Modifying existing file content in Java

Question

I want to replace the second line file content, can somebody help please based on the below file format and listener method.

1324254875443
1313131
Paid
0.0

2nd line is long and want to replace to currentTimeMillis().

/************** Pay Button Listener **************/
public class payListener implements ActionListener {
    public void actionPerformed(ActionEvent e) {

        ArrayList<String> lines = new ArrayList<String>();
        String line = null;

        try {
            FileReader fr = new FileReader("Ticket/" + ticketIDNumber + ".dat");
            BufferedReader br = new BufferedReader(fr);
            FileWriter fw = new FileWriter("Ticket/" + ticketIDNumber + ".dat");
            BufferedWriter bw = new BufferedWriter(fw);
            while ((line = br.readLine()) != null) {
                if (line.contains("1313131"))
                    line.replace(System.currentTimeMillis();
                lines.add(line);
                bw.write(line);
            }  //end if                 

        }     //end try
        catch (Exception e) {

        }  //end catch   
    }    //end while
}//end method

maltesmann · Answer 1 · 2016-03-17T14:28:22.103

35

Although this question is very old I'd like to add that this can be achieved much easier since Java 1.7 with java.nio.file.Files:

List<String> newLines = new ArrayList<>();
for (String line : Files.readAllLines(Paths.get(fileName), StandardCharsets.UTF_8)) {
    if (line.contains("1313131")) {
       newLines.add(line.replace("1313131", ""+System.currentTimeMillis()));
    } else {
       newLines.add(line);
    }
}
Files.write(Paths.get(fileName), newLines, StandardCharsets.UTF_8);

edited Mar 17 '16 at 14:28

answered Mar 17 '16 at 09:45

maltesmann

445
1
6
13

Thanks for this tip. It's 2017 and I had still been relying on commons io – Sridhar Sarnobat Jun 09 '17 at 07:35
This should be the accepted answer. Straight forward and doesn't encounter an issue I had where the file went blank after reading from it with BufferedReader. – Carlos Medina May 27 '21 at 06:43

score 32 · Accepted Answer · edited May 23 '17 at 11:47

As proposed in the accepted answer to a similar question:

open a temporary file in writing mode at the same time, and for each line, read it, modify if necessary, then write into the temporary file. At the end, delete the original and rename the temporary file.

Based on your implementation, something similar to:

import java.io.BufferedReader;
import java.io.BufferedWriter;
import java.io.File;
import java.io.FileReader;
import java.io.FileWriter;
import java.io.IOException;

public class ReplaceFileContents {
   public static void main(String[] args) {
     new ReplaceFileContents().replace();
   }

   public void replace() {
      String oldFileName = "try.dat";
      String tmpFileName = "tmp_try.dat";

      BufferedReader br = null;
      BufferedWriter bw = null;
      try {
         br = new BufferedReader(new FileReader(oldFileName));
         bw = new BufferedWriter(new FileWriter(tmpFileName));
         String line;
         while ((line = br.readLine()) != null) {
            if (line.contains("1313131"))
               line = line.replace("1313131", ""+System.currentTimeMillis());
            bw.write(line+"\n");
         }
      } catch (Exception e) {
         return;
      } finally {
         try {
            if(br != null)
               br.close();
         } catch (IOException e) {
            //
         }
         try {
            if(bw != null)
               bw.close();
         } catch (IOException e) {
            //
         }
      }
      // Once everything is complete, delete old file..
      File oldFile = new File(oldFileName);
      oldFile.delete();

      // And rename tmp file's name to old file name
      File newFile = new File(tmpFileName);
      newFile.renameTo(oldFile);

   }
}

no idea how this worked for you?! since for me it doesn't even create a file. To fix I create a new FileWriter parsing in a File instance rather than a file name string like this code does. So obviously move `File newFile = new File(tmpFileName)` before creating the FileWriter and use that File instance for the FileWriter. fixed it for me. — dapperwaterbuffalo, Mar 29 '12 at 10:12

score 3 · Answer 3 · edited Dec 19 '11 at 16:21

3

I could suggest to use Apache Commons IO library. There you'll find the class org.apache.commons.io.FileUtils. You can use it:

File file = new File("... your file...");
List<String> lines = FileUtils.readLines(file);
lines.set(1, ""+System.currentTimeMillis());
FileUtils.writeLines(file, lines);

This code reads entire file contents into a List of Strings and changes the second line's content, then writes the list back to the file.

edited Dec 19 '11 at 16:21

melihcelik

4,531
1
21
25

answered Dec 19 '11 at 15:20

Valchev

1,490
14
17

what library isit in, its not working, List is not recognised either. – user1106130 Dec 19 '11 at 19:55

score 2 · Answer 4 · answered Dec 19 '11 at 15:20

2

I'm not sure reading and writing the same file simultaneously is a good idea. I think it would be better to read the file line by line into a String array, replace the second line and then write the String array back into the file.

answered Dec 19 '11 at 15:20

Tudor

61,523
12
102
142

im sure it can be done like above, i get the following error? any ideas? – user1106130 Dec 19 '11 at 19:03

Modifying existing file content in Java

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