ClassFanOutComplexity

Since Checkstyle 3.4

Description

Checks the number of other types a given class/record/interface/enum/annotation relies on. Also, the square of this has been shown to indicate the amount of maintenance required in functional programs (on a file basis) at least.

This check processes files in the following way:

  1. Iterates over all tokens that might contain type reference.
  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.

Properties

name description type default value since
excludeClassesRegexps Specify user-configured regular expressions to ignore classes. Pattern[] ^$ 7.7
excludedClasses Specify user-configured class names to ignore. String[] ArrayIndexOutOfBoundsException, ArrayList, Boolean, Byte, Character, Class, Collection, Deprecated, Deque, Double, DoubleStream, EnumSet, Exception, Float, FunctionalInterface, HashMap, HashSet, IllegalArgumentException, IllegalStateException, IndexOutOfBoundsException, IntStream, Integer, LinkedHashMap, LinkedHashSet, LinkedList, List, Long, LongStream, Map, NullPointerException, Object, Optional, OptionalDouble, OptionalInt, OptionalLong, Override, Queue, RuntimeException, SafeVarargs, SecurityException, Set, Short, SortedMap, SortedSet, Stream, String, StringBuffer, StringBuilder, SuppressWarnings, Throwable, TreeMap, TreeSet, UnsupportedOperationException, Void, boolean, byte, char, double, float, int, long, short, var, void 5.7
excludedPackages Specify user-configured packages to ignore. String[] {} 7.7
max Specify the maximum threshold allowed. int 20 3.4

Examples

To configure the check: 

<module name="Checker">
  <module name="TreeWalker">
    <module name="ClassFanOutComplexity"/>
  </module>
</module>

Example:

The check passes without violations in the following: 

class InputClassComplexity {
  Set set = new HashSet(); // Set, HashSet ignored due to default excludedClasses property
  Map map = new HashMap(); // Map, HashMap ignored due to default excludedClasses property
  Date date = new Date(); // Counted, 1
  Time time = new Time(); // Counted, 2
  Place place = new Place(); // Counted, 3
  int value = 10; // int is ignored due to default excludedClasses property
  void method() {
    var result = "result"; // var is ignored due to default excludedClasses property
  }
}

The check results in a violation in the following: 

class InputClassComplexity {
  Set set = new HashSet(); // Set, HashSet ignored due to default excludedClasses property
  Map map = new HashMap(); // Map, HashMap ignored due to default excludedClasses property
  Date date = new Date(); // Counted, 1
  Time time = new Time(); // Counted, 2
  // mention of 18 other user defined classes
  Place place = new Place(); // violation, total is 21
}

To configure the check with a threshold of 2: 

<module name="Checker">
  <module name="TreeWalker">
    <module name="ClassFanOutComplexity">
      <property name="max" value="2"/>
    </module>
  </module>
</module>

Example:

The check passes without violations in the following: 

// A is ignored because the superclass doesn't depend on the permitted subclasses
sealed class InputClassComplexity permits A {
  Set set = new HashSet(); // Set, HashSet ignored due to default excludedClasses property
  Map map = new HashMap(); // Map, HashMap ignored due to default excludedClasses property
  Date date = new Date(); // Counted, 1
  Time time = new Time(); // Counted, 2
}

final class A extends InputClassComplexity { } // InputClassComplexity is counted, 1

The check results in a violation in the following: 

// A is ignored because the superclass doesn't depend on the permitted subclasses
sealed class InputClassComplexity permits A {
  Set set = new HashSet(); // Set, HashSet ignored due to default excludedClasses property
  Map map = new HashMap(); // Map, HashMap ignored due to default excludedClasses property
  Date date = new Date(); // Counted, 1
  Time time = new Time(); // Counted, 2
  Place place = new Place(); // violation, total is 3
}

final class A extends InputClassComplexity { } // InputClassComplexity is counted, 1

To configure the check with three excluded classes HashMap, HashSet and Place: 

<module name="Checker">
  <module name="TreeWalker">
    <module name="ClassFanOutComplexity">
      <property name="excludedClasses" value="HashMap, HashSet, Place"/>
    </module>
  </module>
</module>

Example:

The check passes without violations in the following: 

class InputClassComplexity {
  Set set = new HashSet(); // Set counted 1, HashSet ignored
  Map map = new HashMap(); // Map counted 2, HashMap ignored
  Date date = new Date(); // Counted, 3
  Time time = new Time(); // Counted, 4
  // mention of 16 other user defined classes
  Place place = new Place(); // Ignored
}

The check results in a violation in the following: 

class InputClassComplexity {
  Set set = new HashSet(); // Set counted 1, HashSet ignored
  Map map = new HashMap(); // Map counted 2, HashMap ignored
  Date date = new Date(); // Counted, 3
  Time time = new Time(); // Counted, 4
  // mention of 16 other user defined classes
  Space space = new Space(); // violation, total is 21
}

To configure the check to exclude classes with a regular expression .*Reader$: 

<module name="Checker">
  <module name="TreeWalker">
    <module name="ClassFanOutComplexity">
      <property name="excludeClassesRegexps" value=".*Reader$"/>
    </module>
  </module>
</module>

Example:

The check passes without violations in the following: 

class InputClassComplexity {
  Set set = new HashSet(); // Set, HashSet ignored due to default excludedClasses property
  Map map = new HashMap(); // Map, HashMap ignored due to default excludedClasses property
  Date date = new Date(); // Counted, 1
  Time time = new Time(); // Counted, 2
  // mention of 18 other user defined classes
  BufferedReader br; // Ignored
}

The check results in a violation in the following: 

class InputClassComplexity {
  Set set = new HashSet(); // Set, HashSet ignored due to default excludedClasses property
  Map map = new HashMap(); // Map, HashMap ignored due to default excludedClasses property
  Date date = new Date(); // Counted, 1
  Time time = new Time(); // Counted, 2
  // mention of 18 other user defined classes
  File file; // violation, total is 21
}

To configure the check with an excluded package java.io: 

<module name="Checker">
  <module name="TreeWalker">
    <module name="ClassFanOutComplexity">
      <property name="excludedPackages" value="java.io"/>
    </module>
  </module>
</module>

Example:

The check passes without violations in the following: 

import java.io.BufferedReader;

class InputClassComplexity {
  Set set = new HashSet(); // Set, HashSet ignored due to default excludedClasses property
  Map map = new HashMap(); // Map, HashMap ignored due to default excludedClasses property
  Date date = new Date(); // Counted, 1
  Time time = new Time(); // Counted, 2
  // mention of 18 other user defined classes
  BufferedReader br; // Ignored
}

The check results in a violation in the following: 

import java.util.StringTokenizer;

class InputClassComplexity {
  Set set = new HashSet(); // Set, HashSet ignored due to default excludedClasses property
  Map map = new HashMap(); // Map, HashMap ignored due to default excludedClasses property
  Date date = new Date(); // Counted, 1
  Time time = new Time(); // Counted, 2
  // mention of 18 other user defined classes
  StringTokenizer st; // violation, total is 21
}

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 note, 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="Checker">
  <module name="TreeWalker">
    <module name="ClassFanOutComplexity">
      <property name="excludedPackages" value="a.b"/>
    </module>
  </module>
</module>

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

package a.b;

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

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

  class Data {
    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

Violation Messages

All messages can be customized if the default message doesn't suit you. Please see the documentation to learn how to.

Package

com.puppycrawl.tools.checkstyle.checks.metrics

Parent Module

TreeWalker