How can I read a large text file line by line using Java?

Question

I need to read a large text file of around 5-6 GB line by line using Java.

How can I do this quickly?

Answer 1

You can use this code:

import java.io.BufferedReader;
import java.io.File;
import java.io.FileReader;
import java.io.IOException;

public class ReadTextFile {

    public static void main(String[] args) throws IOException {

        try {

            File f = new File("src/com/data.txt");

            BufferedReader b = new BufferedReader(new FileReader(f));

            String readLine = "";

            System.out.println("Reading file using Buffered Reader");

            while ((readLine = b.readLine()) != null) {
                System.out.println(readLine);
            }

        } catch (IOException e) {
            e.printStackTrace();
        }

    }

}

Answer 2

A common pattern is to use

try (BufferedReader br = new BufferedReader(new FileReader(file))) {
    String line;
    while ((line = br.readLine()) != null) {
       // process the line.
    }
}

You can read the data faster if you assume there is no character encoding. e.g. ASCII-7 but it won't make much difference. It is highly likely that what you do with the data will take much longer.

EDIT: A less common pattern to use which avoids the scope of line leaking.

try(BufferedReader br = new BufferedReader(new FileReader(file))) {
    for(String line; (line = br.readLine()) != null; ) {
        // process the line.
    }
    // line is not visible here.
}

---

UPDATE: In Java 8 you can do

try (Stream<String> stream = Files.lines(Paths.get(fileName))) {
        stream.forEach(System.out::println);
}

NOTE: You have to place the Stream in a try-with-resource block to ensure the #close method is called on it, otherwise the underlying file handle is never closed until GC does it much later.

Answer 3

FileReader won't let you specify the encoding, use InputStreamReaderinstead if you need to specify it:

try {
    BufferedReader br = new BufferedReader(new InputStreamReader(new FileInputStream(filePath), "Cp1252"));         

    String line;
    while ((line = br.readLine()) != null) {
        // process the line.
    }
    br.close();

} catch (IOException e) {
    e.printStackTrace();
}

If you imported this file from Windows, it might have ANSI encoding (Cp1252), so you have to specify the encoding.

Answer 4

You can use streams to do it more precisely:

Files.lines(Paths.get("input.txt")).forEach(s -> stringBuffer.append(s);

Answer 5

What you can do is scan the entire text using Scanner and go through the text line by line. Of course you should import the following:

import java.io.File;
import java.io.FileNotFoundException;
import java.util.Scanner;
public static void readText throws FileNotFoundException {
    Scanner scan = new Scanner(new File("samplefilename.txt"));
    while(scan.hasNextLine()){
        String line = scan.nextLine();
        //Here you can manipulate the string the way you want
    }
}

Scanner basically scans all the text. The while loop is used to traverse through the entire text.

The .hasNextLine() function is a boolean that returns true if there are still more lines in the text. The .nextLine() function gives you an entire line as a String which you can then use the way you want. Try System.out.println(line) to print the text.

Side Note: .txt is the file type text.

Answer 6

BufferedReader br;
FileInputStream fin;
try {
    fin = new FileInputStream(fileName);
    br = new BufferedReader(new InputStreamReader(fin));

    /*Path pathToFile = Paths.get(fileName);
    br = Files.newBufferedReader(pathToFile,StandardCharsets.US_ASCII);*/

    String line = br.readLine();
    while (line != null) {
        String[] attributes = line.split(",");
        Movie movie = createMovie(attributes);
        movies.add(movie);
        line = br.readLine();
    }
    fin.close();
    br.close();
} catch (FileNotFoundException e) {
    System.out.println("Your Message");
} catch (IOException e) {
    System.out.println("Your Message");
}

It works for me. Hope It will help you too.