如何识别文本文件的行分隔符？ 并且可以在写入该文本文件时将其复制吗？

Question

Working in Java, a homework problem is asking me to

read a file, manipulate the strings and lines in it, (not the problem) write to the file, but keep the line separator that was used in the original file and use it when writing back to the file (big problem), Junit tests will use multiple different line separators for their file inputs.

My question is what methods can I use to identify what line separator the text file is using?

The file texts that are being inputted and read by my project can have /r, /n, /r/n, or a + System.lineSeparator(). When I write to the text file, i also need to match the original line separator so it can be OS friendly.

  String data = "";
    try { data = new String(Files.readAllBytes(Paths.get(path)));
    } catch (IOException e) { e.printStackTrace(); }
    //System.out.println("Text file as String in Java");
    //System.out.println(data);
    String linesep= "";
    if (data.contains("\n") == true){
        linesep="\n";
        //System.out.println("n detected");
    }else if((data.contains("\r") == true)){
        linesep="\r";
        //System.out.println("r detected");
         }
    else {
        linesep= System.lineSeparator();
        //System.out.println("line separator detected");
    }

Answer 1

The requirements as you showed them to us say:

Junit tests will use multiple different line separators for their file inputs.

They do NOT say that each test will use a consistent line separator throughout the file. They also do NOT say that last line will always end with a separator.

If you write your code to try to find out which separator is used, it will break when the file has mixed separators.

So what you need to do is to preserve the separator at the end of each line including empty lines. And you need to deal with the final line which may have no separator.

Hint: the line separator characters are just characters, so you could include them in the line strings ... if you decide to split the input into lines at all.

... and can you copy it when writing into that text file?

See above!

Answer 2

As far as I know, there is no way to tell without question what the line separator for a particular file is, without some additional information that is not in the file itself.

As others have pointed out, the carriage-return and line-feed characters are just characters - there's nothing special about them. It just convention that these are separators, and the convention is different on Windows than it is on Linux and Mac OS.

However, especially since it sounds like your program will be writing the files, you can try to make some assumptions:

• Each file will use one and only one of the three "standard" line separators: \\r, \\n, or \\r\\n

• Each file will not contain any of the other two line separators

If you can safely make those assumptions, then you can simply read the file (as a binary file, not as a text file) and inspect the characters, looking for one of the line endings.

To append new lines to the file with a matching separator, you can set the system line separator with, eg

System.setProperty("line.separator", "\r\n");

using the line separator that you found in the file.

That should cause the new separator to be used when you write to the file normally.