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This
abstract class is intended to act as a superclass for character
output streams that filter the data written to them before writing it
to some other character output stream.
FilterWriter is declared
abstract so that it cannot be instantiated. But
none of its methods are themselves abstract: they all simply invoke
the corresponding method on the output stream that was passed to the
FilterWriter constructor. If you were allowed to
instantiate a FilterWriter object,
you'd find that it acts as a null filter (i.e., it
simply passes the characters written to it along, without any
filtration).Because FilterWriter implements a null filter, it
is an ideal superclass for classes that want to implement simple
filters without having to override all of the methods of
Writer. In order to create your own filtered
character output stream, you should subclass
FilterWriter and
override all its write( ) methods to perform the
desired filtering operation. Note that you can implement two of the
write( ) methods in terms of the third and thus
implement your filtering algorithm only once. In some cases, you may
want to override other Writer methods and add
other methods or constructors that are specific to your subclass.
FilterWriter is the character-stream analog of
FilterOutputStream.
Figure 9-24. java.io.FilterWriter
public abstract class FilterWriter extends Writer {
// Protected Constructors
protected FilterWriter (Writer out );
// Public Methods Overriding Writer
public void close ( ) throws IOException;
public void flush ( ) throws IOException;
public void write (int c ) throws IOException;
public void write (char[ ] cbuf , int off , int len ) throws IOException;
public void write (String str , int off , int len ) throws IOException;
// Protected Instance Fields
protected Writer out ;
}