Output Streams

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 methods that call native code to write files. 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 this 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 some kind of OutputStream.

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 API 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 method it inherits from its superclass.