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 · Accepted Answer · 2016-03-17 14:28:22Z

39

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, 2016 at 14:28

answered Mar 17, 2016 at 9:45

maltesmann

4951 gold badge7 silver badges14 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

Sridhar Sarnobat Over a year ago

Thanks for this tip. It's 2017 and I had still been relying on commons io

girraiffe Over a year ago

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.

Community · Accepted Answer · 2017-05-23 11:47:11Z

32

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);

   }
}

edited May 23, 2017 at 11:47

CommunityBot

11 silver badge

answered Dec 19, 2011 at 15:22

melihcelik

4,5991 gold badge23 silver badges25 bronze badges

1 Comment

dapperwaterbuffalo Over a year ago

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.

melihcelik · Accepted Answer · 2011-12-19 16:21:20Z

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, 2011 at 16:21

melihcelik

4,5991 gold badge23 silver badges25 bronze badges

answered Dec 19, 2011 at 15:20

Valchev

1,53014 silver badges17 bronze badges

1 Comment

user1106130 Over a year ago

what library isit in, its not working, List is not recognised either.

Tudor · Accepted Answer · 2011-12-19 15:20:44Z

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, 2011 at 15:20

Tudor

62.6k13 gold badges105 silver badges148 bronze badges

1 Comment

user1106130 Over a year ago

im sure it can be done like above, i get the following error? any ideas?

Collectives™ on Stack Overflow

Modifying existing file content in Java

4 Answers 4

2 Comments

1 Comment

1 Comment

1 Comment

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

4 Answers 4

2 Comments

1 Comment

1 Comment

1 Comment

Your Answer

Sign up or log in

Post as a guest

Linked

Related