Replace Command with Function - Instance to Static

This is one of the refactorings in Martin Fowler’s book. The basic idea is to change a method that needs an instance of a class with a freestanding function. In languages like Java and C# you’d probably call it ‘instance to static’. This is the inverse of Replace Function with Command

Examine

Usually the prompt for this refactoring is unnecessary complexity - a plain function can be simpler than constructing an object and calling a method on it.

Prepare

Identify the command you want to turn into a function, and find all the places it is used.

Implement

• For the first place where the command is used, use ‘extract function’ to create a new function that both constructs the object and calls the command.
• Introduce parameters for all the necessary data used by the new function
• (optional) Move the new function to a place where it can access object internals for the command it is calling (ie to the class of the object it is constructing)
• For all the other places where the command is used, have them call the new function instead.
• Tests all passing.
• In the new function, inline the call to the command.
• In the new function, inline the call to the constructor and instead use parameters directly.
• Tests all passing.
• Remove any dead code

Clear

Check again whether the new function is in the right place.

Follow up

Check whether the function has feature envy on any of its arguments. Perhaps it should be moved and become a method on that class - in which case you can use the inverse of this refactoring - Replace Function with Command.

Prompted by Code Smells

• Lazy Element

Relevant Learning Hours

• Change Signature

Sources

• This refactoring is included Martin Fowler's book "Refactoring", 2nd Edition

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