Beginning with Java 7, it is possible to add underscores ('_') to numeric literals to enhance readability. The addition of underscores in this manner has no semantic meaning, but makes it easier for maintainers to understand the code.

The number of digits to the left of a decimal point needed to trigger this rule varies by base.

Base Minimum digits Ideal spacing
binary 9 every 8
decimal 5 every 3
hexadecimal 9 every 4

It is only the presence of underscores, not their spacing that is scrutinized by this rule.

Note that this rule is automatically disabled when the project's sonar.java.source is lower than 7.

Noncompliant Code Example

int i = 10000000;  // Noncompliant; is this 10 million or 100 million?
int  j = 0b01101001010011011110010101011110;  // Noncompliant
long l = 0x7fffffffffffffffL;  // Noncompliant

Compliant Solution

int i = 10_000_000;
int  j = 0b01101001_01001101_11100101_01011110;
long l = 0x7fff_ffff_ffff_ffffL;