Java 7 underscore in numeric literals

Question

When we must use a _ to separate digits in a number I don't understand the following case in which I can't use it:

In positions where a string of digits is expected

(as documented in the JDK7 guide here)

Some examples?

Answer 1

You don't have to use "_", you can. And examples given in the proposal are credit card numbers, phone numbers, or simply numbers for which it makes sense to have a separator in the code.

For the "In positions where a string of digits is expected" it's simply in places where it's supposed to start (or end) with a digit. Here are some examples.

Note that according to this proposal, underscores can only be placed between digits. They cannot be placed by themselves in positions where a string of digits would normally be expected:

int x1 = _52; // This is an identifier, not a numeric literal.

int x2 = 5_2; // OK. (Decimal literal)

int x2 = 52_; // Illegal. (Underscores must always be between digits)

int x3 = 5_______2; // OK. (Decimal literal.)

int x4 = 0_x52; // Illegal. Can't put underscores in the "0x" radix prefix.

int x5 = 0x_52; // Illegal. (Underscores must always be between digits)

int x6 = 0x5_2; // OK. (Hexadecimal literal)

int x6 = 0x52_; // Illegal. (Underscores must always be between digits)

int x6 = 0x_; // Illegal. (Not valid with the underscore removed)

int x7 = 0_52; // OK. (Octal literal)

int x7 = 05_2; // OK. (Octal literal)

int x8 = 052_; // Illegal. (Underscores must always be between digits)

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Resources:

• OpenJDK - Project Coin - PROPOSAL: Underscores in Numbers (Version 2)
• Joe Darcy's blog - Project Coin: Project Coin: Literal Grammar Hackery