Removing a lot of package files.

[ifi-stolz-refaktor.git] / thesis / master-thesis-erlenkr.tex

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\title{Refactoring}
\subtitle{An essay}
\author{Erlend Kristiansen}

\bibliography{bibliography/master-thesis-erlenkr-bibliography}

\begin{document}
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\frontmatter{}


\chapter*{Abstract}
Empty document.

\tableofcontents{}
\listoffigures{}
\listoftables{}

\chapter*{Preface}

\mainmatter{}

\listoftodos

\part{Introduction}

\chapter{Refactoring in general}

\section{What is refactoring?}

This question is best answered dividing the answer into two parts.  First 
defining the concept of a refactoring, then discuss what the dicipline of 
refactoring is all about. And to make it clear already from the beginning: The 
dicussions in this report must be seen in the context of object oriented 
programming languages. It may be obvious, but much of the material will not make 
much sense otherwise, although some of the techniques may be applicable to 
sequential \todo{sequential?} languages, then possibly in other forms.

\subsection{Defining refactoring}
Martin Fowler, in his masterpiece on refactoring~\cite{refactoring}, defines a 
refactoring like this:
\begin{quote}
  \emph{Refactoring} (noun): a change made to the \todo{what does he mean by 
  internal?} internal structure of software to make it easier to understand and 
  cheaper to modify without changing its observable 
  behaviour.~\cite{refactoring} % page 53
\end{quote}
This definition gives additional meaning to the word \emph{refactoring}, beyond 
its \todo{original?} original meaning. Fowler is mixing the \emph{motivation} 
behind refactoring into his definition. Instead it could be made clean, only 
considering the mechanical and behavioural aspects of refactoring. That is to 
factor the program again, putting it together in a different way than before, 
while preserving the behaviour of the program. An alternative definition could 
then be: 

\definition{A refactoring is a transformation
done to a program without altering its external behaviour.}

So a refactoring primarily changes how the \emph{code} of a program is percepted 
by the \emph{programmer}, and not the behaviour experienced by any user of the 
program. Although the logical meaning is preserved, such changes could 
potentially alter the program's behaviour when it comes to performance gain or 
penalties. So any logic depending on the performance of a program could make the 
program behave differently after a refactoring.

In the extreme case one could argue that such a thing as \emph{software 
obfuscation} is to refactor. If we where to define it as a refactoring, it could 
be defined as a composite refactoring \see{intro_composite}, consisting of, for 
instance, a series of rename refactorings. (But it could of course be much more 
complex, and the mechanics of it would not exactly be carved in stone.) To 
perform some serious obfuscation one would also take advantage of techniques not 
found among established refactorings, such as removing whitespace. This might 
not even generate a different syntax tree for languages not sensitive to 
whitespace, placing it in the gray area of what transformations is to be 
considered refactorings.

Finally, to \emph{refactor} is (quoting Martin Fowler)
\begin{quote}
  \ldots to restructure software by applying a series of refactorings without 
  changing its observable behaviour.~\cite{refactoring} % page 54, definition
\end{quote}

% subsection with the history of refactoring?

\subsection{Motivation} % better headline?
To get a grasp of what refactoring is all about, we can answer this question: 
\emph{Why do people refactor?} Possible answers could include: ``To remove 
duplication'' or ``to break up long methods''.  Practitioners of the art of 
Design Patterns~\cite{dp} could say that they do it to introduce a long-needed 
pattern to their program's design.  So it's safe to say that peoples' intentions 
are to make their programs \emph{better} in some sense. But what aspects of the 
programs are becoming improved?

As already mentioned, people often refactor to get rid of duplication. Moving 
identical or similar code into methods, and maybe pushing those up or down in 
their hierarchies. Making template methods for overlapping algorithms 
\todo{better?: functionality} and so on.  It's all about gathering what belongs 
together and putting it all in one place.  And the result? The code is easier to 
maintain. When removing the implicit coupling between the code snippets, the 
location of a bug is limited to only one place, and new functionality need only 
to be added this one place, instead of a number of places people might not even 
remember.

The same people find out that their program contains a lot of long and 
hard-to-grasp methods. Then what do they do? They begin dividing their methods 
into smaller ones, using the \emph{Extract Method} 
refactoring~\cite{refactoring}.  Then they may discover something about their 
program that they weren't aware of before; revealing bugs they didn't know about 
or couldn't find due to the complex structure of their program. \todo{Proof?} 
Making the methods smaller and giving good names to the new ones clarifies the 
algorithms and enhances the \emph{understandability} of the program. This makes 
simple refactoring an excellent method for exploring unknown program code, or 
code that you had forgotten that you wrote!

The word \emph{simple} came up in the last section. In fact, most basic 
refactorings are simple. The true power of them are revealed first when they are 
combined into larger --- higher level --- refactorings, called \emph{composite 
refactorings} \see{intro_composite}. Often the goal of such a serie of 
refactorings is a design pattern. Thus the \emph{design} can be evolved 
throughout the lifetime of a program, opposed to designing up-front.  It's all 
about being structured and taking small steps to improve the design.

Many refactorings are aimed at lowering the coupling between different classes 
and different layers of logic. Say for instance that the coupling between the 
user interface and the business logic of a program is lowered. Then the business 
logic of the program could much easier be the target of automated tests, 
increasing the productivity in the software development process. It would also 
be much easier to distribute the different parts of the program if they were 
decoupled.

Another effect of refactoring is that with the increased separation of concerns 
coming out of many refactorings, the \emph{performance} is improved.  When 
profiling programs, the problem parts are narrowed down to smaller parts of the 
code, which are easier to tune, and optimization can be performed only where 
needed and in a more effective way.

Refactoring program code --- with a goal in mind --- can give the code itself 
more value. That is in the form of robustness to bugs, understandability and 
maintainability. With the first as an obvious advantage, but with the following 
two being also very important in software development. By incorporating 
refactoring in the development process, bugs are found faster, new functionality 
is added more easily and code is easier to understand by the next person exposed 
to it, which might as well be the person who wrote it. So, refactoring can also 
add to the monetary value of a business, by increased productivity of the 
develompment process in the long run.  Where this last point also should open 
the eyes of some nearsighted managers who seldom see beyond the next milestone.


\section{Classification of refactorings} 
% only interesting refactorings
% with 2 detailed examples? One for structured and one for intra-method?
% Is replacing Bubblesort with Quick Sort considered a refactoring?

\subsection{Structural refactorings}

\subsubsection{Basic refactorings}

% Composing Methods
\explanation{Extract Method}{You have a code fragment that can be grouped 
together.}{Turn the fragment into a method whose name explains the purpose of 
the method.}

\explanation{Inline Method}{A method's body is just as clear as its name.}{Put 
the method's body into the body of its callers and remove the method.}

\explanation{Inline Temp}{You have a temp that is assigned to once with a simple 
expression, and the temp is getting in the way of other refactorings.}{Replace 
all references to that temp with the expression}

% Moving Features Between Objects
\explanation{Move Method}{A method is, or will be, using or used by more 
features of another class than the class on which it is defined.}{Create a new 
method with a similar body in the class it uses most. Either turn the old method 
into a simple delegation, or remove it altogether.}

\explanation{Move Field}{A field is, or will be, used by another class more than 
the class on which it is defined}{Create a new field in the target class, and 
change all its users.}

% Organizing Data
\explanation{Replace Magic Number with Symbolic Constant}{You have a literal 
number with a particular meaning.}{Create a constant, name it after the meaning, 
and replace the number with it.}

\explanation{Encapsulate Field}{There is a public field.}{Make it private and 
provide accessors.}

\explanation{Replace Type Code with Class}{A class has a numeric type code that 
does not affect its behaviour.}{Replace the number with a new class.}

\explanation{Replace Type Code with Subclasses}{You have an immutable type code 
that affects the behaviour of a class.}{Replace the type code with subclasses.}

\explanation{Replace Type Code with State/Strategy}{You have a type code that 
affects the behaviour of a class, but you cannot use subclassing.}{Replace the 
type code with a state object.}

% Simplifying Conditional Expressions
\explanation{Consolidate Duplicate Conditional Fragments}{The same fragment of 
code is in all branches of a condtional expression.}{Move it outside of the 
expression.}

\explanation{Remove Control Flag}{You have a variable that is acting as a 
control flag fro a series of boolean expressions.}{Use a break or return 
instead.}

\explanation{Replace Nested Conditional with Guard Clauses}{A method has 
conditional behaviour that does not make clear the normal path of 
execution.}{Use guard clauses for all special cases.}

\explanation{Introduce Null Object}{You have repeated chacks for a null 
value.}{Replace the null value with a null object.}

\explanation{Introduce Assertion}{A section of code assumes something about the 
state of the program.}{Make the assumption explicit with an assertion.}

% Making Method Calls Simpler
\explanation{Rename Method}{The name of a method does not reveal its 
purpose.}{Change the name of the method}

\explanation{Add Parameter}{A method needs more information from its 
caller.}{Add a parameter for an object that can pass on this information.}

\explanation{Remove Parameter}{A parameter is no longer used by the method 
body.}{Remove it.}

%\explanation{Parameterize Method}{Several methods do similar things but with 
%different values contained in the method.}{Create one method that uses a 
%parameter for the different values.}

\explanation{Preserve Whole Object}{You are getting several values from an 
object and passing these values as parameters in a method call.}{Send the whole 
object instead.}

\explanation{Remove Setting Method}{A field should be set at creation time and 
never altered.}{Remove any setting method for that field.}

\explanation{Hide Method}{A method is not used by any other class.}{Make the 
method private.}

\explanation{Replace Constructor with Factory Method}{You vant to do more than 
simple construction when you create an object}{Replace the consgtructor with a 
factory method.}

% Dealing with Generalization
\explanation{Pull Up Field}{Two subclasses have the same field.}{MOve the field 
to the superclass.}

\explanation{Pull Up Method}{You have methods with identical results on 
subclasses.}{Move them to the superclass.}

\explanation{Push Down Method}{Behaviour on a superclass is relevant only for 
some of its subclasses.}{Move it to those subclasses.}

\explanation{Push Down Field}{A field is used only by some subclasses.}{Move the 
field to those subclasses}

\explanation{Extract Interface}{Several clients use the same subset of a class's 
interface, or two classes have part of their interfac|es in common.}{Extract the 
subset into an interface.}

\explanation{Replace Inheritance with Delegation}{A subclass uses only part of a 
superclasses 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.}

\explanation{Replace Delegation with Inheritance}{You're using delegation and 
are often writing many simple delegations for the entire interface}{Make the 
delegating class a subclass of the delegate.}

\subsubsection{Composite refactorings}

% Composing Methods
% \explanation{Replace Method with Method Object}{}{}

% Moving Features Between Objects
\explanation{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.}

\explanation{Inline Class}{A class isn't doing very much.}{Move all its features 
into another class and delete it.}

\explanation{Hide Delegate}{A client is calling a delegate class of an 
object.}{Create Methods on the server to hide the delegate.}

\explanation{Remove Middle Man}{A class is doing to much simple delegation.}{Get 
the client to call the delegate directly.}

% Organizing Data
\explanation{Replace Data Value with Object}{You have a data item that needs 
additional data or behaviour.}{Turn the data item into an object.}

\explanation{Change Value to Reference}{You have a class with many equal 
instances that you want to replace with a single object.}{Turn the object into a 
reference object.}

\explanation{Encapsulate Collection}{A method returns a collection}{Make it 
return a read-only vew and provide add/remove methods.}

% \explanation{Replace Array with Object}{}{}

\explanation{Replace Subclass with Fields}{You have subclasses that vary only in 
methods that return constant data.}{Change the methods to superclass fields and 
eliminate the subclasses.}

% Simplifying Conditional Expressions
\explanation{Decompose Conditional}{You have a complicated conditional 
(if-then-else) statement.}{Extract methods from the condition, then part, an 
else part.}

\explanation{Consolidate Conditional Expression}{You have a sequence of 
conditional tests with the same result.}{Combine them into a single conditional 
expression and extract it.}

\explanation{Replace Conditional with Polymorphism}{You have a conditional that 
chooses different behaviour depending on the type of an object.}{Move each leg 
of the conditional to an overriding method in a subclass. Make the original 
method abstract.}

% Making Method Calls Simpler
\explanation{Replace Parameter with Method}{An object invokes a method, then 
passes the result as a parameter for a method. The receiver can also invoke this 
method.}{Remove the parameter and let the receiver invoke the method.}

\explanation{Introduce Parameter Object}{You have a group of parameters that 
naturally go together.}{Replace them with an object.}

% Dealing with Generalization
\explanation{Extract Subclass}{A class has features that are used only in some 
instances.}{Create a subclass for that subset of features.}

\explanation{Extract Superclass}{You have two classes with similar 
features.}{Create a superclass and move the common features to the 
superclass.}

\explanation{Collapse Hierarchy}{A superclass and subclass are not very 
different.}{Merge them together.}

\explanation{Form Template Method}{You have two methods in subclasses that 
perform similar steps in the same order, yet the steps are different.}{Get the 
steps into methods with the same signature, so that the original methods become 
the same. Then you can pull them up.}


\subsection{Functional refactorings}

\explanation{Substitute Algorithm}{You want to replace an algorithm with one 
that is clearer.}{Replace the body of the method with the new algorithm.}


\section{The impact on software quality}

\subsection{What is meant by quality?}
The term \emph{software quality} has many meanings. It all depends on the 
context we put it in. If we look at it with the eyes of a software developer, it 
usually mean that the software is easily maintainable and testable, or in other 
words, that it is \emph{well designed}. This often correlates with the 
management scale, where \emph{keeping the schedule} and \emph{customer 
satisfaction} is at the center. From the customers point of view, in addition to 
good usability, \emph{performance} and \emph{lack of bugs} is always 
appreciated, measurements that are also shared by the software developer. (In 
addition, such things as good documentation could be measured, but this is out 
of the scope of this document.)

\subsection{The impact on performance}
\begin{quote}
  Refactoring certainly will make software go more slowly, but it also makes the 
  software more amenable to performance tuning.~\cite{refactoring} % page 69
\end{quote}
There is a common belief that refactoring compromises performance, due to 
increased degree of indirection and that polymorphism is slower than 
conditionals.

In a survey, Demeyer~\cite{demeyer2002} disproves this view in the case of 
polymorphism. He is doing an experiment on, what he calls, ``Transform Self Type 
Checks'' where you introduce a new polymorphic method and a new class hierarchy 
to get rid of a class' type checking of a ``type attribute``. He uses this kind 
of transformation to represent other ways of replacing conditionals with 
polymorphism as well. The experiment is performed on the C++ programming 
language and with three different compilers and platforms. \todo{But is the 
result better?} Demeyer concludes that, with compiler optimization turned on, 
polymorphism beats middle to large sized if-statements and does as well as 
case-statements.  (In accordance with his hypothesis, due to similarities 
between the way C++ handles polymorphism and case-statements.)
\begin{quote}
  The interesting thing about performance is that if you analyze most programs, 
  you find that they waste most of their time in a small fraction of the code.  
  ~\cite{refactoring}
\end{quote}
So, although an increased amount of method calls could potentially slow down 
programs, one should avoid premature optimization and sacrificing good design, 
leaving the performance tuning until after profiling the software and having 
isolated the actual problem areas.



\section{Correctness of refactorings} 
% Volker's example?

\section{Composite refactorings} \label{intro_composite}
% motivation, example(s)
% manual vs automated?
% what about refactoring in a very large code base?

\section{Software metrics}


%\part{The project}
%\chapter{Planning the project}
%\part{Conclusion}
%\chapter{Results}                   

\backmatter{}
\printbibliography
\end{document}