How to Read File Line by Line in Java - Simple Guide

To read a file line by line in Java, use BufferedReader with a FileReader. Call readLine() in a loop until it returns null, which means the file ended.

Syntax

Use BufferedReader wrapped around a FileReader to read text files efficiently. The readLine() method reads one line at a time and returns null when no more lines exist.

• BufferedReader br = new BufferedReader(new FileReader("filename.txt")); - opens the file.
• String line = br.readLine(); - reads one line.
• while (line != null) { ... } - loops until end of file.
• br.close(); - closes the file to free resources.

java

BufferedReader br = new BufferedReader(new FileReader("filename.txt"));
String line;
while ((line = br.readLine()) != null) {
    // process line
}
br.close();

Example

This example reads a file named example.txt line by line and prints each line to the console.

java

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

public class ReadFileLineByLine {
    public static void main(String[] args) {
        try (BufferedReader br = new BufferedReader(new FileReader("example.txt"))) {
            String line;
            while ((line = br.readLine()) != null) {
                System.out.println(line);
            }
        } catch (IOException e) {
            System.err.println("Error reading file: " + e.getMessage());
        }
    }
}

Output

Hello, this is line 1.
This is line 2.
And this is line 3.

Common Pitfalls

Common mistakes include not closing the BufferedReader, which can cause resource leaks, and not handling IOException. Also, using FileReader alone without buffering can be slower for large files.

Another mistake is reading lines without checking for null, which leads to NullPointerException.

java

/* Wrong way: Not closing BufferedReader and no exception handling */
BufferedReader br = new BufferedReader(new FileReader("file.txt"));
String line = br.readLine();
while (line != null) {
    System.out.println(line);
    line = br.readLine();
}

/* Right way: Using try-with-resources and handling exceptions */
try (BufferedReader br = new BufferedReader(new FileReader("file.txt"))) {
    String line;
    while ((line = br.readLine()) != null) {
        System.out.println(line);
    }
} catch (IOException e) {
    e.printStackTrace();
}

Quick Reference

• Use BufferedReader with FileReader for efficient reading.
• Always check if readLine() returns null to detect end of file.
• Use try-with-resources to auto-close the reader.
• Handle IOException to catch file errors.

Key Takeaways

Use BufferedReader with FileReader to read files line by line efficiently.

Always check for null from readLine() to avoid errors.

Use try-with-resources to automatically close the file reader.

Handle IOException to manage file reading errors gracefully.

Avoid reading files without buffering for better performance.