Java - Underscores in Numeric Literals

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Why using Underscores in Numeric Literals?

Using underscores in big numbers makes them easier to read.

You can use any number of underscores between two digits in numeric literals.

For example, an int literal 99999 can be written as 9_9999, 999_99, 9999_9, 99___999.

The use of underscores is also allowed in octal, hexadecimal, and binary formats.

Illegal use of underscore

You cannot use underscores in the beginning or end of a numeric literal.

You cannot use underscores with prefixes such as 0x for hexadecimal format and 0b for binary format, and suffixes such as L for long literal and F for float literal.

You CAN use underscores after the first zero in an int literal in octal format. For example, write 02654 as 0_2654.

The following examples show the invalid uses of underscores in numeric literals:

LiteralError
_1969;An error. Underscore in the beginning
1969_; An error. Underscore in the end
0x_DEAD; An error. Underscore after prefix 0x
0_xDEAD;An error. Underscore inside prefix 0x
9999_L;An error. Underscore with suffix L
9999_.0919; An error. Underscore before decimal
9999._0919;An error. Underscore after decimal

Valid usage

The following examples show the valid uses of underscores in numeric literals: 

x1 = 99_999;            // Underscore in deciaml format 
x2 = 99__999;           // Multiple consecutive underscores 
x3 = 03_123;           // Underscore in octal literal 
x4 = 0b0111_1011_0001; // Underscore in binary literal 
x5 = 0x7_B_1_E;          // Underscores in hexadecimal literal 
b1 = 1_2_7;           // Underscores in decimal format 
d1 = 99_999.09_19;  // Underscores in double literal

Demo

public class Main {
  public static void main(String[] args) {
    int x1 = 99_999;            // Underscore in deciaml format 
    int x2 = 99__999;           // Multiple consecutive underscores 
    int x3 = 03_123;           // Underscore in octal literal 
    int x4 = 0b0111_1011_0001; // Underscore in binary literal 
    int x5 = 0x7_B_1_E;          // Underscores in hexadecimal literal 
    byte x6 = 1_2_7;           // Underscores in decimal format 
    double x7  = 99_999.09_19;  // Underscores in double literal 

    System.out.println(x1);// w  w  w  .j a va  2  s  . c o  m
    System.out.println(x2);
    System.out.println(x3);
    System.out.println(x4);
    System.out.println(x5);
    System.out.println(x6);
    System.out.println(x7);
  }
}

Result