasses.

classDiagram
  class Unit
  class Soldier
  class Tank {
    fuel
  }

  Unit <|-- Soldier
  Unit <|-- Tank

Why Refactor §

Although it was planned to use a field universally for all classes, in reality the field is used only in some subclasses. This situation can occur when planned features fail to pan out, for example.

This can also occur due to extraction (or removal) of part of the functionality of class hierarchies.

Benefits §

• Improves internal class coherency. A field is located where it’s actually used.

• When moving to several subclasses simultaneously, you can develop the fields independently of each other. This does create code duplication, yes, so push down fields only when you really do intend to use the fields in different ways.

How to Refactor §

1. Declare a field in all the necessary subclasses.

2. Remove the field from the superclass.