Java IO.pdf - Nguyen Dang Binh

Chapter 1. Introducing I/O
Java I/O
Input and output, I/O for short, are fundamental to any computer operating system or
programming language. Only theorists find it interesting to write programs that don't require
input or produce output. At the same time, I/O hardly qualifies as one of the more "thrilling"
topics in computer science. It's something in the background, something you use every day—
but for most developers, it's not a topic with much sex appeal.
There are plenty of reasons for Java programmers to find I/O interesting. Java includes a
particularly rich set of I/O classes in the core API, mostly in the java.io package. For the
most part I/O in Java is divided into two types: byte- and number-oriented I/O, which is
handled by input and output streams; and character and text I/O, which is handled by readers
and writers. Both types provide an abstraction for external data sources and targets that allows
you to read from and write to them, regardless of the exact type of the source. You use the
same methods to read from a file that you do to read from the console or from a network
connection.
But that's just the tip of the iceberg. Once you've defined abstractions that let you read or
write without caring where your data is coming from or where it's going to, you can do a lot
of very powerful things. You can define I/O streams that automatically compress, encrypt,
and filter from one data format to another, and more. Once you have these tools, programs can
send encrypted data or write zip files with almost no knowledge of what they're doing;
cryptography or compression can be isolated in a few lines of code that say, "Oh yes, make
this an encrypted output stream."
In this book, I'll take a thorough look at all parts of Java's I/O facilities. This includes all the
different kinds of streams you can use. We're also going to investigate Java's support for
Unicode (the standard multilingual character set). We'll look at Java's powerful facilities for
formatting I/O—oddly enough, not part of the java.io package proper. (We'll see the reasons
for this design decision later.) Finally, we'll take a brief look at the Java Communications API
(javax.comm), which provides the ability to do low-level I/O through a computer's serial and
parallel ports.
I won't go so far as to say, "If you've always found I/O boring, this is the book for you!" I will
say that if you do find I/O uninteresting, you probably don't know as much about it as you
should. I/O is the means for communication between software and the outside world
(including both humans and other machines). Java provides a powerful and flexible set of
tools for doing this crucial part of the job.
Having said that, let's start with the basics.
1.1 What Is a Stream?
A stream is an ordered sequence of bytes of undetermined length. Input streams move bytes
of data into a Java program from some generally external source. Output streams move bytes
of data from Java to some generally external target. (In special cases streams can also move
bytes from one part of a Java program to another.)
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