Java IO.pdf - Nguyen Dang Binh

Java I/O
Text editors that work with non-ASCII character sets like MacRoman, Arabic, or Big-5
Chinese can integrate with existing Java compilers by providing a preprocessing phase where
the natively encoded data is translated to Unicode-escaped ASCII before being passed to
Sun's javac compiler. Alternately, they can hand off the translation work to javac (1.1 and
later) by using its -encoding flag. For example, to specify that the file MyClass.java is
written in the ISO 8859-9 character set (essentially Latin-1 with the Turkish characters , , ,
, , and replacing the Icelandic characters þ, , ý, Ý, ð, and Ð) you would type:
% javac -encoding 8859_9 MyClass.java
Table B.4 lists the encodings that Java 1.1 understands.
14.4 UTF-8
Since every Unicode character is encoded in exactly two bytes, Unicode is a fairly simple
encoding. The first two bytes of a file are the first character. The next two bytes are the
second character, and so on. This makes parsing Unicode data relatively simple compared to
schemes that use variable-width characters. The downside is that Unicode is far from the most
efficient encoding possible. In a file containing mostly English text, the high bytes of almost
all the characters will be 0. These bytes can occupy as much as half of the file. If you're
sending data across the network, Unicode data can take twice as long.
A more efficient encoding can be achieved for files that are composed primarily of ASCII text
by encoding the more common characters in fewer bytes. UTF-8 is one such format that
encodes the non-null ASCII characters in a single byte, characters between 128 and 2047 and
ASCII null in two bytes, and the remaining characters in three bytes. While theoretically this
encoding might expand a file's size by 50%, because most text files contain primarily ASCII,
in practice it's almost always a huge savings. Therefore, Java uses UTF-8 in string literals,
identifiers, and other text data in compiled byte code. UTF-8 is also a common encoding for
XML files and the native encoding of Bell Labs' experimental Plan 9 operating system.
To better understand UTF-8, consider a typical Unicode character as a sequence of 16 bits:
x15 x14 x13 x12 x11 x10 x9 x8
x7 x6 x5 x4 x3 x2 x1 x0
Each ASCII character except the null character (each character between 1 and 127) has its
upper nine bits equal to 0:
0 0 0 0 0 0 0 0
0 x6 x5 x4 x3 x2 x1 x0
Therefore, it's easy to encode an ASCII character as a single byte. Just drop the high-order
byte:
0 x6 x5 x4 x3 x2 x1 x0
Now consider characters between 128 and 2047. These all have their top five bits equal to 0,
as shown here:
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