7

I found this code from another question

private void updateLine(String toUpdate, String updated) throws IOException {
    BufferedReader file = new BufferedReader(new FileReader(data));
    String line;
    String input = "";

    while ((line = file.readLine()) != null)
        input += line + "\n";

    input = input.replace(toUpdate, updated);

    FileOutputStream os = new FileOutputStream(data);
    os.write(input.getBytes());

    file.close();
    os.close();
}

This is my file before I replace some lines

example1
example2
example3

But when I replace a line, the file now looks like this

example1example2example3

Which makes it impossible to read the file when there are a lot of lines in it.

How would I go about editing the code above to make my file look what it looked like at the start?

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5
  • @AstroCB Exactly what I said. My file looks clean, everything there line by line, but when I replace any of the lines, it will mess up and put all of the lines together, as seen in the second example. Commented Aug 9, 2014 at 15:57
  • Are you running on a windows based system? So you have to use \r\n as line delimiter. Commented Aug 9, 2014 at 15:57
  • @Jens Yes I am running Windows. I've managed to fix this by using System.getProperty("line.separator"), but I'll keep that in mind as well. Thanks. Commented Aug 9, 2014 at 16:00
  • 1
    Please use readers , writers and write string inside the loop, dont use + to append strings Commented Aug 9, 2014 at 16:09
  • 1
    @KalpeshSoni This is not my code, I found it on a similar question. I'm not too familiar with this stuff so I don't know what I should edit. Commented Aug 9, 2014 at 16:12

3 Answers 3

Reset to default
10

Use System.lineSeparator() instead of \n.

while ((line = file.readLine()) != null)
    input += line + System.lineSeparator();

The issue is that on Unix systems, the line separator is \n while on Windows systems, it's \r\n.

In Java versions older then Java 7, you would have to use System.getProperty("line.separator") instead.

As pointed out in the comments, if you have concerns about memory usage, it would be wise to not store the entire output in a variable, but write it out line-by-line in the loop that you're using to process the input.

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5 Comments

I use Java6, but using System.getProperty("line.separator") instead of \n worked perfectly. Thanks.
Please point out that op's code to read entire file in one string is terrible, he should do the writes in the while loop
@KalpeshSoni Why don't you add a comment to the question yourself? And, it's only terrible if OP actually needs to worry about memory usage, which might not be the case.
developers will never need to worry about memory usage, their system admins will
What is the output of an assignment, such as this (line = file.readLine())? I suppose it is the contents of line, but I didn't know you could do that. Can you confirm?
3

If you read and modify line by line this has the advantage, that you dont need to fit the whole file in memory. Not sure if this is possible in your case, but it is generally a good thing to aim for streaming. In your case this would in addition remove the need for concatenate the string and you don't need to select a line terminator, because you can write each single transformed line with println(). It requires to write to a different file, which is generally a good thing as it is crash safe. You would lose data if you rewrite a file and get aborted.

private void updateLine(String toUpdate, String updated) throws IOException {
    BufferedReader file = new BufferedReader(new FileReader(data));
    PrintWriter writer = new PrintWriter(new File(data+".out"), "UTF-8");
    String line;

    while ((line = file.readLine()) != null)
    {
        line = line.replace(toUpdate, updated);
        writer.println(line);
    }
    file.close();
    if (writer.checkError())
        throw new IOException("cannot write");
    writer.close();
}

In this case, it assumes that you need to do the replace only on complete lines, not multiple lines. I also added an explicit encoding and use a writer, as you have a string to output.

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3 Comments

Sorry had to fix my sample, I was copying to much from the original question: it is supposed to use println(), of course.
Writer does not have a method called println, however a PrintWriter does. Using this code gives me a StackOverflowError on line PrintWriter writer = new PrintWriter(new FileWriter(data + ".out"));
Yes it is PrintWriter. Not sure about the StackOverflow.
1

This is because you use OutputStream which is better for handling binary data. Try using PrintWriter and don't add any line terminator at the end of the lines. Example is here

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