Java in a Nutshell, 5th Edition [Electronic resources]

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.

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 ;  
}