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copied!<p>If you know the number of lines in your file and if you're randomising complete rows, you can just randomise by line number and then read that selected row. Just select a random line via the <a href="http://download.oracle.com/javase/6/docs/api/java/util/Random.html" rel="nofollow noreferrer">Random</a> class and store the list of random numbers, so you don't pick one twice.</p> <pre><code>BufferedReader reader = new BufferedReader(new FileReader(new File("file.cvs"))); BufferedWriter chosen = new BufferedWriter(new FileWriter(new File("chosen.cvs"))); BufferedWriter notChosen = new BufferedWriter(new FileWriter(new File("notChosen.cvs"))); int numChosenRows = 10000; long numLines = 1000000000; Set<Long> chosenRows = new HashSet<Long>(numChosenRows+1, 1); for(int i = 0; i < numChosenRows; i++) { while(!chosenRows.add(nextLong(numLines))) { // add returns false if the value already exists in the Set } } String line; for(long lineNo = 0; (line = reader.readLine()) != null; lineNo++){ if(chosenRows.contains(lineNo)){ // Do nothing for the moment } else { notChosen.write(line); } } // Randomise the set of chosen rows // Use RandomAccessFile to write the rows in that order </code></pre> <p>See <a href="https://stackoverflow.com/questions/2546078/java-random-long-number-in-0-x-n-range/2546186#2546186">this answer</a> for the nextLong method, which produces a random long scaled to a particular size.</p> <p><strong>Edit:</strong> As most people, I overlooked the requirement for writing the randomly selected lines in a random order. I'm presuming that <a href="http://download.oracle.com/javase/6/docs/api/java/io/RandomAccessFile.html" rel="nofollow noreferrer">RandomAccessFile</a> would help with that. Just randomise the List with the chosen rows and access them in that order. As for the unchosen ones, I edited the code above to simply ignore the chosen ones. </p>

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