Java Language Using underscore to improve readability


Since Java 7 it has been possible to use one or more underscores (_) for separating groups of digits in a primitive number literal to improve their readability.

For instance, these two declarations are equivalent:

Java SE 7
int i1 = 123456;
int i2 = 123_456;
System.out.println(i1 == i2); // true

This can be applied to all primitive number literals as shown below:

Java SE 7
byte color = 1_2_3;
short yearsAnnoDomini= 2_016;
int socialSecurtyNumber = 999_99_9999;
long creditCardNumber = 1234_5678_9012_3456L;
float piFourDecimals =  3.14_15F;
double piTenDecimals =  3.14_15_92_65_35;

This also works using prefixes for binary, octal and hexadecimal bases:

Java SE 7
short binary= 0b0_1_0_1;
int octal = 07_7_7_7_7_7_7_7_0;
long hexBytes = 0xFF_EC_DE_5E;

There are a few rules about underscores which forbid their placement in the following places:

  • At the beginning or end of a number (e.g. _123 or 123_ are not valid)
  • Adjacent to a decimal point in a floating point literal (e.g. 1._23 or 1_.23 are not valid)
  • Prior to an F or L suffix (e.g. 1.23_F or 9999999_L are not valid)
  • In positions where a string of digits is expected (e.g. 0_xFFFF is not valid)