-- Scenario:
    The purpose of a reader is to interpret a low-level byte stream (ByteArrayInputStream, StringInputStream, FileInputStream and so on) as a character stream and provid character input to whatever class needs it. And it is very simple to convert an inputstream to a reader:

Reader reader = new InputStreamReader( in ); //in is an instance of class InputStream or derived classes

But the issue is sometimes we need convert a reader to inputstream, think about following scenaros:
1.  the original inputstream has been filtered by certian reader, now we need save back filtered content into database by inputstream: we can not use original inputstream but filtered stream which can only get from your reader.
2.  Given a class who contains a reader to access a streaming content after complex parsering or downloading, we want to utilize the streaming content in this class while not repeating complex operations for content analysis, so we need employ some wrapper methods to get inputstream from reader.

-- Solution:
1. write own InputStream implementation, such as following:

class MyInputStream extends InputStream
{
private Reader rd;
public  MyInputStream(Reader rd)
{
super();
     this.rd = rd;
}
 
 
// implement the read() method to make this all work
publicint read()
{
int t = rd.read();
// you can do your processing on the inputReader here
// fiddle with the values and return
return t;
 
}
}

Note: Applications that need to define a subclass of InputStream must always provide a method that returns the next byte of input.
(refer to http://java.sun.com/j2se/1.4.2/docs/api/java/io/InputStream.html)

-- anything else? BTW, for parsering xml-based input stream by SAX, I am glad to see that the inputSource constructor can take either InputStream or Reader (refer to http://java.sun.com/j2se/1.4.2/docs/api/org/xml/sax/InputSource.html)