What is the most efficient way to read a large text file line by line in Java, and how can I handle memory effectively for files around 5-6 GB?

General Discussions
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1

I need to Java read file line by line without loading the entire file into memory. What are the best practices or built-in classes I should use to do this quickly and efficiently?

2

When dealing with huge files—like 5 to 6 GB—you definitely don’t want to load everything into memory at once. Over the years, I’ve found that the best approach is to process files java read file line by line without ever bringing the entire file into memory. This is where Java’s built-in tools come into play, and for large files, BufferedReader is your best friend.

Here’s the code for that:

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

try (BufferedReader br = new BufferedReader(new FileReader("largefile.txt"))) {
    String line;
    while ((line = br.readLine()) != null) {
        // process the line
        System.out.println(line);
    }
} catch (IOException e) {
    e.printStackTrace();
}

Why it works so well? BufferedReader reads chunks of characters at a time, which significantly reduces memory usage. You’re not reading the entire file, just one line at a time. This makes it ideal for handling large files like the ones you’re describing.

3

I totally agree with Joe, and if you’re working with Java 8 or later, there’s an even more modern way to handle it using Streams. I’ve been using this approach lately because it feels much more elegant, and it’s perfect for those who appreciate a functional programming style. Plus, it’s still very memory efficient—java read file line by line works just as well with this method.

Here’s how you’d use it:

import java.nio.file.Files;
import java.nio.file.Paths;
import java.io.IOException;
import java.util.stream.Stream;

try (Stream<String> stream = Files.lines(Paths.get("largefile.txt"))) {
    stream.forEach(System.out::println); // or any other processing logic
} catch (IOException e) {
    e.printStackTrace();
}

The cool part? The Files.lines() method lazily loads each line, meaning you won’t run into memory issues with large files. However, a small tip—if you chain too many stream operations, you might end up using more memory than necessary. So, it’s a good idea to keep it simple when dealing with huge files.

4

Both of the previous approaches are solid, but if you’re after something a bit simpler, you could also use the Scanner class. It’s lightweight and easy to use, which makes it a good option for smaller tasks or if you’re just getting started. I personally turn to it when I don’t need the complexity of streams or buffered readers.

Here’s an example for java read file line by line:

import java.io.File;
import java.io.FileNotFoundException;
import java.util.Scanner;

try (Scanner scanner = new Scanner(new File("largefile.txt"))) {
    while (scanner.hasNextLine()) {
        String line = scanner.nextLine();
        System.out.println(line);
    }
} catch (FileNotFoundException e) {
    e.printStackTrace();
}

When to use this: It’s ideal for simpler tasks, but just a heads-up—Scanner can be a bit slower compared to BufferedReader when dealing with large files. For massive files, you’ll likely notice a performance drop, but it works perfectly fine for smaller projects or where performance isn’t as critical.