does your design smell - tushar sharma

Does Your Design Smell?

Tushar Sharma

http://www.tusharma.in

Cities grow, cities evolve, (...), each cityhas to be renewed in order to meet theneeds of its populace. From time to timecities are intentionally refactored (...);most times cities are refactored in bits(...), but sometimes the literal andfigurative debt is so great a once greatcity falls into despair (...).

Grady Booch

Missing road

Unrestricted access

Hub-like road

Footpath is not a road

Deadlock!!

Multitasking

Useless… unnecessary

Complex

Beyond capacity

Why?

Every engineering discipline has to deal withnon-optimal and sub-optimal solutions.

Such solutions violate fundamentalprinciples of the discipline.

Smells

Design smells

Design smells are certain structures in the design thatindicate violation of fundamental design principles and

negatively impact design quality.

Missing road

This smell arises when clumps ofdata or encoded strings are usedinstead of creating a class or an

interface.

Missing abstraction

public class Throwable {public void printStackTrace(); //other methods elided.

}

(Java 1)

Throwable class in JDK

public class Throwable {public void printStackTrace();public StackTraceElement[] getStackTrace();//other methods elided.

}

public final class StackTraceElement {public String getFileName();public int getLineNumber();public String getClassName();public String getMethodName();public boolean isNativeMethod();

}

Refactored in Java 1.4

(Java 1.4)

Unrestricted access

This smell occurs when the declared accessibilityof one or more members of an abstraction ismore permissive than actually required.

Deficient Encapsulation

In JDK7• 508 classes - at least one

public field• 96 classes - at least five

public fields

Hub-like road

This smell arises when an abstraction has dependencies(both incoming and outgoing) with a large number ofother abstractions.

Hub-likeModularization

Component class in JDK

Footpath is not a road

This smell arises when a supertype and its subtypeconceptually do not share an “IS-A” relationshipresulting in broken substitutability.

Broken Hierarchy

Stack is not a Vector

This smell arises when two or more abstractionsdepend on each other directly or indirectly.

Cyclically-dependentModularization

Deadlock!! Cycle!!

This smell arises when an abstraction has more thanone responsibility assigned to it.

Multifaceted Abstraction

Multitasking

Image class• load• process• render• store

Example

multiplereasons tochange

granularity of a responsibility

This smell occurs when an abstraction that is actuallynot needed (and thus could have been avoided) getsintroduced in a software design.

Unnecessary Abstraction

Useless… unnecessary

public interface WindowConstants {/**The do-nothing default window close operation*/public static final int DO_NOTHING_ON_CLOSE = 0;/**The hide-window default window close operation*/public static final int HIDE_ON_CLOSE = 1;/**The dispose-window default window close operation*/public static final int DISPOSE_ON_CLOSE = 2;/**The exit application default window close operation.*/public static final int EXIT_ON_CLOSE = 3;

}

Example

In javax.swing.WindowConstants

InsufficientModularization

Beyond capacity!!Complex!!

This smell arises when an abstraction exists that has notbeen completely decomposed, and a furtherdecomposition could reduce its size, implementationcomplexity, or both.

InsufficientModularization

Examples• SocketPermission

getMask method – CC=81• Component

259 public methods

Tools to detect design smells

Infusion

Tools to detect design smellsDesignite

http://www.designite-tools.com

Tools to detect design smells

Designite

http://www.designite-tools.com

Themessage

Every engineering discipline has to deal withnon-optimal and sub-optimal solutions.

Software engineering is not an exception;in fact, it is more prone to such smells.

Design smells – know them, avoidthem, and refactor them!

References

“Refactoring for software designsmells: Managing technical debt”By Girish Suryanarayana, Ganesh SG,and Tushar Sharma.Morgan Kaufmann. ISBN 978-0128013977

http://www.designsmells.com

Tushar Sharma@[email protected]://www.tusharma.in