CODEWITHSUNDEEP
java
Bialecki
Bialecki

Reputation: 31041

Replace first line of a text file in Java

I have a text file where I want to change only the first line of the file. The file could be millions of rows long, so I'd rather not have to loop over everything, so I'm wondering if there is another way to do this.

I'd also like to apply some rules to the first line so that I replace instances of certain words with other words.

Is this possible?

Upvotes: 11

Views: 33242

Answers (5)

Godwince Owiti Adera
Godwince Owiti Adera

Reputation: 1

public void main() throws IOException {
    String newString = "The New String";
    File myFile = new File("PathToFile");
    // An array to store each line in the file
    ArrayList<String> fileContent = new ArrayList<String>();
    Scanner myReader = new Scanner(myFile);
    while (myReader.hasNextLine()) {
        // Reads the file content into an array
        fileContent.add(myReader.nextLine());
    }
    myReader.close();
    // Removes Original Line
    fileContent.remove(0);
    // Enters New Line
    fileContent.add(0, newString);
    // Writes the new content to file
    FileWriter myWriter = new FileWriter("PathToFile");
    for (String eachLine : fileContent) {
        myWriter.write(eachLine + "\n");
    }
    myWriter.close();
}

Upvotes: 0

volley
volley

Reputation: 6711

A RandomAccessFile will do the trick, unless the length of the resulting line is different from the length of the original line.

If it turns out you are forced to perform a copy (where the first line is replaced and the rest of the data shall be copied as-is), I suggest using a BufferedReader and BufferedWriter. First use BufferedReader's readLine() to read the first line. Modify it and write it to the BufferedWriter. Then use a char[] array to perform a brute-force copy of the remainder of the file. This will be more efficient than doing the copy line by line. Let me know if you need details..

Another option is to perform the reading and writing inside the same file. It'll be a bit more complex though. :) Let me know if you need details on this as well..

Upvotes: 16

David Schmitt
David Schmitt

Reputation: 59346

If the new line has a different amount of characters (bytes) than the original first line, you will have to re-write the whole file to get rid of the gap or avoid overwriting part of the second line.

Of course, various tools like String.replaceFirst(String regex, String replacement) (javadoc) or the RandomAccessFile (javadoc) can help you with this task.

Upvotes: 4

anjanb
anjanb

Reputation: 13867

apply a regex only once. String.replaceFirst("regex", "replacementstring") : http://java.sun.com/j2se/1.5.0/docs/api/java/lang/String.html#replaceFirst(java.lang.String,%20java.lang.String)

Open the file as RandomAccessFile. Read the 1st line into a string and then apply the change and then write the string back.

Upvotes: 1

jjnguy
jjnguy

Reputation: 138874

You want a RandomAccesssFile. Using the file you can read and write wherever you want in the file.

It is much like an InputStream and OutputStream, but it allows reading and writing wherever you like.

Upvotes: 3

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