Since Checkstyle 3.4

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.

Structure | Complexity 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 label | 1 |

break | 1 |

Expressions | Number of && and || operators in expression. No operators - 0 |

continue | 1 |

return | 1 |

Statement (even sequential statements) | 1 |

Empty block {} | 1 |

Function call | 1 |

Function(Method) declaration or Block | P(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.

name | description | type | default value | since |
---|---|---|---|---|

max | Specify the maximum threshold allowed. | int | `200` |
3.4 |

To configure the check:

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

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="Checker"> <module name="TreeWalker"> <module name="NPathComplexity"> <property name="max" value="100"/> </module> </module> </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); }

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