How to read random lines?

Question

Hi everyone,
I am trying to write a program that will take a random lines from a number text files and copy them to a new document.
Is there a way to read specific lines in a document?
For example if I set an int array to hold 20 random numbers, (23, 504, 498, etc...)
can i then jump to each of these lines in the document and output the data on each line??
I have tried the RandomAccessFile seek method but that only moves a certain number of bytes(not lines).
I have also tried:
String tempLine = null;
// where numLines is the total number of lines in the document
// where randInput is my RandomAccessFile
For (int i = 0; i < numLines; i++)
tempLine = (randInput.readLine())
for(int k = 0; k < 20; k++)
if(i == randNums[k])
out.println(tempLine);
but this method is too slow, as some of my input files are over 500,000 lines in length.
Can anyone please suggest a faster more efficient method??
Many Thanks

Answer 1

If that is really the requirement then I suggest you
pre-process the file into some other form so it is
easier to fulfil the requirement.
For example you could copy the lines into a database
table, then choose random records from the table. Or
you could copy the lines into a new file where all
the lines had the same length (pad with blanks or
nulls or whatever you need to get back the original
line), which would make it possible to seek to line N.How about cache first the file, by reading the each line in the file and store that line in an array?
ie.
package com.yahoo.ronilloang.idunoifthisright;
import java.io.BufferedReader;
import java.io.BufferedWriter;
import java.io.File;
import java.io.FileOutputStream;
import java.io.FileReader;
import java.io.IOException;
import java.io.OutputStreamWriter;
import java.text.SimpleDateFormat;
import java.util.Date;
import java.util.Locale;
import java.util.Vector;
* How about caching first then randomly read? This maybe slow or definitely slow in performance.
* @author     Ronillo Ang
public class CacheFirstThenRandomlyPick{
     private String[] lines;
     private int lineCount;
     public CacheFirstThenRandomlyPick(String fileName) throws IOException{
          File file = new File(fileName);
          if(file.exists()){
               BufferedReader bufferedReader = new BufferedReader(new FileReader(file));
               String line = bufferedReader.readLine();
               while(line != null){
                    addLine(line);
                    line = bufferedReader.readLine();
               trim();
               randomlyPickAndWrite();
               bufferedReader.close();
     private void addLine(String line){
          if(lines == null)
               lines = new String[10];
          checkAndExpandCapacityAsNeeded(lineCount + 1);
          lines[lineCount++] = line;
     private void checkAndExpandCapacityAsNeeded(int minCapacity){
          int oldCapacity = lines.length;
          if(minCapacity > oldCapacity){
               int newCapacity = oldCapacity * 2;
               while(minCapacity >= newCapacity)
                    newCapacity += 10;
               String[] oldLines = lines;
               lines = new String[newCapacity];
               System.arraycopy(oldLines, 0, lines, 0, lineCount);
     private void trim(){
          if(lineCount > 0 && lineCount < lines.length){
               String[] oldLines = lines;
               lines = new String[lineCount];
               System.arraycopy(oldLines, 0, lines, 0, lineCount);
     private void randomlyPickAndWrite() throws IOException{
          Vector drawnNumber = new Vector();
          BufferedWriter bufferedWriter = new BufferedWriter(new OutputStreamWriter(new FileOutputStream(new File("out.txt")), "8859_1"));
          for(int index = 0; index < lineCount; ++index){
               int whatLineToRead = (int)(Math.random() * lineCount);
               while(drawnNumber.contains(String.valueOf(whatLineToRead)))
                    whatLineToRead = (int)(Math.random() * lineCount);
               drawnNumber.addElement(String.valueOf(whatLineToRead));
               bufferedWriter.write(lines[whatLineToRead]);
               bufferedWriter.newLine();
          bufferedWriter.flush();
          bufferedWriter.close();
     public static void main(String[] args) throws IOException{
          if(args.length == 1){
               SimpleDateFormat sdf = new SimpleDateFormat("hh:mm:ss", Locale.getDefault());
               System.out.println("Program start at " + sdf.format(new Date()));
               new CacheFirstThenRandomlyPick(args[0]);
               System.out.println("Program ended at " + sdf.format(new Date()));
          else
               System.out.println("Usage: java com.yahoo.ronilloang.idunoifthisright.CacheFirstThenRandomlyPick <text file>");
          System.out.println("\ndone.");
}What do you think? Definitely wrong? Okay. This just what I think. Never mind.
Thank you. -Ronillo