Mastering Refactoring: When and How to Move Methods, Fields, and Classes

In object‑oriented programming, clearly define each object's responsibilities; a class should do only what it is meant to do and fulfill its duties.

1. Move Method

Class behavior must follow the single‑responsibility principle; avoid overstepping. If a function interacts more with another class than its own, create a similar function in that other class, delegate the original, or remove it.

Moving functions is a pillar of refactoring. When a class has too many responsibilities or is highly coupled with another, move functions to simplify classes. Identify functions used more on another object than its own, examine callers and callees, and decide where to move.

2. Move Field

Fields should reside where they are most used. If a field is used more by another class, create the field in the target class and update all users to use the new field.

3. Extract Class

A class should be a clear abstraction handling specific responsibilities. When a class grows too large, create a new class and move related fields and methods to it. This prevents a “big ball of mud” and improves understandability.

4. Inline Class

If a small class no longer has enough responsibility, move its features into another class and delete the original.

5. Hide Delegate

Clients call another object through a delegate class. Build all needed functions on the service class to hide the delegation, reducing client knowledge of the underlying structure and easing future changes.

6. Remove Middle Man

When a service class becomes a simple pass‑through, let clients call the delegated class directly, eliminating unnecessary middle‑man code.

7. Introduce Foreign Method

If you need a new function for a service class you cannot modify, add the function in a client class and pass the service instance as the first parameter. This avoids duplicating code across clients.

8. Introduce Local Extension

When multiple extra functions are needed for a class you cannot change, create a new class that extends or wraps the original, providing all additional features in one place.