Java IO.pdf - Nguyen Dang Binh

Java I/O
rush to snap up all the good ones like "TEXT" and "HTML". The list of codes that a particular
Macintosh understands is stored in the Desktop database, a file users never see and only rarely
have to worry about. Overall, this is a pretty good system that's worked incredibly well for
more than a decade. Neither Windows nor Unix has anything nearly as simple and troublefree.
Because Windows and Unix have not adopted Mac-style type and creator codes, Java
does not have any standard means for accessing them.
None of these solutions is perfect. On a Mac you're likely to want to use Photoshop to create
GIF files but use JPEGView or Netscape to view them. You can drag and drop the file onto
the desired application, but only if both the file and the application you want to view it with
are on the screen at the same time, which is not necessarily true if both are stored several
folders deep. Furthermore, it's relatively hard to say that you want all text files opened in
BBEdit. On the other hand, the Windows solution is prone to user error; filename extensions
are too exposed. For example, novice HTML coders often can't understand why their HTML
files painstakingly crafted in Notepad open as plaintext in Navigator. Notepad surreptitiously
inserts a .txt extension on all the files it saves unless the filename is enclosed in double quote
marks. For instance, a file saved as HelloWorld.html actually becomes HelloWorld.html.txt ,
while a file saved as "HelloWorld.html" is saved with the expected name. Furthermore,
filename extensions make it easy for a user to lie about the contents of a file, potentially
confusing and crashing applications. (You can lie about a file type on a Mac too, but it takes a
lot more work.) Finally, Windows provides absolutely no support for saying that you want
one group of GIF images opened in Photoshop and another group opened in DeBabelizer.
There are some algorithms that can attempt to determine a file's type from its contents, though
these are also error-prone. Many file formats require files to begin with a particular magic
number that uniquely identifies the format. For instance, all compiled Java class files begin
with the number 0xCAFEBABE (in hexadecimal). If the first four bytes of a file aren't
0xCAFEBABE, then it's definitely not a Java class file. Furthermore, barring deliberate fraud,
there's only about a one in four billion chance that a random, non-Java file will begin with
those four bytes. Unfortunately, only a few file formats require magic numbers. Text files, for
instance, can begin with any four ASCII characters. There are some heuristics you can apply
to identify such files. For example, a file of pure ASCII should not contain any bytes with
values between 128 and 255 and should have a limited number of control characters with
values less than 32. But such algorithms are complicated to devise and far from reliable. Even
if you are able to identify a file as ASCII text, how would you determine whether it contains
Java source code or a letter to your mother? Worse yet, how could you tell whether it contains
Java source code or C source code? It's not impossible, barring deliberately perverse files like
a concatenation of a C program with a Java program, but it's difficult and often not worth your
time.
One possible solution to the problem of identifying file types across platforms is using
MIME. The file-extension-content type mappings listed in Table 12.1 are de facto standards.
MIME, the Multipurpose Internet Mail Extensions, is a de jure (RFCs 2045-2049)
specification for embedding and identifying arbitrary data types in Internet email. MIME is
also used by HTTP servers that want to identify the kinds of data they're sending to a client.
And, in the BeOS, it's used as a Mac-like means of identifying file types. A MIME type
consists of a primary type like "text" or "image," followed by a forward slash, followed by a
subtype like "html" or "gif." "text/html" is a typical MIME content type that indicates a file of
textual information in the HTML format. MIME also uses x-types, like "application/x-tar"
and "application/x-mif," to allow ad hoc extensions to the standard. There may be more than
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