Java File read and write in reverse [byte by byte]

Question

I need to read from this text file source.txt and write the content in reverse in this text file destination.txt. The read and write must be done using byte-by-byte!

I did this exercise using BufferedReader & BufferedWriter which gives you a whole line as a string then it's very simple to reverse it!

But I don't know how to write in reverse order using byte-by-byte! Thank you for your help!

source.txt has this text: "Operating Systems"

And the result on destination.txt should be the reverse of source.txt: "smetsyS gnitarepO"

Here's the code:

import java.io.FileInputStream; import java.io.FileNotFoundException; import java.io.FileOutputStream; import java.io.IOException; public class Main { public static void main(String[] args) throws IOException{ FileInputStream in = null; FileOutputStream out = null; try { in = new FileInputStream("source.txt"); out = new FileOutputStream("destination.txt"); int c; while ((c = in.read()) != -1) { out.write(c); } } catch (FileNotFoundException e) { e.printStackTrace(); } finally { if (in != null) { in.close(); } if (out != null) { out.close(); } } } }

Read the file into a byte array and then write out the byte array in the reverse order. — yole
– yole, Commented Mar 7, 2018 at 10:52
If your file is in ASCII, then it's pretty simple - every byte is basically the same as a char. If it's UTF-8 or other multibyte encoding, it may be a challenge. Anyway, hint: keep the bytes you read until you reach the end of a line or the end of the file. — RealSkeptic
– RealSkeptic, Commented Mar 7, 2018 at 11:42
@RealSkeptic: If what is asked for is byte for byte reversal, encoding does not matter, the file should be processed at a byte level as is it was binaty. — Serge Ballesta
– Serge Ballesta, Commented Mar 7, 2018 at 12:30

Evgeniy Dorofeev · Accepted Answer · 2018-03-07 12:04:34Z

You can use RandomAccesFile for reading:

... in = new RandomAccessFile("source.txt", "r"); out = new FileOutputStream("destination.txt"); for(long p = in.length() - 1; p >= 0; p--) { in.seek(p); int b = in.read(); out.write(b); } ...

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