Java IO.pdf - Nguyen Dang Binh

Java I/O
There are also two unrelated mnemonic constants, File.pathSeparator and
File.pathSeparatorChar. The path separator string is set from the system property
path.separator. As with the separator character, File.pathSeparatorChar is the first
character in File.pathSeparator.
public static final String pathSeparator =
System.getProperty("path.separator");
public static final char pathSeparatorChar = pathSeparator.charAt(0);
The path separator is used to separate two files (generally with complete pathnames) in a list
of paths such as a class path. For example, with a separator of a slash and a path separator of a
colon, my class path looks like this:
.:/usr/local/java/lib:/home/users/elharo/:/home/users/elharo/JavaDis/
These four mnemonic constants are one of the few instances in the core
API where JavaSoft violates its convention of using all caps for public
final static fields. You'd expect these constants to be named
File.SEPARATOR, File.SEPARATOR_CHAR, File.PATH_SEPARATOR, and
File.PATH_SEPARATOR_CHAR. Perhaps a future version of Java will
conform to the standard naming conventions.
Now the bad news: although JavaSoft has provided a fairly powerful abstraction layer so that
programmers don't need to hardcode explicit separators and path separators, few programmers
actually use this. Many programmers simply assume that the file separator is a slash and the
path separator is a colon and hardcode those constants as "/" and ":" or '/' and ':'.
Therefore, to avoid breaking all this third-party code, Java 1.1 VMs on the Mac and Windows
generally use a slash for the separator and a colon for the path separator, then make the
appropriate conversions in the native code that underlies Java. Java 2 VMs pass pathnames
through a normalization phase that attempts to recognize the separator conventions and
convert those to the conventions of the local platform.
This isn't a big problem for Windows, since both the slash and the backslash are illegal
characters in filenames. However, on the Mac a filename can contain both a slash and a
backslash. Macintosh virtual machines have adopted a number of different and incompatible
schemes to distinguish between slashes that are part of filenames and slashes that represent
separators. One early scheme was to replace slashes by colons and colons by slashes.
However, all but one of the alternative Mac VM vendors have canceled their own VM efforts
in favor of Apple's own Macintosh Runtime for Java (MRJ), and thus the scheme that MRJ
uses seems likely to become the standard way to handle unusual filenames on the Mac.
Apple's translation scheme is loosely based on the x-www-form-urlencoded format for
encoding URLs. Any characters that are likely to cause problems in a filename are replaced
by a percent sign (%) followed by the two hexadecimal digits corresponding to the ASCII
value of that character. For example, the space is ASCII 32 (decimal) or 20 (hexadecimal).
Thus, it's encoded as %20. The forward slash is ASCII 47 (decimal) or 2f (hexadecimal).
Thus, it's encoded as %2f. The pathname for this chapter, Macintosh HD:Java:Java I/O:12
Working with Files/12 Working with Files.doc, would be encoded as
/Macintosh%20HD/Java/Java%20I%2fO/
12%20Working%20with%20Files/12%20Working%20with%20Files.doc. Table 12.2 gives a
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