NUHMAN.com

1. Introduction

In Java, if you want to read a file line by line and delete specific lines as you read, you typically need to:

• Read the file line by line using a BufferedReader.
• Store only the lines you want to keep.
• Overwrite the file with the filtered content using a BufferedWriter.

This method ensures that the unwanted line(s) are removed from the file, while the remaining content is preserved.

2. Step-by-Step Approach

✅ Steps:

1. Open the file for reading.
2. Read each line and decide whether to keep or skip it.
3. Store the lines to keep in a list or collection.
4. Overwrite the file with only the lines you kept.

3. Java Code Example

Here is a full example that reads a file and deletes a specific line:

java

import java.io.*;
import java.util.ArrayList;
import java.util.List;

public class FileReadAndDeleteLine {
    public static void main(String[] args) {
        String filePath = "path/to/your/file.txt";
        String lineToDelete = "line you want to delete"; // Replace with your line content

        File file = new File(filePath);
        List<String> lines = new ArrayList<>();

        // Step 1: Read the file and collect lines to keep
        try (BufferedReader reader = new BufferedReader(new FileReader(file))) {
            String line;
            while ((line = reader.readLine()) != null) {
                if (!line.equals(lineToDelete)) {
                    lines.add(line);
                }
            }
        } catch (IOException e) {
            e.printStackTrace();
        }

        // Step 2: Write the kept lines back to the file
        try (BufferedWriter writer = new BufferedWriter(new FileWriter(file))) {
            for (String line : lines) {
                writer.write(line);
                writer.newLine();
            }
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

4. How It Works

• Reading the file:
• The BufferedReader reads each line of the file. If the line matches the target line to delete, it's skipped.
• Storing lines:
• Lines that do not match are added to a list.
• Writing back:
• After reading is complete, the BufferedWriter rewrites the file with only the kept lines.

5. Output Example

Assume the file originally contains:

nginx

Hello
World
Test
Java

If lineToDelete = "World", after execution, the file will become:

nginx

Hello
Test
Java

6. Notes

• Overwriting the file: This approach fully replaces the file content.
• Exact match: The deletion is based on exact string match (String.equals()).
• Case sensitivity: This comparison is case-sensitive. If needed, convert both strings to lower/upper case before comparison.
• Large files: For huge files, consider using a temporary file to reduce memory usage or use streaming techniques.

7. Extensions

• Delete lines based on regex patterns.
• Keep a log of deleted lines.
• Support bulk deletion (e.g., delete all lines that start with "temp").