Package com.puppycrawl.tools.checkstyle.checks.metrics

Class NPathComplexityCheck

java.lang.Object
com.puppycrawl.tools.checkstyle.AbstractAutomaticBean
com.puppycrawl.tools.checkstyle.api.AbstractViolationReporter
com.puppycrawl.tools.checkstyle.api.AbstractCheck
com.puppycrawl.tools.checkstyle.checks.metrics.NPathComplexityCheck
All Implemented Interfaces:
Configurable, Contextualizable
public final class NPathComplexityCheck extends AbstractCheck
Checks the NPATH complexity against a specified limit.

The NPATH metric computes the number of possible execution paths through a function(method). It takes into account the nesting of conditional statements and multipart boolean expressions (A && B, C || D, E ? F :G and their combinations).

The NPATH metric was designed base on Cyclomatic complexity to avoid problem of Cyclomatic complexity metric like nesting level within a function(method).

Metric was described at "NPATH: a measure of execution pathcomplexity and its applications". If you need detailed description of algorithm, please read that article, it is well written and have number of examples and details.

Here is some quotes:

An NPATH threshold value of 200 has been established for a function. The value 200 is based on studies done at AT&T Bell Laboratories [1988 year].
Some of the most effective methods of reducing the NPATH value include:
  • distributing functionality;
  • implementing multiple if statements as a switch statement;
  • creating a separate function for logical expressions with a high count of variables and (&&) and or (||) operators.
Although strategies to reduce the NPATH complexity of functions are important, care must be taken not to distort the logical clarity of the software by applying a strategy to reduce the complexity of functions. That is, there is a point of diminishing return beyond which a further attempt at reduction of complexity distorts the logical clarity of the system structure.
Examples
StructureComplexity expression
if ([expr]) { [if-range] }NP(if-range) + 1 + NP(expr)
if ([expr]) { [if-range] } else { [else-range] } NP(if-range)+ NP(else-range) + NP(expr)
while ([expr]) { [while-range] }NP(while-range) + NP(expr) + 1
do { [do-range] } while ([expr])NP(do-range) + NP(expr) + 1
for([expr1]; [expr2]; [expr3]) { [for-range] } NP(for-range) + NP(expr1)+ NP(expr2) + NP(expr3) + 1
switch ([expr]) { case : [case-range] default: [default-range] } S(i=1:i=n)NP(case-range[i]) + NP(default-range) + NP(expr)
when[expr]NP(expr) + 1
[expr1] ? [expr2] : [expr3]NP(expr1) + NP(expr2) + NP(expr3) + 2
goto label1
break1
Expressions Number of && and || operators in expression. No operators - 0
continue1
return1
Statement (even sequential statements)1
Empty block {}1
Function call1
Function(Method) declaration or BlockP(i=1:i=N)NP(Statement[i])

Rationale: Nejmeh says that his group had an informal NPATH limit of 200 on individual routines; functions(methods) that exceeded this value were candidates for further decomposition - or at least a closer look. Please do not be fanatic with limit 200 - choose number that suites your project style. Limit 200 is empirical number base on some sources of at AT&T Bell Laboratories of 1988 year.

  • Property max - Specify the maximum threshold allowed. Type is int. Default value is 200.

Parent is com.puppycrawl.tools.checkstyle.TreeWalker

Violation Message Keys:

  • npathComplexity
Since:
3.4

  • Field Details

  • Constructor Details

  • Method Details

    • setMax

      public void setMax(int max)
      Setter to specify the maximum threshold allowed.
      Parameters:
      max - the maximum threshold
      Since:
      3.4

    • getDefaultTokens

      public int[] getDefaultTokens()
      Description copied from class: AbstractCheck
      Returns the default token a check is interested in. Only used if the configuration for a check does not define the tokens.
      Specified by:
      getDefaultTokens in class AbstractCheck
      Returns:
      the default tokens
      See Also:

    • getAcceptableTokens

      public int[] getAcceptableTokens()
      Description copied from class: AbstractCheck
      The configurable token set. Used to protect Checks against malicious users who specify an unacceptable token set in the configuration file. The default implementation returns the check's default tokens.
      Specified by:
      getAcceptableTokens in class AbstractCheck
      Returns:
      the token set this check is designed for.
      See Also:

    • getRequiredTokens

      public int[] getRequiredTokens()
      Description copied from class: AbstractCheck
      The tokens that this check must be registered for.
      Specified by:
      getRequiredTokens in class AbstractCheck
      Returns:
      the token set this must be registered for.
      See Also:

    • beginTree

      public void beginTree(DetailAST rootAST)
      Description copied from class: AbstractCheck
      Called before the starting to process a tree. Ideal place to initialize information that is to be collected whilst processing a tree.
      Overrides:
      beginTree in class AbstractCheck
      Parameters:
      rootAST - the root of the tree

    • visitToken

      public void visitToken(DetailAST ast)
      Description copied from class: AbstractCheck
      Called to process a token.
      Overrides:
      visitToken in class AbstractCheck
      Parameters:
      ast - the token to process

    • leaveToken

      public void leaveToken(DetailAST ast)
      Description copied from class: AbstractCheck
      Called after all the child nodes have been process.
      Overrides:
      leaveToken in class AbstractCheck
      Parameters:
      ast - the token leaving

    • visitConditional

      private void visitConditional(DetailAST ast, int basicBranchingFactor)
      Visits if, while, do-while, for and switch tokens - all of them have expression in parentheses which is used for calculation.
      Parameters:
      ast - visited token.
      basicBranchingFactor - default number of branches added.

    • visitWhenExpression

      private void visitWhenExpression(DetailAST ast, int basicBranchingFactor)
      Visits when expression token. There is no guarantee that when expression will be bracketed, so we don't use visitConditional method.
      Parameters:
      ast - visited token.
      basicBranchingFactor - default number of branches added.

    • visitUnitaryOperator

      private void visitUnitaryOperator(DetailAST ast, int basicBranchingFactor)
      Visits ternary operator (?:) and return tokens. They differ from those processed by visitConditional method in that their expression isn't bracketed.
      Parameters:
      ast - visited token.
      basicBranchingFactor - number of branches inherently added by this token.

    • leaveUnitaryOperator

      private void leaveUnitaryOperator()
      Leaves ternary operator (?:) and return tokens.

    • leaveConditional

      private void leaveConditional()
      Leaves while, do, for, if, ternary (?::), return or switch.

    • leaveBranch

      private void leaveBranch()
      Leaves else, default or case group tokens.

    • leaveMethodDef

      private void leaveMethodDef(DetailAST ast)
      Process the end of a method definition.
      Parameters:
      ast - the token type representing the method definition

    • leaveAddingConditional

      private void leaveAddingConditional()
      Leaves catch.

    • pushValue

      private void pushValue(Integer expressionValue)
      Pushes the current range value on the range value stack. Pushes this token expression value on the expression value stack.
      Parameters:
      expressionValue - value of expression calculated for current token.

    • popValue

      private NPathComplexityCheck.Values popValue()
      Pops values from both stack of expression values and stack of range values.
      Returns:
      pair of head values from both of the stacks.

    • leaveMultiplyingConditional

      private void leaveMultiplyingConditional()
      Leaves try.

    • countConditionalOperators

      private static int countConditionalOperators(DetailAST ast)
      Calculates number of conditional operators, including inline ternary operator, for a token.
      Parameters:
      ast - inspected token.
      Returns:
      number of conditional operators.
      See Also:

    • getLastToken

      private static DetailAST getLastToken(DetailAST ast)
      Finds a leaf, which is the most distant from the root.
      Parameters:
      ast - the root of tree.
      Returns:
      the leaf.

    • countCaseTokens

      private static int countCaseTokens(DetailAST ast)
      Counts number of case tokens subject to a case group token.
      Parameters:
      ast - case group token.
      Returns:
      number of case tokens.

    • countCaseConstants

      private static int countCaseConstants(DetailAST ast)
      Counts number of case constants tokens in a switch labeled rule.
      Parameters:
      ast - switch rule token.
      Returns:
      number of case constant tokens.