Filter input streams read data from a preexisting input stream such as a
FileInputStream and have an opportunity to work with or change the data before it is delivered to the client program. Filter output streams write data to a preexisting output stream such as a
FileOutputStream and have an opportunity to work with or change the data before it is written onto the underlying stream. Multiple filters can be chained onto a single underlying stream. Filter streams are used for encryption, compression, translation, buffering, and much more.
The word filter is derived by analogy with a water filter. A water filter sits between the pipe and faucet, filtering out impurities. A stream filter sits between the source of the data and its eventual destination and applies a specific algorithm to the data. As drops of water are passed through the water filter and modified, so too are bytes of data passed through the stream filter. Of course, there are some big differences—most notably, a stream filter, in addition to removing things you don’t want, can add data or some other kind of annotation to the stream; it may even produce a stream that is completely different than its original input (for example, by compressing the original data).
are concrete superclasses for input and output stream subclasses that somehow modify or manipulate data of an underlying stream:
public class FilterInputStream ...