Java FilterWriter Tutorial with Examples

FilterWriter
Examples

1. FilterWriter

FilterWriter is an abstract subclass of the Writer class. It is the base class for creating subclasses used to selectively write the required characters.

FilterWriter does not directly write characters to the target (e.g. file), instead it manages another Writer that is responsible for writing the data to the target. FilterWriter filters or processes the characters written to it, then writes the results to the Writer it manages.

Take a look at the source code of the FilterWriter class: All the methods that it inherits from the superclass are overridden to act as a proxy of the Writer object it manages:

FilterWriter

public abstract class FilterWriter extends Writer {  
    protected Writer out;
    
    protected FilterWriter(Writer out) {
        super(out);
        this.out = out;
    }
    public void write(int c) throws IOException {
        out.write(c);
    }
    public void write(char cbuf[], int off, int len) throws IOException {
        out.write(cbuf, off, len);
    }
    public void write(String str, int off, int len) throws IOException {
        out.write(str, off, len);
    }
    public void flush() throws IOException {
        out.flush();
    }
    public void close() throws IOException {
        out.close();
    }
}

FilterWriter constructors

protected FilterWriter(Writer out)

Java FilterReader

2. Examples

ROT13 is a simple letter substitution cipher, replacing each letter with another letter that is 13 position after it on the alphabet. ROT13 is a simple case of the Caesar cipher.

Below is a table of letters and their corresponding substitutions as the result of using the ROT13 algorithm.

The standard Latin alphabet consists of 26 letters (2x13). Basically, ROT13 algorithm substitutes one letter for another, and uses this same algorithm to reverse the result. For example, algorithm ROT13 turns the letter "A" to "N", and uses ROT13 algorithm for "N" to get "A" as the substitute.

ROT13 is often considered a prime example of a weak encryption.

The following ROT13 class provides a static rotate(int) method that accepts a letter as input and returns a substitute letter as output using ROT13 algorithm:

ROT13.java

package org.o7planning.filterwriter.ex;

public class ROT13 {

    /**
     * <pre>
     *   a <==> n
     *   b <==> o
     *   c <==> p
     *   d <==> q
     *   e <==> r
     *   ...
     *   y <==> l
     *   z <==> m
     * </pre>
     */
    public static int rotate(int inChar) {
        int outChar;

        if (inChar >= (int) 'a' && inChar <= (int) 'z') {
            outChar = (((inChar - 'a') + 13) % 26) + 'a';
        } else if (inChar >= (int) 'A' && inChar <= (int) 'Z') {
            outChar = (((inChar - 'A') + 13) % 26) + 'A';
        } else {
            outChar = inChar;
        }
        return outChar;
    }

    // Test
    public static void main(String[] args) {
        for (char ch = 'a'; ch <= 'z'; ch++) {
            char m = (char) rotate(ch);
            System.out.println(ch + " ==> " + m);
        }
        System.out.println(" --- ");
        for (char ch = 'A'; ch <= 'Z'; ch++) {
            char m = (char) rotate(ch);
            System.out.println(ch + " ==> " + m);
        }
    }
}

Write ROT13Writer class extended from FilterWriter class. Characters written in ROT13Writer will be substituted using ROT13 algorithm:

ROT13Writer.java

package org.o7planning.filterwriter.ex;

import java.io.FilterWriter;
import java.io.IOException;
import java.io.Writer;
 
public class ROT13Writer extends FilterWriter {
 
    public ROT13Writer(Writer out) {
        super(out);
    }  
    
    @Override
    public void write(int outChar) throws IOException {
        super.write(ROT13.rotate(outChar));
    }
 
    @Override
    public void write(char[] cbuf, int offset, int length) throws IOException {
        char[] tempbuf = new char[length];
        for (int i = 0; i < length; i++) {
            tempbuf[i] = (char) ROT13.rotate(cbuf[offset + i]);
        }
        super.write(tempbuf, 0, length);
    }  
    
    @Override
    public void write(String str, int off, int len) throws IOException {
        char[] cbuf = str.toCharArray();
        this.write(cbuf, off, len);
    }
}

An example for using ROT13Writer:

ROT13WriterTest1.java

package org.o7planning.filterwriter.ex;

import java.io.FilterWriter;
import java.io.IOException;
import java.io.StringWriter;
import java.io.Writer;

public class ROT13WriterTest1 {

    public static void main(String[] args) throws IOException {
        Writer targetWriter = new StringWriter();
        FilterWriter filterWriter = new ROT13Writer(targetWriter);
        
        String inputString = "Hello";
        
        filterWriter.write(inputString);
        filterWriter.close();
        
        String outputString = targetWriter.toString();
        System.out.println(inputString + " ==> " + outputString);
    }
}

Output:

Hello ==> Uryyb

An example of using ROT13Writer to write data to the file:

ROT13WriterTest2.java

package org.o7planning.filterwriter.ex;

import java.io.File;
import java.io.FileWriter;
import java.io.FilterWriter;
import java.io.IOException;
import java.io.Writer;

public class ROT13WriterTest2 {

    public static void main(String[] args) throws IOException {
        // Windows: C:/somedir/rot13-out-file.txt
        File outFile = new File("/Volumes/Data/test/rot13-out-file.txt");
        outFile.getParentFile().mkdirs();
        
        Writer targetWriter = new FileWriter(outFile);
        FilterWriter filterWriter = new ROT13Writer(targetWriter);
        
        String inputString = "Uryyb";
        
        filterWriter.write(inputString);
        filterWriter.close();
    }
}

Output:

rot13-out-file.txt

Hello

Java FilterWriter Tutorial with Examples

1. FilterWriter

2. Examples

Java IO Tutorials

Java IO Tutorials

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