doubts about underscore usage in numeric values

[Mauri Mura]

I'm starting to learn java and I can't understand why a numeric variable can have a single underscore, but when they are two, java it does not compile anymore.
For example, this is correct:

int i1 = 1_234;
double d3 = 1_234.0;

but this is not:

double d1 = 1_234_.0;
double d2 = 1_234._0;

[Scott Shipp]

It's because the underscore is a substitute character for a comma. It's easier to read long numbers when they're broken up. 123456789 is less readable to humans than 123,456,789. I think it was in Java 7, they allowed underscores in numbers to make larger numbers like this more readable. "1_234_" is therefore the same as writing "1,234," which is not a valid value. The compiler is trying to be helpful and indicate this. Just think of it this way: if you replace the underscores with commas, is it a valid number that makes sense? Otherwise you'll get the illegal underscore message from the compiler.

So minor nitpick: you can have more than one underscore, if your number still makes sense: e.g. 1_234_567 would be valid.

[Scott Shipp]

I found the Oracle/Java technote on this: https://docs.oracle.com/javase/7/docs/technotes/guides/language/underscores-literals.html - hope that helps

[Liutauras Vilda]

No.

Answer is, because placing an underscore has some rules.
1. It can't be before very first digit
2. It can't be after the last digit
3. It can't be prior or next to radix point

[Campbell Ritchie]

Scott Shipp wrote:. . . I think it was in Java 7 . . .

Yes, it was. The link you found explains it all and there is a section in the Java™ Tutorials which appears to say exactly the same
Not being in USA, I don't have a social security number, but it appears that the format of those numbers may be threeDigits‑twoDigits‑fourDigits; If you have a “number” whose format is specified like that, I think it ceases to be a number and becomes a String containing digits and no letters. The same applies to house numbers. If it's a number, you should be able to do arithmetic with it and vice versa. You never do arithmetic with house numbers.

[Emrik SjöKa]

Hi Mauricio,
You have already gotten answers for your question but i would like to add a bit more info.
If you'll be use Binary, Octal or Hexadecimal numbers you should be aware also some points.

You might know binary system use 0 and 1(base 2). Same rules are valid as Liutauras Vilda's mentioned.
You can't use underscore beginig or ending ex;
int i = 0b_111; // won't compile
int i1 = 0b1_11 // compilse

Octal number uses between 0 to 7 (base 8)
int i2 = 0_123; // won't compiles
int i3 = _0123;  // compiles

Hexidecimal num uses 0 to 9 and letter A or a to F or f ( A is 11 B is 12 ... 15 is F)
int i4 = 0x_9A; // won't compile
int i5 = 0x9_A; // compiles

By the way do not use underscore for object arguments
ex; Integer i = new Integer("123_456"); // NumberFormatException

[Mauri Mura]

Thanks a lot. Each one of all answers has been very useful.

[Emrik SjöKa]

Emrik SjöKa wrote:Hi Mauricio,
You have already gotten answers for your question but i would like to add a bit more info.
If you'll be use Binary, Octal or Hexadecimal numbers you should be aware also some points.

You might know binary system use 0 and 1(base 2). Same rules are valid as Liutauras Vilda's mentioned.
You can't use underscore beginig or ending ex;
int i = 0b_111; // won't compile
int i1 = 0b1_11 // compile

Octal number uses between 0 to 7 (base 8)
[strike]int i2 = 0_123; //  compiles
int i3 = _0123;  // won't  compile

Hexidecimal num uses 0 to 9 and letter A or a to F or f ( A is 11 B is 12 ... 15 is F)
int i4 = 0x_9A; // won't compile
int i5 = 0x9_A; // compiles

By the way do not use underscore for object arguments
ex; Integer i = new Integer("123_456"); // NumberFormatException

[Campbell Ritchie]

Emrik SjöKa wrote:. . .
int i5 = 0x9_A; // compiles

. . . but isn't a number literal. Have a look at the two links posted yesterday.

. . .  do not use underscore for object arguments
ex; Integer i = new Integer("123_456"); // NumberFormatException

That is because nobody writes numbers 123_456 in real life. Only in programming.

If I remember correctly, that format was permitted in the Eiffel language 30 years ago.