Here is an example of how to use the AbstractCharacterFilterReader class from the Apache Commons IO library in Java:

import java.io.IOException;
import org.apache.commons.io.input.AbstractCharacterFilterReader;

public class MyCharacterFilterReader extends AbstractCharacterFilterReader {
    public MyCharacterFilterReader(Reader in) {
        super(in);
    }

    @Override
    protected int processChar(int c) throws IOException {
        if (c == 'a') {
            return 'b';
        }
        return c;
    }
}

In this example, MyCharacterFilterReader is a custom implementation of AbstractCharacterFilterReader that replaces all occurrences of the letter 'a' with the letter 'b' when reading from the input Reader.

Here's how you can use it:

Reader input = new StringReader("abc");
Reader filtered = new MyCharacterFilterReader(input);
int character;
while ((character = filtered.read()) != -1) {
    System.out.print((char) character);
}

This will output: "bbc"

Please note that this is a simple example and in real-world usage, you might want to do more complex processing on the characters before returning them.