Java 7 introduced the ability to use a digit separator (``++_++``) to split a literal number into groups of digits for better readability.
To ensure that readability is really improved by using digit separators, this rule verifies:
* *Homogeneity*
** Except for the left-most group, which can be smaller, all groups in a number should contain the same number of digits. Mixing group sizes is at best confusing for maintainers, and at worst a typographical error that is potentially a bug.
* *Standardization*
** It is also confusing to regroup digits using a size that is not standard. This rule enforce the following standards:
*** Decimal numbers should be separated using groups of 3 digits.
*** Hexadecimal numbers should be separated using groups of 2 or 4 digits.
*** Octal and Binary should be separated using groups of 2, 3 or 4 digits.
Furthermore, using groups with more than 4 consecutive digits is not allowed because they are difficult for maintainers to read.
=== Noncompliant code example
[source,java]
----
long decimal_int_value = 1_554_3124L; // Noncompliant; mixing groups of 3 and 4 digits
double decimal_float_value = 7_91_87_14.3456d; // Noncompliant; using groups of 2 instead of 3 digits
long hexadecimal_value = 0x8_3A3_248_6E2L; // Noncompliant; using groups of 3 instead of 2 or 4 digits
long octal_value = 0442_03433_13726L; // Noncompliant; using groups of 5 instead of 2, 3 or 4 digits.
long binary_value = 0b01010110_11101010L; // Noncompliant; using groups of 8 instead of 2, 3 or 4 digits.
