Java IO.pdf - Nguyen Dang Binh

Chapter 12. Working with Files
Java I/O
You've already learned how to read and write data in files using file input streams and file
output streams. That's not all there is to files. Files can be created, moved, renamed, copied,
deleted, and otherwise manipulated without respect to their contents. Files are also often
associated with meta-information that's not strictly part of the contents of the file, such as the
time the file was created, the icon for the file, the permissions that determine which users can
read or write to the file, and even the name of the file.
While the abstraction of the contents of a file as an ordered sequence of bytes used by file
input and output streams is almost standard across platforms, the meta-information is not. The
java.io.File class attempts to provide a platform-independent abstraction for common file
operations and meta-information. Unfortunately, this class really shows its Unix roots. It
works well on Unix, adequately on Windows and OS/2—with a few caveats—and fails
miserably on the Macintosh. Java 2 improves things, but there's still a lot of history—and
coming up with something that genuinely works on all platforms is an extremely difficult
problem.
File manipulation is thus one of the real difficulties of cross-platform Java programming.
Before you can hope to write truly cross-platform code, you need a solid understanding of the
filesystem basics on all the target platforms. This chapter tries to cover those basics for the
major platforms that support Java—Unix; DOS/Windows 3.x ; Windows 95, 98, and NT;
OS/2; and the Mac—then it shows you how to write your file code so that it's as portable as
possible.
12.1 Understanding Files
As far as a Java program knows, a file is a sequential set of bytes stored on a disk like a hard
drive or a CD-ROM. There is a first byte in the file, a second byte, and so on, until the end of
the file. In this way a file is similar to a stream. However, a program can jump around in a
file, reading first one part of a file, then another. This isn't possible with a stream.
Macintosh files are a little different. Mac files are divided into two forks, each of which is
equivalent to a separate file on other platforms. The first part of a Mac file is called the data
fork and contains the text, image data, or other basic information of the file. The second part
of the file is called the resource fork and typically contains localizable strings, pictures, icons,
graphical user interface components like menubars and dialogs, executable code, and more.
On a Macintosh, all the standard java.io classes work exclusively with the data fork.
12.1.1 Filenames
Every file has a name. The format of the filename is determined by the operating system. For
example, in DOS and Windows 3.1, filenames are case-insensitive, (though generally
rendered as all capitals), eight ASCII characters long with a three-letter extension.
README.TXT is a valid DOS filename, but Read me before you run this program or your
hard drive will get trashed is not. All ASCII characters from 32 up (that is, noncontrol
characters), except for the 15 punctuation characters (+=/][":;,?*\|) and the space
character, may be used in filenames. A period may be used only as a separator between the
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