This abstract class is intended to act as a superclass for character input streams that read data from some other character input stream, filter it in some way, and then return the filtered data when a read( ) method is called. FilterReader is declared abstract so that it cannot be instantiated. But none of its methods are themselves abstract: they all simply call the requested operation on the input stream passed to the FilterReader( ) constructor. If you were allowed to instantiate a FilterReader, you’d find that it is a null filter (i.e., it simply reads characters from the specified input stream and returns them without any kind of filtering).

Because FilterReader implements a null filter, it is an ideal superclass for classes that want to implement simple filters but do not want to override all the methods of Reader. In order to create your own filtered character input stream, you should subclass FilterReader and override both its read( ) methods to perform the desired filtering operation. Note that you can implement one of the read( ) methods in terms of the other, and thus only implement the filtration once. Recall that the other read( ) methods defined by Reader are implemented in terms of these methods, so you do not need to override those. In some cases, you may need to override other methods of FilterReader and provide methods or constructors that are specific to your subclass. FilterReader is the character-stream analog to FilterInputStream ...

Get Java in a Nutshell, 5th Edition now with O’Reilly online learning.

O’Reilly members experience live online training, plus books, videos, and digital content from 200+ publishers.