How to get a random line of a text file in Java?

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Say there is a file too big to be put to memory. How can I get a random line from it? Thanks.

Update: I want to the probabilities of getting each line to be equal.

Here's a solution. Take a look at the choose() method which does the real thing (the main() method repeatedly exercises choose(), to show that the distribution is indeed quite uniform).

The idea is simple: when you read the first line it has a 100% chance of being chosen as the result. When you read the 2nd line it has a 50% chance of replacing the first line as the result. When you read the 3rd line it has a 33% chance of becoming the result. The fourth line has a 25%, and so on....

import java.util.*;

public class B {

  public static void main(String[] args) throws FileNotFoundException {
     Map<String,Integer> map = new HashMap<String,Integer>();
     for(int i = 0; i < 1000; ++i)
        String s = choose(new File("g:/temp/a.txt"));
           map.put(s, 0);
        map.put(s, map.get(s) + 1);


  public static String choose(File f) throws FileNotFoundException
     String result = null;
     Random rand = new Random();
     int n = 0;
     for(Scanner sc = new Scanner(f); sc.hasNext(); )
        String line = sc.nextLine();
        if(rand.nextInt(n) == 0)
           result = line;         

     return result;      

How to Fetch Random Line From Text File In Java, import java. util. Random; import java. io. File; import java. io. FileNotFoundException; import java. util. Scanner; public class RandomFileReading { public static void main (String args ) { try{ File files = new File("codespeedy.txt"); If if you don’t know the clue to fetch or get the random line from the text file through a simple java code then you are at the right spot to figure out your problem. We are going to bring the irregular line from the content document through the java code. Fetch Random Line From Text File In Java. Let’s learn with some easy examples.

Reading the entire file if you want only one line seems a bit excessive. The following should be more efficient:

  1. Use RandomAccessFile to seek to a random byte position in the file.
  2. Seek left and right to the next line terminator. Let L the line between them.
  3. With probability (MIN_LINE_LENGTH / L.length) return L. Otherwise, start over at step 1.

This is a variant of rejection sampling.

Line lengths include the line terminator character(s), hence MIN_LINE_LENGTH >= 1. (All the better if you know a tighter bound on line length).

It is worth noting that the runtime of this algorithm does not depend on file size, only on line length, i.e. it scales much better than reading the entire file.

Picking random lines out of text files, Read the file line by line into a List or a Map, then fetch a random entry from here. (Judging by readAllLines(Paths.get("icaocodes.txt"));. Note that Files is available in Java 7 and later but, the readAllLines version I use above is in Java 8 only. The answer uses 8 API classes, you can use the following links to see more code examples. - - java.util.HashMap - java.lang.String

Either you

  1. read the file twice - once to count the number of lines, the second time to extract a random line, or

  2. use reservoir sampling

How to get a random line of a text file in Java?, Get a random line of a text file in Java. Code example extracted from Stack Overflow: import*;  Read the file line by line into a List or a Map, then fetch a random entry from here. (Judging by the brief details of what you're working on, a Map of different objects might be a good choice if you'd like to parse a file into different objects for flights, etc.)

Looking over Itay's answer, it looks as though it reads the file a thousand times over after sampling one line of the code, whereas true reservoir sampling should only go over the 'tape' once. I've devised some code to go over code once with real reservoir sampling, based on this and the various descriptions on the web.

import java.util.List;

public class reservoirSampling {

    public static void main(String[] args) throws FileNotFoundException, IOException{
        Sampler mySampler = new Sampler();
        List<String> myList = mySampler.sampler(10);
        for(int index = 0;index<myList.size();index++){

import java.util.ArrayList;
import java.util.List;
import java.util.Random;
import java.util.Scanner;

public class Sampler {

    public Sampler(){}
    public List<String> sampler (int reservoirSize) throws FileNotFoundException, IOException
        String currentLine=null;
        //reservoirList is where our selected lines stored
        List <String> reservoirList= new ArrayList<String>(reservoirSize); 
        // we will use this counter to count the current line number while iterating
        int count=0; 

        Random ra = new Random();
        int randomNumber = 0;
        Scanner sc = new Scanner(new File("Open_source.html")).useDelimiter("\n");
        while (sc.hasNext())
            currentLine =;
            count ++;
            if (count<=reservoirSize)
            else if ((randomNumber = (int) ra.nextInt(count))<reservoirSize)
                reservoirList.set(randomNumber, currentLine);
        return reservoirList;

The basic premise is that you fill up the reservoir, and then go back to it and fill in random lines with a 1/ReservoirSize chance. I hope this provides more efficient code. Please let me know if this doesn't work for you, as I've literally knocked it up in half an hour.

Random Number Generation in Java, How do you read a specific line from a text file in Java? Suppose, we have a text file “colors.txt” which is of 5 lines and the content of the text file is given below: As you can see, the text file “colors.txt” contains with five colors, one in each line. Now we are going to choose a random line and get the text from that line.

Use RandomAccessFile:

  1. Construct a RandomAccessFile, file
  2. Get the length of that file, filelen, by calling file.length()
  3. Generate a random number, pos, between 0 and filelen
  4. Call to seek to the random position
  5. Call file.readLine() to get to the end of the current line
  6. Read the next line by calling file.readLine() again

Using this method, I've been sampling lines from the Brown Corpus at random, and can easily retrieve a 1000 random samples from randomly chosen files in a few seconds. If I tried to do the same by reading through each file line-by-line it would take me much longer.

The same principle can be used for selecting random elements from a list. Rather than reading through the list and stopping at a random place, if you generate a random number between 0 and the length of the list, then you can index directly into the list.

java, How do you read a random line from a text file in Python? If you add all of these of a List you can then get the size and select a random number between 0 and size(), then use this to get the String for the collection. You now have a random word from the file that you just read in.

Print A Random Line From A Text File, Use a BufferedReader with the readLine() method. You can either skip lines as you read them, or place each line into an ArrayList, and process  Java – Read File Line by Line – LineNumberReader. If you want to read a file’s content line by line only in certain requirement the LineNumberReader could be perfect choice for you. LineNumberReader is a subclass of the BufferedReader class and allows you to keep track of which line you’re currently processing. Line numbering begins at 0.

random « text file « Java I/O Q&A, Hi, I do not understand how to print a random line from a text file. So far in my program i have been able to read the file but not to print out a  you would then need to allow for the inner loop jumping in steps of 15, (you will get 105 random numbers currently) suggest you add a break if count = 100 in the inner loop

Answers to Questions and Exercises: (The Java™ Tutorials , Say there is a file too big to be put to memory. How can I get a random line from it​? Thanks. Update: I want to the probabilities of getting each line to Store the lines of the text file in a string vector using push_back() function. If you don’t know about push_back() function, See this: push_back() and pop_back() function in C++; Generate a random number lying between 0 and count of total lines using rand() function and modulus operator. Fetch the line for which the line index is equal to the random number. Assuming the indexing starts from 0.

  • An implementation of reservoir sampling
  • Amazing. Never heard about reservoir sampling. What about if my file is MBs? Are there any performace issues? If yes, are there alternatives to avoid a full file scan?
  • Am I right and assuming this is for a fixed n=1, where n is number of 'samples'? Is there a way to get choose to choose more than one at once? as it stands you 'loop over the tape' more than once, or at least attempt to which seems inefficient.
  • Excellent! If the file will be be sampled repeatedly, use a single pass to collect a List<Integer> of offsets, which may then be randomized via Collections.shuffle().
  • This should be the best answer.
  • I've put this up for review.
  • How do I ensure the file isn't over when I want to stop? I.e. how do I know the number of lines if a file?
  • Also, I want the probalilities of getting each single line to be equal.
  • @Dinuk, so if the file is smaller than the others, I'll get the last line too often, if the file is larger - I'll get it too seldom
  • THen you have to read the file twice or if all lines have an equal length, you can calculate the number of lines from the file size