Refactoring Method 30: replace type code with class (replace type code with Class)

 

Class has a numeric type code,But it does not affect the behavior of the class. Replace the Value Type Code with a new class.

Motivation: C-basedProgramming LanguageType Code or enumeration value is very common. If there is a meaningful symbolic name, the type code is quite readable. The problem is that the symbol name is only an alias. The value behind the symbol is what the compiler sees and checks the type. Any function that accepts the type code as a parameter actually expects a value and cannot force the use of the symbol name. This greatly reducesCodeBecome the source of a bug.

If such a value is changed to a class, the compiler can perform type checks on the class. By providing a factory function for this class, you can always ensure that only valid instances are common and that they are passed to the correct host object.

However, before using replace type code with class (replace type code with class), you should consider other replacement methods of type code. Only when the type code is pure data (that is, the type code will not cause behavior changes in the switch statement) Can you replace it with a class. More importantly, any switch statement should be removed using replace conditional with polymorphism (replacing conditional expressions with polymorphism. To perform such refactoring, you must first use replace type code with subclass (replace type code with subclass) or replace type code with State/Strategy (replace type code with State policy ), process the type code.

Even if a type code does not cause behavior differences due to its numeric values, some behaviors in the host class may be more suitable for the Type Code class, therefore, you should also pay attention to whether it is necessary to use the move method to move one or two functions.