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;  
}