Filtering Character Streams
FilterReader is an abstract
class that defines a null filter; it reads characters from a specified
Reader and returns them with no
modification. In other words, FilterReader defines no-op implementations
of all the Reader methods. A
subclass must override at least the two read(
) methods to perform whatever sort of filtering is
necessary. Some subclasses may override other methods as well. Example 3-6 shows RemoveHTMLReader, which is a custom subclass
of FilterReader that reads HTML
text from a stream and filters out all of the HTML tags from the text
it returns.
In the example, we implement the HTML tag filtration in
the three-argument version of read(
), and then implement the no-argument version in terms of
that more complicated version. The example includes an inner Test class with a main( ) method that shows how you might use
the RemoveHTMLReader class.
Note that we could also define a RemoveHTMLWriter class by performing the
same filtration in a FilterWriter
subclass. Or, to filter a byte stream instead of a character stream,
we could subclass FilterInputStream
and FilterOutputStream. RemoveHTMLReader is only one example of a
filter stream. Other possibilities include streams that count the
number of characters or bytes processed, convert characters to
uppercase, extract URLs, perform search-and-replace operations,
convert Unix-style LF line terminators to Windows-style CRLF line
terminators, and so on.
Example 3-6. RemoveHTMLReader.java ...
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