Java Reference
In-Depth Information
in UTF-8 or UTF-16 unless the server tells it otherwise. However, a web server that uses
PrintStream
may well send the files encoded in CP1252 from a U.S.-localized Windows
system or SJIS from a Japanese-localized system, whether the client expects or under‐
stands those encodings or not.
PrintStream
doesn't provide any mechanism for chang‐
ing the default encoding. This problem can be patched over by using the related
Print
Writer
class instead. But the problems continue.
The third problem is that
PrintStream
eats all exceptions. This makes
PrintStream
suitable for textbook programs such as HelloWorld, because simple console output can
be taught without burdening students with first learning about exception handling and
all that implies. However, network connections are much less reliable than the console.
Connections routinely fail because of network congestion, phone company misfea‐
sance, remote systems crashing, and many other reasons. Network programs must be
prepared to deal with unexpected interruptions in the flow of data. The way to do this
is by handling exceptions. However,
PrintStream
catches any exceptions thrown by the
underlying output stream. Notice that the declaration of the standard five
Output
Stream
methods in
PrintStream
does not have the usual
throws IOException
decla‐
ration:
public
abstract
void
write
(
int
b
)
public
void
write
(
byte
[]
data
)
public
void
write
(
byte
[]
data
,
int
offset
,
int
length
)
public
void
flush
()
public
void
close
()
Instead,
PrintStream
relies on an outdated and inadequate error flag. If the underlying
stream throws an exception, this internal error flag is set. The programmer is relied
upon to check the value of the flag using the
checkError()
method:
public
boolean
checkError
()
To do any error checking at all on a
PrintStream
, the code must explicitly check every
call. Furthermore, once an error has occurred, there is no way to unset the flag so further
errors can be detected. Nor is any additional information available about the error. In
short, the error notification provided by
PrintStream
is wholly inadequate for unreli‐
able network connections.
Data Streams
The
DataInputStream
and
DataOutputStream
classes provide methods for reading and
writing Java's primitive data types and strings in a binary format. The binary formats
used are primarily intended for exchanging data between two different Java programs
through a network connection, a datafile, a pipe, or some other intermediary. What a
data output stream writes, a data input stream can read. However, it happens that the
formats are the same ones used for most Internet protocols that exchange binary num‐
bers. For instance, the time protocol uses 32-bit big-endian integers, just like Java's
int
