How to replace a specific line in a file using Java?

Question

How do I write over a specific line in a text file using FileWriter and PrintWriter? I don't want to have to make a new file every time.

Edit: Can I just cycle through the file, get the length of the String at the indicated line number, and then use that length to backspace once I get to that line (to delete the String), and write in the new data?

public static void setVariable(int lineNumber, String data) {
    try { 
        // Creates FileWriter. Append is on.
        FileWriter fw = new FileWriter("data.txt", true);       

        PrintWriter pw = new PrintWriter(fw);       

        //cycles through file until line designated to be rewritten is reached
        for (int i = 1; i <= lineNumber; i++) {     
            //TODO: need to figure out how to change the append to false to overwrite data
            if (i == lineNumber) {
                pw.println(data);
                pw.close();
            } else {          
                // moves printwriter focus to next line (doesn't overwrite)
                pw.println(""); 
            }
        } 
    }
}

Maybe this answer will help? stackoverflow.com/a/6477893/1797341 — Hugo M. Zuleta
– Hugo M. Zuleta, Commented Jul 13, 2015 at 5:36

Paul Vargas · Accepted Answer · 2015-07-13 06:20:03Z

11

If you are using Java 7 or higher and if lineNumber starts in 1, you can do the following:

public static void setVariable(int lineNumber, String data) throws IOException {
    Path path = Paths.get("data.txt");
    List<String> lines = Files.readAllLines(path, StandardCharsets.UTF_8);
    lines.set(lineNumber - 1, data);
    Files.write(path, lines, StandardCharsets.UTF_8);
}

Obviously if lineNumber starts in 0, then:

lines.set(lineNumber, data);

edited Jul 13, 2015 at 6:20

answered Jul 13, 2015 at 5:50

Paul Vargas

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5 Comments

Telle Miller Over a year ago

Would this work with any line? E.g. I have 90 lines in the file and I want to write over the 34th.

Telle Miller Over a year ago

Do I import java.awt.List or java.util.List ?

Paul Vargas Over a year ago

Ah! The imports are import java.nio.file.Files; import java.nio.file.Path; import java.nio.file.Paths; import java.util.List;. :)

Telle Miller Over a year ago

This causes two errors. The first is "The method readAllLines(Path, Charset) in the type Files is not applicable for the arguments (Path)" and the second is "The method write(Path, byte[], OpenOption...) in the type Files is not applicable for the arguments (Path, List<String>)"

Paul Vargas Over a year ago

Ah! Sorry! The code is for Java 8. Updated to Java 7. Try again. :)

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