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ClassFanOutComplexity.md

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Pattern: Class relies on too many other classes

Issue: -

Description

In other other words, class fan-out complexity is too high. It has direct impact on amount of maintenance required in functional programs. This check processes files in the following way:

  1. Iterates over the list of tokens (defined below) and counts all mentioned classes.
- [PACKAGE_DEF](http://checkstyle.sourceforge.net/apidocs/com/puppycrawl/tools/checkstyle/api/TokenTypes.html#PACKAGE_DEF)
- [IMPORT](http://checkstyle.sourceforge.net/apidocs/com/puppycrawl/tools/checkstyle/api/TokenTypes.html#IMPORT)
- [CLASS_DEF](http://checkstyle.sourceforge.net/apidocs/com/puppycrawl/tools/checkstyle/api/TokenTypes.html#CLASS_DEF)
- [INTERFACE_DEF](http://checkstyle.sourceforge.net/apidocs/com/puppycrawl/tools/checkstyle/api/TokenTypes.html#INTERFACE_DEF)
- [ENUM_DEF](http://checkstyle.sourceforge.net/apidocs/com/puppycrawl/tools/checkstyle/api/TokenTypes.html#ENUM_DEF)
- [TYPE](http://checkstyle.sourceforge.net/apidocs/com/puppycrawl/tools/checkstyle/api/TokenTypes.html#TYPE)
- [LITERAL_NEW](http://checkstyle.sourceforge.net/apidocs/com/puppycrawl/tools/checkstyle/api/TokenTypes.html#LITERAL_NEW)
- [LITERAL_THROWS ](http://checkstyle.sourceforge.net/apidocs/com/puppycrawl/tools/checkstyle/api/TokenTypes.html#LITERAL_THROWS)
- [ANNOTATION_DEF ](http://checkstyle.sourceforge.net/apidocs/com/puppycrawl/tools/checkstyle/api/TokenTypes.html#ANNOTATION_DEF)
  2. If a class was imported with direct import (i.e. `import java.math.BigDecimal`), or the class was referenced with the package name (i.e. `java.math.BigDecimal value`) and the package was added to the `excludedPackages` parameter, the class does not increase complexity. 
  3. If a class name was added to the `excludedClasses` parameter, the class does not increase complexity.

Examples

To configure the check:

<module name="ClassFanOutComplexity"/>

To configure the check with a threshold of 10:

<module name="ClassFanOutComplexity">
    <property name="max" value="10"/>
</module>

Override property excludedPackages to mark some packages as excluded. Each member of excludedPackages should be a valid identifier:

  • java.util - valid, excludes all classes inside java.util, but not from the subpackages.
  • java.util. - invalid, should not end with a dot.
  • java.util.* - invalid, should not end with a star.

Note, that checkstyle will ignore all classes from the java.lang package and its subpackages, even if the java.lang was not listed in the excludedPackages parameter.

Also node, that excludedPackages will not exclude classes, imported via wildcard (e.g. import java.math.*). Instead of wildcard import you should use direct import (e.g. import java.math.BigDecimal).

Also note, that checkstyle will not exclude classes within the same file even if it was listed in the excludedPackages parameter. For example, assuming the config is

<module name="ClassDataAbstractionCoupling">
    <property name="excludedPackages" value="a.b"/>
</module>

And the file a.b.Foo.java is:

package a.b;
 

import a.b.Bar;
import a.b.c.Baz;
 

public class Foo {
    public Bar bar; // Will be ignored, located inside ignored a.b package
    public Baz baz; // Will not be ignored, located inside a.b.c package
    public Data data; // Will not be ignored, same file
 

    class Data {
        public Foo foo; // Will not be ignored, same file
    }
}

The bar member will not be counted, since the a.b added to the excludedPackages. The baz member will be counted, since the a.b.c was not added to the excludedPackages. The data and foo members will be counted, as they are inside same file.

Example of usage:

<module name="ClassFanOutComplexity">
    <property name="excludedPackages" value="java.util, java.math"/>
</module>

Further Reading