Java Tips - How to read file upside/down

The example below reads a file from bottom to top in reverse way. If last line of file is 'hello', it will read 'olleh'. Two methods are used to implement above mentioned functionality. The first one is readinf file into character array and reversing it and the second is using RandomAccessFile and FileChannel.


import java.io.*;

import java.nio.channels.*;

import java.nio.*;



public class FileReaderDownToUp {

    

    public static void main(String[] args) {

        

        try {

            

            //method 1

            File f = new File("hello.txt");

            FileReader fr = new FileReader(f);

            

            char[] c = new char[(int)f.length()];

            char[] cnew = new char[(int)f.length()];

            StringBuffer sbuf = new StringBuffer();

            fr.read(c,0,(int)f.length());

            

            int len = (int)f.length();

            

            for (int i = 0, j = len - 1; i < len ; i++, j--) {

                cnew[i] = c[j];

                sbuf.append(cnew[i]);

            }

            

            System.out.println(sbuf.toString());

            fr.close();

            

            //method 2

            

            /*

            

            RandomAccessFile f = new RandomAccessFile("hello.txt","rw");

            FileChannel fc = f.getChannel();

                         

            // map file to buffer

                         

            MappedByteBuffer mbb = fc.map(FileChannel.MapMode.READ_WRITE, 

             0, f.length());

             

            int len = (int)f.length();

                   for (int i = 0, j = len - 1; i < j; i++, j--)

                   {

                        byte b = mbb.get(i);

                        mbb.put(i, mbb.get(j));

                        mbb.put(j, b);

                   }

                         

                   // finish up

                   fc.close();

             */

            

        } catch (Exception e) {

            e.printStackTrace();

        }

    }

}

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