What does "\1" represent in this Java string?

Question

System.out.println("\1");

I thought it did not compile because of the non-recognized escape sequence.

What does "\1" exactly represent?

Arraign

Answer 1

It's an octal escape sequence, as listed in section 3.10.6 of the JLS. So for example:

String x = "\16";

is equivalent to:

String x = "\u000E";

(As Octal 16 = Hex E.)

So \1 us U+0001, the "start of heading" character.

Octal escape sequences are very rarely used in Java in my experience, and I'd personally avoid them where possible. When I want to specify a character using a numeric escape sequence, I always use \uxxxx.

Mazer

Java is one of those languages that make you swear profusely when you hit the "incidental" bugs it has. Did you know that Java interprets character escapes to be a literal character in the source code? Well this isn't a problem until you try and escape a carriage return... then Java thinks you stuck a newline in your source code when you really didn't want one. – Diley

@PP.: It depends on which character escape you mean. For example, a comment of // This \n is okay is fine, but // This \u000a is not will not compile. – Mazer

Your second code line doesn't compile (on my machine at least): "string literal is not properly closed by a double-quote". – Magnus

Also, since Java does not have raw strings, using regular expressions with back references requires the use of \\1, which can be quite irritating. – Dominations

I disagree with the value of using \u. This is because Java will interpret \u as starting a a hex escape sequence anywhere in your Java code, including in comments and identifiers. The compiler replaces the character before compiling your code, potentially leading to strange bugs. For example: github.com/antlr/antlr4/issues/164 – Antiphon

@Kevin: Yes, but I don't use non-ASCII identifiers, and in a comment I'd use U+000D or something similar instead. Neither of those cases is one where "I want to specify a character using a numeric escape sequence". – Mazer

That makes sense. I guess I haven't run into situations where I need to specify a non-ASCII character in Java. What I'm more afraid of is accidentally typing \uxxxx in a string like a file path (like the bug I posted), not deliberately using it to specify a special character. – Antiphon

@Antiphon Just another reason not to use backslashes in path names. BTW, I guess the octal notation like also the octal int literals are there to make C programmers feel comfortable. Interestingly, octal literals are frowned upon, while hexadecimal ones are fine. – Antipersonnel

Answer 2

In java It is following value

\u0001

Grease