Why can some ASCII characters not be expressed in the form '\uXXXX' in Java source code?

Question

I stumbled over this (again) today:

class Test {
    char ok = \'\\n\';
    char okAsWell = \'\\u000B\';
    char error = \'\\u000A\';
}

It

Answer 1

Unicode escape sequences like \u000a are replaced by the actual characters they represent before the Java compiler does anything else with the source code. And so, your program eventually ends up at

char ch = '
';

So the \u000a in your source code is replaced internally by a linefeed character. Note that this happens before the compiler actually reads and interprets your source code.

Referring to the Java Language Specification:

It is a compile-time error for a line terminator (§3.4) to appear after the opening ' and before the closing '.

And as well all know by heart, \n is a line terminator, quoting:

 LineTerminator:
    the ASCII LF character, also known as "newline"
    the ASCII CR character, also known as "return"
    the ASCII CR character followed by the ASCII LF character

Other symbols that could cause problems are \, ' and " for example.