Java IO.pdf - Nguyen Dang Binh

Chapter 2. Output Streams
2.1 The OutputStream Class
Java I/O
The java.io.OutputStream class declares the three basic methods you need to write bytes
of data onto a stream. It also has methods for closing and flushing streams.
public abstract void write(int b) throws IOException
public void write(byte[] data) throws IOException
public void write(byte[] data, int offset, int length) throws IOException
public void flush() throws IOException
public void close() throws IOException
OutputStream is an abstract class. Subclasses provide implementations of the abstract
write(int b) method. They may also override the four nonabstract methods. For example,
the FileOutputStream class overrides all five methods with native methods that know how
to write bytes into files on the host platform. Although OutputStream is abstract, often you
only need to know that the object you have is an OutputStream ; the more specific subclass
of OutputStream is hidden from you. For example, the getOutputStream() method of
java.net.URLConnection has the signature:
public OutputStream getOutputStream() throws IOException
Depending on the type of URL associated with this URLConnection object, the actual class of
the output stream that's returned may be a sun.net.TelnetOutputStream , a
sun.net.smtp.SmtpPrintStream , a sun.net.www.http.KeepAliveStream , or something
else completely. All you know as a programmer, and all you need to know, is that the object
returned is in fact some instance of OutputStream. That's why the detailed classes that handle
particular kinds of connections are hidden inside the sun packages.
Furthermore, even when working with subclasses whose types you know, you still need to be
able to use the methods inherited from OutputStream. And since methods that are inherited
are not included in the online documentation, it's important to remember that they're there. For
example, the java.io.DataOutputStream class does not declare a close() method, but you
can still call the one it inherits from its superclass.
2.2 Writing Bytes to Output Streams
The fundamental method of the OutputStream class is write():
public abstract void write(int b) throws IOException
This method writes a single unsigned byte of data whose value should be between and 255. If
you pass a number larger than 255 or smaller than zero, it's reduced modulo 256 before being
written.
Example 2.1, AsciiChart, is a simple program that writes the printable ASCII characters (32
to 126) on the console. The console interprets the numeric values as ASCII characters, not as
numbers. This is a feature of the console, not of the OutputStream class or the specific
subclass of which System.out is an instance. The write() method merely sends a particular
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