Why do Java octal escapes only go up to 255?

Question

The Java language specification states that the escapes inside strings are the "normal" C ones like \n and \t, but they also specify octal escapes from \0 to \377. Specifically, the JLS states:

OctalEscape:
    \ OctalDigit
    \ OctalDigit OctalDigit
    \ ZeroToThree OctalDigit OctalDigit

OctalDigit: one of
    0 1 2 3 4 5 6 7

ZeroToThree: one of
    0 1 2 3

meaning that something like \4715 is illegal, despite it being within the range of a Java character (since Java characters are not bytes).

Why does Java have this arbitrary restriction? How are you meant to specify octal codes for characters beyond 255?

Answer 1

It is probably for purely historical reasons that Java supports octal escape sequences at all. These escape sequences originated in C (or maybe in C's predecessors B and BCPL), in the days when computers like the PDP-7 ruled the Earth, and much programming was done in assembly or directly in machine code, and octal was the preferred number base for writing instruction codes, and there was no Unicode, just ASCII, so three octal digits were sufficient to represent the entire character set.

By the time Unicode and Java came along, octal had pretty much given way to hexadecimal as the preferred number base when decimal just wouldn't do. So Java has its \u escape sequence that takes hexadecimal digits. The octal escape sequence was probably supported just to make C programmers comfortable, and to make it easy to copy'n'paste string constants from C programs into Java programs.

Check out these links for historical trivia:

http://en.wikipedia.org/wiki/Octal#In_computers
http://en.wikipedia.org/wiki/PDP-11_architecture#Memory_management