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

Class 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 multi-part 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)
    [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.

    To configure the check: 

     <module name="NPathComplexity"/>

    Example: 

     public abstract class Test {

 final int a = 0;
 int b = 0;

 public void foo() { // OK, NPath complexity is less than default threshold
   // function consists of one if-else block with an NPath Complexity of 3
   if (a > 10) {
     if (a > b) { // nested if-else decision tree adds 2 to the complexity count
       buzz();
     } else {
       fizz();
     }
   } else { // last possible outcome of the main if-else block, adds 1 to complexity
     buzz();
   }
 }

 public void boo() { // violation, NPath complexity is 217 (max allowed is 200)
   // looping through 3 switch statements produces 6^3 + 1 (217) possible outcomes
   for(int i = 0; i < b; i++) { // for statement adds 1 to final complexity
     switch(i) { // each independent switch statement multiplies complexity by 6
       case a:
         // ternary with && adds 3 to switch's complexity
         print(f(i) && g(i) ? fizz() : buzz());
       default:
         // ternary with || adds 3 to switch's complexity
         print(f(i) || g(i) ? fizz() : buzz());
     }
     switch(i - 1) { // multiplies complexity by 6
       case a:
         print(f(i) && g(i) ? fizz() : buzz());
       default:
         print(f(i) || g(i) ? fizz() : buzz());
     }
     switch(i + 1) { // multiplies complexity by 6
       case a:
         print(f(i) && g(i) ? fizz() : buzz());
       default:
         print(f(i) || g(i) ? fizz() : buzz());
     }
   }
 }

 public abstract boolean f(int x);
 public abstract boolean g(int x);
 public abstract String fizz();
 public abstract String buzz();
 public abstract void print(String str);
 }

    To configure the check with a threshold of 100: 

     <module name="NPathComplexity">
   <property name="max" value="100"/>
 </module>

    Example: 

     public abstract class Test1 {
 public void foo() { // violation, NPath complexity is 128 (max allowed is 100)
   int a,b,t,m,n;
   a=b=t=m=n = 0;

   // Complexity is achieved by choosing from 2 options 7 times (2^7 = 128 possible outcomes)
   if (a > b) { // non-nested if-else decision tree multiplies complexity by 2
     bar();
   } else {
     baz();
   }

   print(t > 1 ? bar() : baz()); // 5 ternary statements multiply complexity by 2^5
   print(t > 2 ? bar() : baz());
   print(t > 3 ? bar() : baz());
   print(t > 4 ? bar() : baz());
   print(t > 5 ? bar() : baz());

   if (m > n) { // multiplies complexity by 2
     baz();
   } else {
     bar();
   }
 }

 public abstract String bar();
 public abstract String baz();
 public abstract void print(String str);
 }

    Parent is com.puppycrawl.tools.checkstyle.TreeWalker

    Violation Message Keys:

    • npathComplexity
    Since:
    3.4

    • Field Detail

      • MSG_KEY

        public static final java.lang.String MSG_KEY
        A key is pointing to the warning message text in "messages.properties" file.
        See Also:
        Constant Field Values

    • Constructor Detail

      • NPathComplexityCheck

        public NPathComplexityCheck()

    • Method Detail

      • setMax

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

      • 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:
        TokenTypes

      • 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:
        TokenTypes

      • 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:
        TokenTypes

      • 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

      • 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