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Introduce Null Object

Problem:

Logic for dealing with a null field or variable is duplicated throughout your code.

Solution:

Replace the null logic with a Null Object - an object that provides the appropriate null behavior.

Refactorings

1.

Chain Constructors

2.

Compose Method

3.

Encapsulate Classes With Factory

4.

Encapsulate Composite With Builder

5.

Extract Adapter

6.

Extract Composite

7.

Extract Parameter

8.

Form Template Method

9.

Inline Singleton

10.

Introduce Null Object

11.

Introduce Polymorphic Creation With Factory Method

12.

Limit Instantiation With Singleton

13.

Move Accumulation To Collecting Parameter

14.

Move Accumulation To Visitor

15.

Move Creation Knowledge To Factory

16.

Move Embellishment To Decorator

17.

Replace Conditional Dispatcher With Command

18.

Replace Conditional Logic With Strategy

19.

Replace Constructors With Creation Methods

20.

Replace Hard-Coded Notifications With Observer

21.

Replace Implicit Language With Interpreter

22.

Replace Implicit Tree With Composite

23.

Replace One/Many Distinctions With Composite

24.

Replace State-Altering Conditionals with State

25.

Replace Type Code With Class

26.

Unify Interfaces With Adapter

27.

Unify Interfaces