Java replace specific string in text file

Question

I've got a text file called log.txt. It's got the following data

1,,Mon May 05 00:05:45 WST 2014,textFiles/a.txt,images/download.jpg
2,,Mon May 05 00:05:45 WST 2014,textFiles/a.txt,images/download.jpg

The numbers before the first comma are indexes that specify each item.

What I want to do is to read the file and then replace one part of the string(e.g. textFiles/a.txt) in a given line with another value(e.g. something/bob.txt).

This is what I have so far:

    File log= new File("log.txt");
                    String search = "1,,Mon May 05 00:05:45 WST 2014,textFiles/a.txt,images/download.jpg;
                    //file reading
                    FileReader fr = new FileReader(log);
                    String s;
                    try (BufferedReader br = new BufferedReader(fr)) {
                        
                        while ((s = br.readLine()) != null) {
                            if (s.equals(search)) {
                                //not sure what to do here
                            }
                        }
                    }

Solution 1

One approach would be to use String.replaceAll():

File log= new File("log.txt");
String search = "textFiles/a\\.txt";  // <- changed to work with String.replaceAll()
String replacement = "something/bob.txt";
//file reading
FileReader fr = new FileReader(log);
String s;
try {
    BufferedReader br = new BufferedReader(fr);

    while ((s = br.readLine()) != null) {
        s.replaceAll(search, replacement);
        // do something with the resulting line
    }
}

You could also use regular expressions, or String.indexOf() to find where in a line your search string appears.

Solution 2

You could create a string of total file content and replace all the occurrence in the string and write to that file again.

You could something like this:

File log= new File("log.txt");
String search = "textFiles/a.txt";
String replace = "replaceText/b.txt";

try{
    FileReader fr = new FileReader(log);
    String s;
    String totalStr = "";
    try (BufferedReader br = new BufferedReader(fr)) {

        while ((s = br.readLine()) != null) {
            totalStr += s;
        }
        totalStr = totalStr.replaceAll(search, replace);
        FileWriter fw = new FileWriter(log);
    fw.write(totalStr);
    fw.close();
    }
}catch(Exception e){
    e.printStackTrace();
}

Comments

[Pindo]

once it is replaced how to i write it back to the same file?

[Jason]

You shouldn't write back into the same file while you are reading it. You could write a new file, and then at the end of the process delete the file you read and rename the new one.

[drakonli]

Solution removes line breaks if there are any

[Paulo Pereira]

The entire file must be read to memory. Can be a problem if the file is too big.