What is the difference between afferent couplings and efferent couplings of a class?

Question

Code quality metric tool like Sonar does provide the ability to drill down to a class and find out the number of:

1. Afferent (incoming) couplings
2. Efferent (outgoing) couplings

What are these two parameters? Can you please describe with a simple contrived example?

Answer 1

According to wikipedia:

Afferent Couplings (Ca): The number of classes in other packages that depend upon classes within the package is an indicator of the package's responsibility. Afferent = incoming.

Efferent Couplings (Ce): The number of classes in other packages that the classes in the package depend upon is an indicator of the package's dependence on externalities. Efferent = outgoing.

So, if you have classes (or packages or whatever) with the following structure:

class Foo {
    Quux q;
}

class Bar {
    Quux q;
}

class Quux {
    // ...
}

Then Foo and Bar each have one efferent coupling, and Quux has two afferent couplings.

Answer 2

Since you mentioned Sonar, here is the definition provided by their documentation page

• Afferent couplings : A class afferent couplings is a measure of how many other classes use the specific class.
• Efferent couplings : A class efferent couplings is a measure of how many different classes are used by the specific class.

Answer 3

Coupling is a measure of dependencies.

Afferent Coupling :

• Who depends on you.
• Measure of how many other packages use a specific package.
• Incoming dependencies.

Efferent Coupling :

• Who do you depend on.
• Measure of how many different packages are used by a specific package.
• Outgoing dependencies.