Java IO.pdf - Nguyen Dang Binh

12.1.3 Filename Extensions and File Types
Java I/O
Filename extensions are often used to indicate the type of a file. For example, a file that ends
with the four-letter extension .java is presumed to be a text file containing Java source code; a
file ending in the five-letter extension .class is assumed to contain compiled Java byte code; a
file ending in the three-letter extension .gif is assumed to contain a GIF image. Table 12.1 lists
some of the more common extensions and their associated types.
Table 12.1. Extension Type Mappings
Extension Type Extension Type
.txt ASCII text .sit StuffIt archive
.gif GIF image .bin MacBinary file
.jpg, .jpeg JPEG image .hqx BinHexed Macintosh file
.htm, .html HTML text .tar Unix tar archive
.java Java source code .doc Microsoft Word file
.class compiled Java class .c C source code
.jar JAR archive .pl Perl program
.zip Zip archive .cc, .cpp C++ source code
.Z Unix compressed file .o Prelinked native object code
.gz gzipped file .exe DOS/Windows executable
What does your computer do when you double-click on the file panther.gif? If your computer
is a Macintosh, it opens the file in the program that created the file. That's because the MacOS
stores a four-letter creator code for every file on a disk in the disk's volume catalog. Assuming
the application associated with that creator code can be found (it can't always, though), the
file panther.gif is opened in the creating program. On the other hand, if your computer is a
Windows PC or a Unix workstation, the creating program is not necessarily opened. Instead,
whichever program is registered as the viewer of .gif files is launched and used to view the
file. In command-line environments, like the Unix shell, this isn't really an issue, because you
begin by specifying the program to run; that is, you type xv panther.gif, not simply
panther.gif. But in GUI environments, the program that's opened may not be the program you
want to use.
File extensions have the further disadvantage that they do not really guarantee the content
type of their document and are an unreliable means of determining the type of a file. Users
can easily change them. For example, the simple command copy HelloWorld.java
HelloWorld.gif causes an ASCII text file to be misinterpreted as a GIF image. Filename
extensions are only as reliable as the user that assigned them. What's more, it's hard to
distinguish between files that belong to different applications that have the same type. For
instance, many users are surprised to discover that after installing Internet Explorer, all their
HTML files appear to belong to Explorer instead of Netscape.
The Macintosh solved this problem over a decade ago. Almost every Mac file has a four-letter
type code like "TEXT" and a four-letter creator code like "R*ch". Since each file has both a
type code and a creator code, a Mac can distinguish between files that belong to different
applications but have the same type. Installing Internet Explorer doesn't mean that IE
suddenly thinks it owns all your Netscape documents, as is the case when you install IE on
Windows. Software vendors register codes with Apple so companies don't accidentally step
on each other's toes; and since codes are almost never seen by end users, there's not a huge
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