ClassDataAbstractionCoupling

Since Checkstyle 3.4

Description

Measures the number of instantiations of other classes within the given class or record. This type of coupling is not caused by inheritance or the object-oriented paradigm. Generally speaking, any data type with other data types as members or local variable that is an instantiation (object) of another class has data abstraction coupling (DAC). The higher the DAC, the more complex the structure of the class.

This check processes files in the following way:

  1. Iterates over the list of tokens (defined below) and counts all mentioned classes.
  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
max Specify the maximum threshold allowed. int 7 3.4
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
excludeClassesRegexps Specify user-configured regular expressions to ignore classes. Pattern[] ^$ 7.7
excludedPackages Specify user-configured packages to ignore. String[] {} 7.7

Examples

To configure the check:

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

Example:

The check passes without violations in the following:

class InputClassCoupling {
  Set set = new HashSet(); // HashSet ignored due to default excludedClasses property
  Map map = new HashMap(); // 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
}
        

The check results in a violation in the following:

class InputClassCoupling {
  Set set = new HashSet(); // HashSet ignored due to default excludedClasses property
  Map map = new HashMap(); // HashMap ignored due to default excludedClasses property
  Date date = new Date(); // Counted, 1
  Time time = new Time(); // Counted, 2
  // instantiation of 5 other user defined classes
  Place place = new Place(); // violation, total is 8
}
        

To configure the check with a threshold of 2:

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

Example:

The check passes without violations in the following:

class InputClassCoupling {
  Set set = new HashSet(); // HashSet ignored due to default excludedClasses property
  Map map = new HashMap(); // HashMap ignored due to default excludedClasses property
  Date date = new Date(); // Counted, 1
  Time time = new Time(); // Counted, 2
}
        

The check results in a violation in the following:

class InputClassCoupling {
  Set set = new HashSet(); // HashSet ignored due to default excludedClasses property
  Map map = new HashMap(); // 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
}
        

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

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

Example:

The check passes without violations in the following:

class InputClassCoupling {
  Set set = new HashSet(); // Ignored
  Map map = new HashMap(); // Ignored
  Date date = new Date(); // Counted, 1
  Time time = new Time(); // Counted, 2
  // instantiation of 5 other user defined classes
  Place place = new Place(); // Ignored
}
        

The check results in a violation in the following:

class InputClassCoupling {
  Set set = new HashSet(); // Ignored
  Map map = new HashMap(); // Ignored
  Date date = new Date(); // Counted, 1
  Time time = new Time(); // Counted, 2
  // instantiation of 5 other user defined classes
  Space space = new Space(); // violation, total is 8
}
        

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

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

Example:

The check passes without violations in the following:

class InputClassCoupling {
  Set set = new HashSet(); // HashSet ignored due to default excludedClasses property
  Map map = new HashMap(); // HashMap ignored due to default excludedClasses property
  Date date = new Date(); // Counted, 1
  Time time = new Time(); // Counted, 2
  // instantiation of 5 other user defined classes
  BufferedReader br = new BufferedReader(); // Ignored
}
        

The check results in a violation in the following:

class InputClassCoupling {
  Set set = new HashSet(); // HashSet ignored due to default excludedClasses property
  Map map = new HashMap(); // HashMap ignored due to default excludedClasses property
  Date date = new Date(); // Counted, 1
  Time time = new Time(); // Counted, 2
  // instantiation of 5 other user defined classes
  File file = new File(); // violation, total is 8
}
        

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

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

Example:

The check passes without violations in the following:

import java.io.BufferedReader;

class InputClassCoupling {
  Set set = new HashSet(); // HashSet ignored due to default excludedClasses property
  Map map = new HashMap(); // HashMap ignored due to default excludedClasses property
  Date date = new Date(); // Counted, 1
  Time time = new Time(); // Counted, 2
  // instantiation of 5 other user defined classes
  BufferedReader br = new BufferedReader(); // Ignored
}
        

The check results in a violation in the following:

import java.util.StringTokenizer;

class InputClassCoupling {
  Set set = new HashSet(); // HashSet ignored due to default excludedClasses property
  Map map = new HashMap(); // HashMap ignored due to default excludedClasses property
  Date date = new Date(); // Counted, 1
  Time time = new Time(); // Counted, 2
  // instantiation of 5 other user defined classes
  StringTokenizer st = new StringTokenizer(); // violation, total is 8
}
        

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="ClassDataAbstractionCoupling">
      <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