Code Smell Research:
History and Future Directions
Nikolaos Tsantalis
Computer Science & Software Engineering
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Changes aiming to improve the future
maintainability and reliability of the
software system.
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• Refactoring Object-oriented Frameworks,
University of Illinois at Urbana-Champaign
– Introduced the concept of refactorings as behaviorpreserving program restructuring operations.
– Introduced the concept of preconditions as
conditions that should be examined to determine
whether a refactoring can be applied safely.
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Duplicated Code:
“If you see the same code
structure in more than one
place, … find a way to unify
them.”
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Code smell definition for Inappropriate Interface
inappropriateInterface(?class,?interface,?subclasses) :findall(<?itf,?scs>,
commonSubclassInterface(?class,?itf,?scs),
?result),
removeDuplicates(?result,?nodups),
member(<?interface,?subclasses>,?nodups)
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• Treat the improvement of object-oriented design
as a search problem in the space of
alternative designs
• The move in the search space is achieved by
modeling refactorings (inheritance-related)
• Apply search algorithms using QMOOD as the
fitness function (rank alternative designs)
• The outcome is a sequence of refactorings
leading to the optimal design
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Many code smell definitions are based on
historical change information
– Shotgun Surgery: When every time you make a
kind of change, you have to make a lot of little
changes to a lot of different classes.
– Parallel Inheritance Hierarchies: When every time
you make a subclass of one class, you also have to
make a subclass of another.
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• 12 code smells
• 6 Professional developers were hired to
implement change requests
• 4 medium-sized Java systems with known
code smells
• 4 weeks of development
• Daily interviews and think-aloud sessions
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Advantages:
Feasible and behavior preserving solutions
to design problems
Ranking and prioritizing solutions based on
their expected effect on design quality
A holistic approach for preventive
maintenance
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1. Identify places where software should be refactored
(known as bad smells).
2. Determine which refactoring(s) should be applied to the
identified places.
3. Guarantee that the applied refactoring preserves
behavior.
4. Apply the refactoring.
5. Assess the effect of the refactoring on quality
characteristics of the software.
6. Maintain the consistency between the refactored code
and other software artifacts.
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Context
State
state
- state
int
type ::int
- STATE_A : int = 1
- STATE_B : int = 2
+ method() {
state.method();
if state == STATE_A
} doStateA();
else
if state == STATE_B
doStateB();
}
+method()
StateA
StateB
+method() {
+method() {
}
}
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Since the beginning of 2011 …
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• “Debt” is the effect of incomplete, immature,
or inadequate maintenance activities
• Delayed tasks may bring a short-term benefit
(higher productivity, shorter release time)
• Might have to be paid back in the future with
“Interest” (increased effort)
• “Principal” is the effort to pay off the debt
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•
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•
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Tracking clones as project evolves
Verifying the consistent modification of clones
Updating clones and groups as project evolves
Assessing the harmfulness of clones
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• So far, we used
– Static source code analysis
– Semantic analysis
– Change history analysis
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Every code smell involves a certain risk
for future maintainability
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• Likelihood of occurrence of the risky event
• Exposure of the system to the event
• Consequence of the event
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• Risky events  changes driven by code smells
– Ex. 1: Fixing the same bug in a clone group
– Ex. 2: Making a method more “envy” to another class
• Intuition: code that changes frequently due to
a design flaw imposes a high risk to the
maintainability of a system
• Likelihood  proneness to code smell driven
changes (estimated from the history of changes)
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• Intuition: the more extensively a module is used,
the more extensive the propagation of code
smell driven changes to dependent modules
• Exposure  the number and strength of
incoming dependencies (estimated from static
and dynamic analysis)
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• Consequence  cost of code smell driven
changes (effort, time, money).
From the opposite perspective:
• Consequence  effect of removing the code
smell.
• Estimated by computing the impact of the
corresponding refactoring on metrics.
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Given a set of different types of refactoring
opportunities in a software system
• Conflicts
• Dependencies
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