Streams
Java programs perform I/O through streams. A stream is an abstraction that either produces
or consumes information. A stream is linked to a physical device by the Java I/O system.
All streams behave in the same manner, even if the actual physical devices to which they are
linked differ. Thus, the same I/O classes and methods can be applied to any type of device.
This means that an input stream can abstract many different kinds of input: from a disk file,
a keyboard, or a network socket. Likewise, an output stream may refer to the console, a disk
file, or a network connection. Streams are a clean way to deal with input/output without
having every part of your code understand the difference between a keyboard and a network,
for example. Java implements streams within class hierarchies defined in the java.io package.
Byte Streams and Character Streams
Java defines two types of streams: byte and character. Byte streams provide a convenient
means for handling input and output of bytes. Byte streams are used, for example, when
reading or writing binary data. Character streams provide a convenient means for handling
input and output of characters. They use Unicode and, therefore, can be internationalized.
Also, in some cases, character streams are more efficient than byte streams.
The original version of Java (Java 1.0) did not include character streams and, thus, all
I/O was byte-oriented. Character streams were added by Java 1.1, and certain byte-oriented
classes and methods were deprecated. This is why older code that doesn't use character streams
should be updated to take advantage of them, where appropriate.
One other point: at the lowest level, all I/O is still byte-oriented. The character-based
streams simply provide a convenient and efficient means for handling characters.
An overview of both byte-oriented streams and character-oriented streams is presented
in the following sections.
The Byte Stream Classes
Byte streams are defined by using two class hierarchies. At the top are two abstract classes:
InputStream and OutputStream. Each of these abstract classes has several concrete subclasses
that handle the differences between various devices, such as disk files, network connections,
and even memory buffers. The byte stream classes are shown in Table 13-1. A few of these
classes are discussed later in this section. Others are described in Part II. Remember, to use
the stream classes, you must import java.io.
The abstract classes InputStream and OutputStream define several key methods that
the other stream classes implement. Two of the most important are read( ) and write( ),
which, respectively, read and write bytes of data. Both methods are declared as abstract
inside InputStream and OutputStream. They are overridden by derived stream classes.
The Character Stream Classes
Character streams are defined by using two class hierarchies. At the top are two abstract
classes, Reader and Writer. These abstract classes handle Unicode character streams. Java
has several concrete subclasses of each of these. The character stream classes are shown in
Table 13-2.
The abstract classes Reader and Writer define several key methods that the other stream
classes implement. Two of the most important methods are read( ) and write( ), which read
and write characters of data, respectively. These methods are overridden by derived stream
classes.
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