Source-Code Activities

• Fix a bug (Debugging)
• Add a new feature (Development)
• Understand what some part of the code is doing (Comprehension)
• Change for a new API (Adaptive Maintenance)

Levels of Software Changes

• High-Level Software Changes
• Add features to a system
• E.g., Support a new output format by adding a class
• Intermediate-Level Software Changes
• Change design
• E.g., Move a method from one class to another
• Low-Level Software Changes
• Change individual statements
• E.g., Convert a deeply nested if to separate if statements

Mathematics: fac.tor

One of two or more quantities that divides a given quantity without a remainder, e.g., 2 and 3 are factors of 6; a and b are factors of ab

• Multiple equivalent forms:
• 2 × 3 = 6 and 1 × 6 = 6
• x^2 - 4 = (x - 2)(x + 2)
• The best form depends on the problem to solve

Mathematics: fac.tor.ing

To determine or indicate explicitly the factors of

• Multiple equivalent forms:
• 2 × 3 = 6 and 1 × 6 = 6
• x^2 - 4 = (x - 2)(x + 2)
• The best form depends on the problem to solve

Software Engineering: fac.tor

The individual items that combined form a complete software system

• Identifiers
• Contents of methods
• Contents of classes
• Relationship of a class to other classes
• Many different ways to factor a software system

Software Engineering: fac.tor.ing

Determining the items, at design time, that make up a software system

• Identifiers
• Contents of methods
• Contents of classes
• Relationship of a class to other classes
• Many different ways to factor a software system

Restructuring: [Opdyke'92]

A program restructuring operation to support the design, evolution, and reuse of object-oriented frameworks that preserve the behavioural aspects of the program

Refactoring: [Fowler99]

Process of changing a software system in such a way that it does not alter the external behavior of the code yet improves its internal structure

• Does not change the behavior of the program
• Type of software transformation
• Source-to-source, remain inside the same language, e.g., Java to Java, C++ to C++
• Originally designed for object-oriented languages, but can also apply to other programming paradigms, e.g., functions, SQL, and other DSL (Domain Specific Languages)

Testing Requirements

Process of changing a software system in such a way that it does not alter the external behavior of the code yet improves its internal structure

• Testing must be in place to make sure that the behavior of the software does not change
• Requires unit testing
• No refactoring (or changes of any kind) before unit testing is in place
• During refactoring, the current, expected behavior of the software must not change

Legacy Code

• Often applied to old systems with "old" technology (e.g., COBOL, Java)
• There is a great deal of software behavior that is:
• Not stated (or clearly stated) in written policies
• No code documentation
• And even no test suite
• Any code that does not have a unit-testing suite is considered legacy code

Refactoring and Process

Process of changing a software system in such a way that it does not alter the external behavior of the code yet improves its internal structure

• Not the same as "cleaning up code" (which may cause changes to the behavior of the program)
• Cleaning up code is not an effective strategy
• Changes to a small context (e.g., individual method) or the entire program
• A key element of most workflows and processes in agile
• Viewpoint is that design is part of an iterative process

Refactoring: Rename Method

Refactoring: Rename Method Step 1

• Copy the declaration of the old method to the new method
• Compile and build, and successfully run tests
• Commit
• Copy the definition of the old method to the new method
• Compile and build, and successfully run tests
• Commit

Refactoring: Rename Method Step 2

• Change the body of the old method to call the new method
• Compile and build, and successfully run tests to make sure no behavioral changes
• Commit

Refactoring: Rename Method Step 3

• Find all calls to the old method and replace them with calls to the new method
• Compile and build, and successfully run tests after each change
• Commit after each change

Refactoring: Rename Method Step 4

• Remove the old method definition (if not a public interface)
• Compile and build, and successfully run tests
• Remove the old method declaration (if not a public interface)
• Compile and build, and successfully run tests
• Commit

Refactoring: Rename Method Timeline

• 1) Copy the declaration of the old method to the new method
• 2) Copy the definition of the old method to the new method
• 3) Change the body of the old method to call the new method
• 4) Find all calls to the old method and replace them with calls to the new method, one at a time
• 5) Remove the old method declaration (if not a public interface)
• 4) Remove the old method definition (if not a public interface)

Refactoring Procedure

• After each step, we compile and build to ensure that there are no build breaks
• We run the tests to make sure that there are no behavioral changes
• We commit after each step so that our changes can be integrated with other changes as early as possible
• If the language does not have a separate declaration/definition, then those steps are combined
• Commits can be combined (i.e., "squashed"), so you cannot commit too frequently.

Refactoring Examples

• Introduce Explaining Variable
• Rename Method
• Move Method
• Pull-Up Method
• Change Value to Reference
• Remove Parameter
• Extract Hierarchy

Refactoring: Split Temporary Variable

Refactoring: Pull-Up Method

• Methods with identical results in subclasses
• Move them to the superclass

Refactoring: Replace Inheritance with Delegation (UML)

• A subclass uses only part of a superclass interface or does not want to inherit data
• Create a field for the superclass, adjust methods to delegate to the superclass, and remove the subclassing

Refactoring: Replace Inheritance with Delegation (Code)

Catalog

• Collected by Fowler
• Refactoring entry composed of:
• Name
• Summary
• Motivation
• Mechanics
• Examples
• Recent edition examples are in Javascript

Categories

• Composing Methods
• Organizing Data
• Moving Features Between Objects
• Simplifying Conditional Expressions
• Simplifying Method Calls
• Dealing with Generalization
• Big Refactorings

Composing Methods: Extract Method

Composing Methods: Inline Method

Replace Nested Conditional with Guard Clauses

Inline Temp

Replace Temp with Query

Why Refactor?

• Design Preservation
• Comprehension
• Debugging
• Faster Programming

Why?

Design Preservation

• Code changes often lead to a loss of the original design
• Loss of design is cumulative:
• Difficulty comprehending design →
  Difficulty preserving design →
  Design decays more rapidly →
  Difficulty comprehending design
• Refactoring improves the design of existing code

Why?

Comprehension

• Developers are most concerned with getting the program to work, not about future developers
• Refactoring makes existing code more readable
• Increases comprehension of existing code, leading to higher levels of code comprehension
• Often applied in stages

Why?

Debugging

• Greater program comprehension leads to easier debugging
• Increased readability leads to the discovery of possible errors
• Developers can put back into the code the understanding gained during debugging

Why?

Faster Programming

• A counterintuitive argument made by Fowler
• Good design is essential for rapid development
• A poor design allows for quick progress but soon slows the process down
• Spend time debugging
• Changes take longer as you understand the system and find duplicate code
• Supported by Lehman's laws

When?

During activities such as …

• adding functionality
• comprehension of an existing program
• preparation for additional functionality
• debugging
• Code Review
• preparation for suggestions to other programmers
• looking for ideas to improve