Puzzle 14: Escape Rout

The following program uses two Unicode escapes, which represent Unicode characters by their hexadecimal numeric codes. What does the program print?

public class EscapeRout {

  public static void main(String[] args) {

    // \u0022 is the Unicode escape for double quote (")

    System.out.println("a\u0022.length() + \u0022b".length());

  }

}

Solution 14: Escape Rout

A naive analysis of the program suggests that it should print 26 because there are 26 characters between the quotation marks that bound the string "a\u0022.length() + \u0022b". A deeper analysis suggests that the program should print 16, as each of the two Unicode escapes requires six characters in the source file but represents only one character in the string. The string is therefore ten characters shorter than it appears. Running the program tells a different story. It prints neither 26 nor 16 but 2.

The key to understanding this puzzle is that Java provides no special treatment for Unicode escapes within string literals. The compiler translates Unicode escapes into the characters they represent before it parses the program into tokens, such as strings literals [JLS 3.2]. Therefore, the first Unicode escape in the program closes a one-character string literal ("a"), and the second one opens a one-character string literal ("b"). The program prints the value of the expression "a".length() + "b".length(), or 2.

If the author of the program had actually wanted this behavior, it would have been much clearer to say:

System.out.println("a".length() + "b".length());

More likely, the author wanted to put the two double quote characters into the string literal. You can't do this with Unicode escapes, but you can do it with escape sequences [JLS 3.10.6]. The escape sequence representing a double quote is a backslash followed by a double quote (\"). If the Unicode escapes in the original program are replaced with this escape sequence, it will print 16 as expected:

System.out.println("a\".length() + \"b".length());

There are escape sequences for many characters, including the single quote (\'), linefeed (\n), tab (\t), and backslash (\\). You can use escape sequences in character literals as well as in string literals. In fact, you can put any ASCII character into a string literal or a character literal by using a special kind of escape sequence called an octal escape, but it is preferable to use normal escape sequences where possible. Both normal escape sequences and octal escapes are far preferable to Unicode escapes because unlike Unicode escapes, escape sequences are processed after the program is parsed into tokens.

All the programs in this book are written using the ASCII subset of Unicode. ASCII is the lowest common denominator of character sets. ASCII has only 128 characters, but Unicode has more than 65,000. A Unicode escape can be used to insert any Unicode character into a program using only ASCII characters. A Unicode escape means exactly the same thing as the character that it represents.

Unicode escapes are designed for use when a programmer needs to insert a character that can't be represented in the source file's character set. They are used primarily to put non-ASCII characters into identifiers, string literals, character literals, and comments. Occasionally, a Unicode escape adds to the clarity of a program by positively identifying one of several similar-looking characters.

In summary, prefer escape sequences to Unicode escapes in string and character literals. Unicode escapes can be confusing because they are processed so early in the compilation sequence. Do not use Unicode escapes to represent ASCII characters. Inside of string and character literals, use escape sequences; outside of these literals, insert ASCII characters directly into the source file.