== Why is this an issue?

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.
----


=== Compliant solution

[source,java]
----
long decimal_int_value     = 15_543_124L;
double decimal_float_value = 7_918_714.3456d;
long hexadecimal_value     = 0x83_A324_86E2L;
long octal_value           = 04_4203_4331_3726L;
long binary_value          = 0b0101_0110_1110_1010L;
----


include::../exceptions.adoc[]


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== Implementation Specification
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include::../message.adoc[]

include::../highlighting.adoc[]

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== Comments And Links
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include::../comments-and-links.adoc[]
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