You must reffer the JLS It clearly says that after \ you can have a maximum of 3 digits (all octal) the first digit can be only 0,1,2,3. So the maximum number possible is 377. Anything more than that is not possible. 377 is equivalent to 255 in decimal. only \u0000 to \u00FF is possible as octal.
The character and string escape sequences allow for the representation of some nongraphic characters as well as the single quote, double quote, and backslash characters in character literals (�3.10.4) and string literals (�3.10.5).
It is a compile-time error if the character following a backslash in an escape is not an ASCII b, t, n, f, r, ", ', \, 0, 1, 2, 3, 4, 5, 6, or 7. The Unicode escape \u is processed earlier (�3.3). (Octal escapes are provided for compatibility with C, but can express only Unicode values \u0000 through \u00FF, so Unicode escapes are usually preferred.)
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