CyclomaticComplexity

Since Checkstyle 3.2

Description

Checks cyclomatic complexity against a specified limit. It is a measure of the minimum number of possible paths through the source and therefore the number of required tests, it is not about quality of code! It is only applied to methods, c-tors, static initializers and instance initializers.

The complexity is equal to the number of decision points + 1. Decision points:

  • if, while, do, for, ?:, catch, switch, case statements.
  • Operators && and || in the body of target.
  • when expression in case labels, also known as guards.

By pure theory level 1-4 is considered easy to test, 5-7 OK, 8-10 consider re-factoring to ease testing, and 11+ re-factor now as testing will be painful.

When it comes to code quality measurement by this metric level 10 is very good level as a ultimate target (that is hard to archive). Do not be ashamed to have complexity level 15 or even higher, but keep it below 20 to catch really bad-designed code automatically.

Please use Suppression to avoid violations on cases that could not be split in few methods without damaging readability of code or encapsulation.

Properties

name description type default value since
max Specify the maximum threshold allowed. int 10 3.2
switchBlockAsSingleDecisionPoint Control whether to treat the whole switch block as a single decision point. boolean false 6.11
tokens tokens to check subset of tokens LITERAL_WHILE , LITERAL_DO , LITERAL_FOR , LITERAL_IF , LITERAL_SWITCH , LITERAL_CASE , LITERAL_CATCH , QUESTION , LAND , LOR , LITERAL_WHEN . LITERAL_WHILE , LITERAL_DO , LITERAL_FOR , LITERAL_IF , LITERAL_SWITCH , LITERAL_CASE , LITERAL_CATCH , QUESTION , LAND , LOR , LITERAL_WHEN . 3.2

Examples

To configure the check: 

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

Example: 

class Example1 {
  int a, b, c, d, e, n;

  public void testMethod1() {
    while (a < b && a > c) {
      fun1();
    }
    if (a == b) {
      do {
        fun1();
      } while (d==a);
    } else if (c == d) {
      while (c > 0) {
        fun1();
      }
      do {
        fun1();
      } while (a==d);
    }
  }
  // violation below, 'Cyclomatic Complexity is 13 (max allowed is 10)'
  public void testMethod2() { // 1, function declaration
    if (a == b) { // 2, if
      fun1();
    } else if (a == 0 // 3, if
      && b == c) { // 4, && operator
      if (c == -1) { // 5, if
        fun1();
      }
    } else if (a == c // 6, if
      || a == d) { // 7, || operator
      fun1();
    } else if (d == e) { //8, if
      try {
        fun1();
      } catch (Exception e) { // 9, catch
      }
    } else {
      switch(n) {
        case 1: // 10, case
          fun1();
          break;
        case 2: // 11, case
          fun1();
          break;
        case 3: // 12, case
          fun1();
          break;
        default:
          break;
      }
    }
    a = a > 0 ? b : c; // 13, ternary operator
  }
  private void fun1() {}
}

To configure the check with a threshold of 4 and check only for while and do-while loops: 

<module name="Checker">
  <module name="TreeWalker">
    <module name="CyclomaticComplexity">
      <property name="max" value="4"/>
      <property name="tokens" value="LITERAL_WHILE, LITERAL_DO"/>
    </module>
  </module>
</module>

Example: 

class Example2 {
  int a, b, c, d, e, n;
  // violation below, 'Cyclomatic Complexity is 5 (max allowed is 4)'
  public void testMethod1() { // 1, function declaration
    while (a < b && a > c) { // 2, while
      fun1();
    }
    if (a == b) {
      do { // 3, do
        fun1();
      } while (d==a);
    } else if (c == d) {
      while (c > 0) { // 4, while
        fun1();
      }
      do { // 5, do-while
        fun1();
      } while (a==d);
    }
  }

  public void testMethod2() {
    if (a == b) {
      fun1();
    } else if (a == 0
      && b == c) {
      if (c == -1) {
        fun1();
      }
    } else if (a == c
      || a == d) {
      fun1();
    } else if (d == e) {
      try {
        fun1();
      } catch (Exception e) {
      }
    } else {
      switch(n) {
        case 1:
          fun1();
          break;
        case 2:
          fun1();
          break;
        case 3:
          fun1();
          break;
        default:
          break;
      }
    }
    a = a > 0 ? b : c;
  }
  private void fun1() {}
}

To configure the check to consider switch-case block as one decision point. 

<module name="Checker">
  <module name="TreeWalker">
    <module name="CyclomaticComplexity">
      <property name="switchBlockAsSingleDecisionPoint" value="true"/>
    </module>
  </module>
</module>

Example: 

class Example3 {
  int a, b, c, d, e, n;

  public void testMethod1() {
    while (a < b && a > c) {
      fun1();
    }
    if (a == b) {
      do {
        fun1();
      } while (d==a);
    } else if (c == d) {
      while (c > 0) {
        fun1();
      }
      do {
        fun1();
      } while (a==d);
    }
  }
  // violation below, 'Cyclomatic Complexity is 11 (max allowed is 10)'
  public void testMethod2() { // 1, function declaration
    if (a == b) { // 2, if
      fun1();
    } else if (a == 0 // 3, if
      && b == c) { // 4, && operator
      if (c == -1) { // 5, if
        fun1();
      }
    } else if (a == c // 6, if
      || a == d) { // 7, || operator
      fun1();
    } else if (d == e) { // 8, if
      try {
        fun1();
      } catch (Exception e) { // 9, catch
      }
    } else {
      switch(n) { // 10, switch
        case 1:
          fun1();
          break;
        case 2:
          fun1();
          break;
        case 3: // 10, case
          fun1();
          break;
        default:
          break;
      }
    }
    a = a > 0 ? b : c; // 11, ternary operator
  }
  private void fun1() {}
}

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