תכנות מונחה עצמים - III

Topics related to OO development Refactoring Design patterns

Refactoring

Agenda Introductory questions Example Refactoring: Focus on its nature, not on techniques  What is refactoring?  Why refactoring?  How refactoring?  Why refactoring hard?  Extreme Programming and refactoring Summary

Introductory Questions מתי אתם לא מרוצים מקוד שכתבתם? מה אתם עושים במצבים כאלה? נניח שאתם עובדים בצוות פיתוח תוכנה.  ראש הצוות שלכם מבקשת מכם לכתוב את הקוד כל שהוא יהיה קריא יותר. כיצד תגיבו?  חבר לצוות מספר שמה שחשוב לו בפיתוח תוכנה הוא שהקוד רץ. לכן, ברגע שהקוד שכתב עובר את כל הבדיקות, הוא עוזב את הקוד ולא משפר את המבנה והעיצוב שלו. כיצד תגיבו?

Example A given design Source: Martin Fowler, Kent Beck (Contributor), John Brant (Contributor), William Opdyke, don Roberts. (2002). Refactoring: Improving the Design of Existing Code, Addison-Wesley.

Example A given design:  Is it well designed?  In what cases may it cause problems?  Would you change it?  If yes: suggest alternative designs.

Example – Reflection How it emerged?  Deal was originally being used to display a single deal.  Someone wanted a table of deals.  The subclass Tabular Active Deal displays a table.  Now you want tables of passive deals.  Another subclass is added.  Small changes in many places.  The code has become complicated, time is pressing,...  Adding a new kind of deal is hard, because the deal logic is tangled with the presentation logic.

Example – Reflection How it emerges? – In general “One day you are adding one little subclass to do a little job. The next day you are adding other subclasses to do the same job in other parts of the hierarchy. A week (or month or year) later you are swimming in spaghetti. Without a paddle.” (Fowler)

Example – Reflection Problems in tangled inheritance:  It leads to code duplication.  It makes changes more difficult: Strategies for solving a certain problem are spread around.  The resulting code is hard to understand.

Example – Reflection How tangled inheritance can be observed?  Spot for a single inheritance hierarchy that is doing 2 jobs. “If every class at a certain level in the hierarchy has subclasses that begin with the same adjective, you probably are doing two jobs with one hierarchy.” Why it can not be coded “correctly” at the first stage? Step-by-step refactoring (Fowler’s style)

Example – Step by Step Refactoring First step: identify the jobs being done by the hierarchy.  Job #1: capturing variation according to type of deal.  Job #2: capturing variation according to presentation style.

Second step: decide which job is more important.  The dealness of the object is far more important than the presentation style.  Leave Deal alone and extract the presentation style to its own hierarchy. Example – Step by Step Refactoring

Third step: use Extract Class to create a presentation style.  Extract Class You have one class doing work that should be done by two. Create a new class and move the relevant fields and methods from the old class into the new class.

Example – Step by Step Refactoring Fourth step: Create subclasses of the extracted class and initialize the instance variable to the appropriate subclass. Adding subclasses of presentation style

Example – Step by Step Refactoring Fifth step: Use Move Method and Move Field to move the presentation-related methods and variables of the deal subclasses to the presentation style subclasses. No code left in the classes Tabular Active Deal and Tabular Passive Deal. Remove them.

Example – Step by Step Refactoring Sixth step: Separate the hierarchies: Distinguish between single and tabular.

Example Original Refactored 

Example - Reflection What did we do? Is there a difference between the two designs? If yes – what is it? How is this change supposed to improve our life? In what way may the change be useful for someone who did not write the code? Couldn’t we write the code refactored from the beginning?

Example - Summary Tease Apart Inheritance  You have an inheritance hierarchy that is doing two jobs at once.  Create two hierarchies and use delegation to invoke one from the other. This format guides Fowler’s book.

Example - Summary Delegation:  The ability of an object to issue a message to another object in response to a message. Delegation can be used as an alternative to inheritance. Contrast: inheritance. Source: OMG Unified Modeling Language Specification.  More about inheritance vs. delegation:

Example - Summary למה refactoring מתאים ליישום בפיתוח תוכנה מונחה עצמים?

Refactoring In what follows:  What is refactoring?  Why refactoring?  When refactoring? When not?  How refactoring?  Why refactoring hard?  XP and refactoring

Refactoring Fowler: Refactoring is the process of changing a software system in such a way that it does not alter the external (observable) behavior of the code yet improves its internal structure, to make it easier to understand and cheaper to modify. Kent (in Fowler, p. 51): Refactoring is the process of taking a running program and adding to its value, not by changing its behavior but by giving it more of these qualities that enable us to continue developing at speed.

Refactoring What do programmers do when refactoring:  remove duplication  improve communication and program comprehension  add simplicity  add flexibility

Refactoring – Metaphors Metaphors for refactoring :  relationships with your program  health

Refactoring – Metaphors I [Refactoring] is like a new kind of relationship with your program. When you really understand refactoring, the design of the system is as fluid and plastic and moldable to you as the individual characters in a source code file. You can feel the whole design at once. You can see how it might flex and change – a little this way and this is possible, a little that way and that is possible. (Kent, in Fowler, p. 333)

Refactoring – Metaphors II Refactoring as health: exercises and eating a proper diet.  The culture we live in.  We can always make excuses, but we are only fooling ourselves if we continue to ignore good behavior.  Near-term and long-term benefits.

Refactoring Main questions:  What is refactoring? OK  Why refactoring?  When refactoring? When not?  How refactoring?  Why refactoring hard? Why people do not do that?  XP and refactoring

Why Refactoring Refactoring improves the design of the software  fosters the examination of the software design  removes duplicated code: reduces the amount of code the code says everything once and only once

Why Refactoring Refactoring makes software easier to understand  helps make your code more readable  increases program comprehension: leads to higher levels of understanding that otherwise may be missed

Why Refactoring Refactoring helps you program faster  sounds counterintuitive  less bugs, no patches  helps correct bugs: errors need to be modified only in one place

Refactoring Main questions:  What is refactoring OK  Why refactoring? OK  When refactoring? When not?  How refactoring?  Why refactoring hard? Why people do not do that?  XP and refactoring

When refactoring You have written some code.  How would you find what to refactor?  What clues in the code may guide you? Fowler, chapter 3 – Bad smells in code

When refactoring Fowler, Chapter 3 – Bad smells in Code Duplicated Code:  “If you see the same code structure in more than one place, you can be sure that your program will be better if you find a way to unify them”.  Extract Method: When you have the same expression in two methods of the same class.

When refactoring Fowler, Chapter 3 – Bad smells in Code Long Method:  “the longer the procedure is, the more difficult it is to understand”.  Extract method: find parts of the methods that seem to go nicely together and make a new method.

When refactoring Fowler, Chapter 3 – Bad smells in Code Comments:  “if you need a comment to explain what a block of code does, try Extract Method. If the method is already extracted but you still need a comment to explain what it does, use Rename Method.”  “when you feel the need to write a comment, first try to refactor the code so that any comment becomes superfluous”.  “a comment is a good place to say why you did something. This kind of information helps future modifiers”.

When shouldn't you refactor? When the code is a mess and it would be better to start from the beginning. Factors that will be discussed later:  Culture  Internal resistance

Refactoring Main questions:  What is refactoring OK  Why refactoring? OK  When refactoring? When not? OK  How refactoring?  Why refactoring hard? Why people do not do that?  XP and refactoring

How Refactoring Most of the time it is done in small and local places Sometimes: a sequence of refactoring Refactoring requires high level of awareness  All the time  Two hats: adding functions and refactoring

How refactoring Resources for specific refactoring:  Refactoring Home Page:  Martin Fowler, Kent Beck (Contributor), John Brant (Contributor), William Opdyke, don Roberts (1999). Refactoring: Improving the Design of Existing Code, Addison-Wesley. Many of the citations in this refactoring presentation are from the book.  Some IDEs (Integrated development environments) offer Refactoring menu Example: Eclipse, IntelliJ

Refactoring Main questions:  What is refactoring OK  Why refactoring? OK  When refactoring? When not? OK  How refactoring?OK  Why refactoring hard? Why people do not refactor?  Extreme Programming (XP) and refactoring

Why refactoring hard? Sub-questions:  Why people do not refactor naturally?  Why does refactoring raise resistance?

Why refactoring hard? Culture:  “ refactoring is an overhead activity. I’m paid to write new, revenue-generating features”.  “What do I tell my manager?” Treat it as part of the profession: This is how you develop code, it is not viewed by you as an additional work.

Why refactoring hard? Internal resistance: Why are developers reluctant to refactor? (Opdyke, in Fowler’s book, p. 313)  it should be executed when the code runs and all the tests pass. It seems that time is wasted now.  if the benefits are long-term, why exert the effort now? In the long term, developers might not be with the project to reap the benefits.  developers might not understand how to refactor.  refactoring might break the existing program.

Refactoring Main questions:  What is refactoring OK  Why refactoring? OK  When refactoring? When not? OK  How refactoring?OK  Why refactoring hard? OK  Extreme Programming and refactoring

Extreme Programming and Refactoring Refactoring is part of eXtreme Programming:  Refactoring can be carried out without XP, but it has additional value with XP  It has similar targets to those that XP inspires  When refactoring is part of XP: refactoring becomes part of the routine it stops feeling like an overhead activity

Extreme Programming and Refactoring Mutual relationships of refactoring and other XP practices Source: Beck, K. (2000). eXtreme Programming explained, Addison Wesley.

Refactoring Main questions:  What is refactoring OK  Why refactoring? OK  When refactoring? When not? OK  How refactoring?OK  Why people do not refactoring? OK  XP and refactoringOK

Refactoring – Summary Refactoring requires awareness! Main Message:  We should not skip refactoring.  Software development is a process that all its details cannot be envisioned in advance. Refactoring may improve programming skills.