Code metrics

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Code quality


What is code quality

• Sample code quality questions:− Are methods and classes clean, readable and short?− Are connections between classes well-designed?− Does your code follow Java conventions?

private static final rather than private final static

− Do you have unit tests? What is the coverage?− Don’t you have some other code-smells?

Eg. if statement with too many || conditions

• Some issues caused by low quality code:− Problems with adding new features− Problems with collaboration− Hidden bugs− Hard to debug

• Static code analysis tools can check all things automatically• Sonar – popular tool for Java


Coding standards


Coding standards – exampleclass FileUtils {

public final static String pngExtension = "png";

public static String getExtension(String filename) { int DOTPos = filename.indexOf(".");

if (filename != null && DOTPos > -1) { String extension = filename.substring(DOTPos+1); return extension; } return null; }}


Cyclomatic complexity


Cyclomatic complexity

• Basic code metric• Describes number of “branches” in method or class• Sequential code (no ifs, fors and whiles): complexity =

1• Each additional condition: +1• If method is too complex, we should split it into smaller ones


House of cards


Complexity for sequential codebyte[] buffer = new byte[1024];InputStream stream = new FileInputStream("/my/file");InputStream buffered = new BufferedInputStream(stream);buffered.read(buffer);buffered.close();

• Complexity: 1• No conditions, simple flow


Complexity with one conditionbyte[] buffer = new byte[1024];File file = new File("/my/file");if (!file.exists()) {return;

}InputStream stream = new FileInputStream(file);InputStream buffered = new BufferedInputStream(stream);buffered.read(buffer);buffered.close();

• Complexity: 2


Complexity: formal definitionFormal definition: Edges – Nodes + 2 Components

for(int i=0;i<3;i++) { System.out.println(i);}

if(j == 42) { System.out.println(”j is the answer”)}Edges: 9, Nodes: 8, Components: 1

E-N+2P=3


Class responsibilities


Single responsibility principle

• Each class should be responsible for only one thing• Example: class Driver:

− 2 fields: car, brain,− 5 methods: drive(), goTo(), stop(), getAngry(), drinkCoffee(),

• This class has 3 responsibilities:− Driving car− Getting angry− Drinking coffe

• It should be split into 3 classes


LCOM4 & cohesion

• LCOM4 – Lack Of Cohesion Methods.• How many responsibilities class?• LCOM4 = 3 means that

− there are 3 not connected sets of methods/properties− class can be split into three new classes.

• Class should be designed to have LCOM4 = 1• Low LCOM4 = high cohesion• Cohesion relates to coupling


Coupling


Tight coupling

• If a class (eg. Driver) invokes methods on some other class (eg. Car) it’s tightly coupled with it

• In this case we can’t easily change Car implementation to something other (eg. Tank)

• Real-life example: iPhone and its batterypublic class Driver { private final Car car;

public Driver(Car car) { this.car = car; }

public void driveCar() { car.startEngine(); car.drive(); }}

public class Car { public void startEngine() { … }

public void drive() { … }}


Loose coupling

• Class doesn’t need to know about other classes existence• It only need to know about contracts (interfaces) provided by

other class• Real-life example: screw and screwdriver

public class Driver { private Vehicle vehicle;

public Driver(Vehicle vehicle) { this.vehicle = vehicle; }

public void drive() { vehicle.startEngine(); vehicle.drive(); }}

public interface Vehicle { void startEngine(); void drive();}

public class Car implements Vehicle { public void startEngine() { … }

public void drive() { … }}


Coupling & cohesion – summary

• Classes must be loosely coupled and highly cohesive.

• Cohesion is the degree to which the methods of a single class are tight together.

− If class is not cohesive (eg. two methods doesn’t share any property), it can be split to two classes.

• Coupling is the degree to which each class is tight to the others.

− Any change you make in one class has impact to some other classes.

− Use interfaces & depdency injection

• Read more…• Read even more…

http://www.sonarsource.org/clean-up-design-at-class-level-with-sonar/

http://www.g3it.com/blog/technical/java/assessing-the-single-responsibility-principle-with-lcom4-and-sonar-2-0/




Technical debt


Technical debt

• „Doing things the quick and dirty way sets us up with a technical debt, which is similar to a financial debt.”

• Metric based on:− duplications,− violations,− (lack of) comments,− coverage,− complexity.

• Absolute debt in mandays and cash• Debt ratio – dependent on the project size.• Useful for project manager• Read more...

http://www.sonarsource.org/evaluate-your-technical-debt-with-sonar/


Other metrics


Other code metrics

• Lines of code• Unit test coverage• Responses for class• …• All these metrics can be checked via Sonar

− Free & very useful software− Can be integrated with Eclipse

• Visual Studio provides integrated metrics as well

http://www.sonarqube.org/

Code metrics

Technology

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