Java IO.pdf - Nguyen Dang Binh

Java I/O
The word stream is derived from an analogy with a stream of water. An input stream is like a
siphon that sucks up water; an output stream is like a hose that sprays out water. Siphons can
be connected to hoses to move water from one place to another. Sometimes a siphon may run
out of water if it's drawing from a finite source like a bucket. On the other hand, if the siphon
is drawing water from a river, it may well provide water indefinitely. So too an input stream
may read from a finite source of bytes like a file or an unlimited source of bytes like
System.in. Similarly an output stream may have a definite number of bytes to output or an
indefinite number of bytes.
Input to a Java program can come from many sources. Output can go to many different kinds
of destinations. The power of the stream metaphor and in turn the stream classes is that the
differences between these sources and destinations are abstracted away. All input and output
are simply treated as streams.
1.1.1 Where Do Streams Come From?
The first source of input most programmers encounter is System.in. This is the same thing as
stdin in C, generally some sort of console window, probably the one in which the Java
program was launched. If input is redirected so the program reads from a file, then System.in
is changed as well. For instance, on Unix, the following command redirects stdin so that
when the MessageServer program reads from System.in, the actual data comes from the file
data.txt instead of the console:
% java MessageServer < data.txt
The console is also available for output through the static field out in the java.lang.System
class, that is, System.out. This is equivalent to stdout in C parlance and may be redirected
in a similar fashion. Finally, stderr is available as System.err. This is most commonly used
for debugging and printing error messages from inside catch clauses. For example:
try {
//... do something that might throw an exception
}
catch (Exception e) { System.err.println(e); }
Both System.out and System.err are print streams, that is, instances of
java.io.PrintStream.
Files are another common source of input and destination for output. File input streams
provide a stream of data that starts with the first byte in a file and finishes with the last byte in
the file. File output streams write data into a file, either by erasing the file's contents and
starting from the beginning or by appending data to the file. These will be introduced in
Chapter 4.
Network connections provide streams too. When you connect to a web server or FTP server
or something else, you read the data it sends from an input stream connected from that server
and write data onto an output stream connected to that server. These streams will be
introduced in Chapter 5.
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