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Java in a Nutshell

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The java.io Package

24.26 java.io.FilterReader (JDK 1.1)

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 their own read() methods are 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 filtering of any kind.

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 of the methods of Reader. In order to create your own filtered character input stream, you should subclass FilterReader and override both of 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 also need to override other methods of FilterReader as well, and you may want to provide methods or constructors that are specific to your subclass.

FilterReader is the character-stream analog to FilterInputStream.

public abstract class FilterReader extends Reader {
    // Protected Constructor
            protected FilterReader(Reader in);
    // Protected Instance Variables
            protected Reader in;
    // Public Instance Methods
            public void close() throws IOException;  // Defines Reader
            public void mark(int readAheadLimit) throws IOException;  // Overrides Reader
            public boolean markSupported();  // Overrides Reader
            public int read() throws IOException;  // Overrides Reader
            public int read(char[] cbuf, int off, int len) throws IOException;  // Defines Reader
            public boolean ready() throws IOException;  // Overrides Reader
            public void reset() throws IOException;  // Overrides Reader
            public long skip(long n) throws IOException;  // Overrides Reader



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