How to Use Files.readAllLines in Java: Simple Guide

Use Files.readAllLines(Path path) to read all lines from a file into a List<String>. Provide the file path as a Path object, and it returns all lines as a list, making file reading simple and direct.

Syntax

The method Files.readAllLines reads all lines from a file and returns them as a List<String>. You need to pass a Path object representing the file location.

• Path path: The file path to read from.
• Returns: List<String> containing all lines.
• Throws IOException if the file cannot be read.

java

List<String> lines = Files.readAllLines(Path.of("filename.txt"));

Example

This example shows how to read all lines from a file named example.txt and print each line to the console.

java

import java.nio.file.Files;
import java.nio.file.Path;
import java.io.IOException;
import java.util.List;

public class ReadAllLinesExample {
    public static void main(String[] args) {
        Path filePath = Path.of("example.txt");
        try {
            List<String> lines = Files.readAllLines(filePath);
            for (String line : lines) {
                System.out.println(line);
            }
        } catch (IOException e) {
            System.out.println("Error reading file: " + e.getMessage());
        }
    }
}

Output

Hello, world!
This is a sample file.
It has multiple lines.

Common Pitfalls

Common mistakes when using Files.readAllLines include:

• Not handling IOException, which is required because file reading can fail.
• Using an incorrect or non-existing file path, causing the method to throw an exception.
• Reading very large files, which can cause memory issues since all lines are loaded at once.

Always check the file path and handle exceptions properly.

java

import java.nio.file.Files;
import java.nio.file.Path;
import java.io.IOException;
import java.util.List;

public class WrongUsage {
    public static void main(String[] args) {
        // This will cause a compile error because IOException is not handled
        // List<String> lines = Files.readAllLines(Path.of("missing.txt"));
    }
}

// Correct usage:

public class CorrectUsage {
    public static void main(String[] args) {
        try {
            List<String> lines = Files.readAllLines(Path.of("missing.txt"));
        } catch (IOException e) {
            System.out.println("File not found or cannot be read.");
        }
    }
}

Quick Reference

• Method: Files.readAllLines(Path path)
• Returns: List<String> of all lines
• Throws: IOException if file is missing or unreadable
• Use case: Small to medium text files
• Alternative: Use Files.lines(Path) for large files (streaming)

Key Takeaways

Use Files.readAllLines with a Path object to read all lines from a file into a List.

Always handle IOException to avoid program crashes when the file is missing or unreadable.

Files.readAllLines loads the entire file into memory, so avoid it for very large files.

Use Path.of("filename") to create the Path object easily in modern Java versions.

For large files, consider using Files.lines for streaming lines instead of reading all at once.