18 — Refactoring Foundations

Driving question: What must remain invariant when internal structure changes?

Learning objectives

• Define refactoring as behavior-preserving restructuring.
• Use preconditions and tests to reduce transformation risk.
• Distinguish refactoring from feature work and optimization.
• Compose small transformations into larger design changes.

Behavior preservation

Refactoring changes internal structure while preserving externally observable behavior under an agreed observation model. That model matters: performance, timing, logging, exception types, serialization order, or reflection behavior may be observable in some systems.

Safe sequence

Characterize → Transform → Test → Commit → Repeat

Small refactorings create checkpoints. When automation is available, precondition checking should prevent transformations known to alter binding, visibility, type resolution, or control/data flow.

Refactoring mechanics

Examples:

• Extract Method / Function;
• Move Method / Field;
• Extract Class / Interface;
• Inline Method / Class;
• Rename;
• Replace Conditional with Polymorphism;
• Introduce Parameter Object;
• Encapsulate Field;
• Pull Up / Push Down members.

Design / research exercise

Choose a refactoring supported by your IDE. Enumerate semantic preconditions that the IDE must check. Then construct one edge case involving inheritance, overload resolution, aliasing, or exceptions.

Suggested reading

• Martin Fowler, Refactoring.
• William Opdyke, early refactoring research.