Complexity is determined by the number of decision points in a method plus one for the method entry. The decision points are 'if', 'while', 'for', and 'case labels'. Generally, 1-4 is low complexity, 5-7 indicates moderate complexity, 8-10 is high complexity, and 11+ is very high complexity. This rule triggers if a method has a complexity of 10 or more.

The higher the cyclomatic complexity, the harder it is to write a set of unit tests that cover all branches. Cyclomatic complexity is an indication how hard it is to test your code.

// Cyclomatic Complexity = 12
public class Foo {
1   public void example()  {
2       if (a == b)  {
3           if (a1 == b1) {
                fiddle();
4           } else if a2 == b2) {
                fiddle();
            }  else {
                fiddle();
            }
5       } else if (c == d) {
6           while (c == d) {
                fiddle();
            }
7        } else if (e == f) {
8           for (int n = 0; n < h; n++) {
                fiddle();
            }
        } else{
            switch (z) {
9               case 1:
                    fiddle();
                    break;
10              case 2:
                    fiddle();
                    break;
11              case 3:
                    fiddle();
                    break;
12              default:
                    fiddle();
                    break;
            }
        }
    }
}